Selected Aspects of Camel Immune System and Immune Responses

The camel economy is of considerable importance for arid countries. In the last decade, studies about camel immune system and immune responses have recorded increasing interest. However, drawing a comprehensive picture of the camel immune system remains far from reached. A major part of this review is to cover the studies of the primary and secondary immune organs and the markers of the camel immune cells and certain lymphoid tissues. At the same time, immune responses to different diseases and the nature of effective immunity were included, with an emphasis on the most important zoonotic diseases in camels such as MERS CoV; brucellosis. New findings on the diversity mechanisms of camel immunoglobulin genes were addressed. How-ever, detail of the mechanism of MHC-restricted cellular immunity and the mechanism of B lymphocyte activation in camels await further attention. In-terestingly, the gross and the histological structure of the


Introduction
In the Qur'an, the camel is a miracle of God, distinct from other animals mentioned in the holy book: "Do (these unbelievers) not observe the camels: how they were created?" (Surah al Ghashiya, Verse 17). Moreover, camel forms an integral part of the culture and agriculture of many people around the world [1].
The camel population was estimated at 35 million heads worldwide [2], of which 89% are dromedary camels (Camelus dromedarius) and 11% are Bactrian (Camelus bactrianus and Camelus ferus) [3]. The Camelidae family is divided into two tribes, the new world camel (Lamini) and the old-world camel (Camelini). Within the Camelini, there are two domesticated species-the twohumped Bactrian camel (Camelus bactrianus) and the single-humped dromedary camel (Camelus dromedarius)-in addition to the critically endangered two-humped wild Bactrian camel (Camelus ferus), which inhabits northern China and southern Mongolia [2].
The camel economy is of considerable importance for arid countries. The dromedary camel can survive and produce a considerable amount of milk during recurrent and prolonged hot and dry periods [4]. Camel milk and meat are considered important sources of protein for a wide-ranging population [5].
Thus, camel milk is considered one of the most valuable food sources due to its nutritional value and medicinal properties, such as treatment of jaundice, tuberculosis, asthma, and anaemia, and as a laxative [6]. In addition, camel milk is rich with several inhibitory proteins, such as its high concentration of lysozyme, lactophorin (proteose peptone component-3), which is a member of the glycosylation-dependent cell adhesion molecule-1 (GlyCAM-1), and the peptidoglycan recognition protein (PGRP), which plays an important role in preventing adhesion and bacterial multiplication [7]. The female camel lactation period varies from 9 -8 months and the estimated annual milk production ranges between 800 and 3600 litres [8]. The level of camel milk production is influenced by various factors, such as type of breed and the husbandry system [8]. The estimated camel meat production was increased from 123,000 tons in 1961 to 356,000 tons in 2009 [9]. The estimated contribution of camel meat to world meat production was 0.13% in general, and 0.45% to red meat production in 2009 [9]. The economic importance of camel meat is of significance as a nutritive value, beside its traditional remedial uses [10]. Camel meat is favored due to its low cholesterol content for those who suffer from problems of diabetes and high cholesterol or related medical conditions [10].
There is no evidence of resistance or susceptibility of certain camel breeds to certain infections. Nevertheless, in general camels are resistant to certain infec-

The Central Lymphatic Organs
The primary lymphoid organs refer to organs in which lymphocytes are generated and undergo development and maturation. (Figure 1(A)) In the dromedary camel, the thymus gland is located in the thoracic cavity, at the anterior superior mediastinum, in front of the heart and behind the sternum [20]. It may extend to the caudal forth part of the neck, where it can be seen between the trachea and the left external jugular vein [21]. Both lobes are surrounded by a thin connective tissue capsule from which thin connective tissue-septa are sent, dividing incompletely the lobe into lobules. These thymic lobules are irregular in outline and consist of an outer cortex and inner medulla ( Figure 1(A)). The cortex is darkly stained, because of numerous T-lymphocytes (thymocytes). Beside the thymocytes, there are epithelial reticular (epithelioreticular) cells, which have stellate-shaped and are numerically much less than thymocytes [22]. In addition, numerous granulated cells of different shapes

Bone Marrow
The literature lacks the histological details of the camel's bone marrow. The scant retrieved data indicated that camel's bone marrow is similar to that of other mammals [24].

Mucosal Associated Lymphatic Tissue (MALT) 1) Diffuse unencapsulated lymphatic tissue
Camels have wide range of unencapsulated lymphoid tissues, mainly distributed in the lamina propria of the glandular stomach in the form of diffuse lymphatic tissue, solitary lymphoid nodules and aggregated lymphoid nodules [25].
On the other hand, there are few lymphoid tissues in the form of sporadic lymphocytes and plasma cells in the oesophagus and the non-glandular stomach.
The Bronchial associated lymphoid tissue (BALT) of the camel is mainly displayed by isolated or aggregates of lymphoid follicles, which are scattered along the bronchial tree in the entire lung. The density of lymphoid tissue increases from the trachea to the lower graded branches (densest in the bronchioles) and then decreases at certain locations around respiratory bronchioles or among the pulmonary mesenchyme [26].

2) Nodular lymphatic Tissue a) Tonsils
In a comprehensive study, Achaaban et al.  the lingual and the velar tonsils-whereas the group nasopharynx includes the   pharyngeal and tubal tonsils, and the final group-the laryngopharynx-is formed of the paraepiglotic tonsil [27].

i) The Palatine Tonsil
This tonsil is well developed in dromedary camels, with several macroscopic nodules in the lateral wall of the oropharynx. It is located within a tonsillar fossulae, which extends between the palatoglossal and palatopharyngeal arches ( Figure 2, Figure 3). Microscopically, the spherical masses of lymphatic cells are arranged into heavily cellular ovoid or spherical nodules and less populated internodular areas. The palatine tonsil is encapsulated within a thick layer of stratified squamous epithelium non-keratinized which is reflected inside the tonsil, forming a blind ended crypt (Figure 1(B)). But the high stratified squamous mucosal epithelium changes to thinner epithelium and is usually infiltrated with lymphocytes, forming a reticular epithelium [19] [27] [28].
ii) The Lingual Tonsil This tonsil is formed of a cluster of spheroidal lymphoid masses at the root of the tongue protrusion into the oropharynx ( Figure 2). The lingual tonsil of the dromedary camel is macroscopically visible at the root of the tongue in the form of several spherical macroscopic nodules protruding into the oropharynx [27] [28]. It has been reported that the crypt is not clearly visible in the lingual tonsil of the dromedary camel [27]. Zidan and Pabst (2019) described more than one crypt in the dromedary's lingual tonsils [29]. Histologically, the lingual tonsil of the camel is made of a cluster of lymphoid nodules and internodular tissues. The mucosal surface of each nodule is covered by a keratinized stratified squamous epithelium, with a similar epithelium of the lingual surface [27] [28].
iii) The Velar (Soft Palate) Tonsil This tonsil is formed of disseminated nodules with crypts on the oropharyngeal (oral) surface of the soft palate, close to the junction with the hard palate ( Figure 2; Figure 4) [27]. The nodules of the velar tonsil are formed of primary     [27].
iv) The Pharyngeal Tonsil In the dromedary camel, this tonsil is a prominent lymphoid structure located within the median part of the dorsal wall of the nasopharynx, behind the nasal septum ( Figure 2). Histologically, this tonsil is represented by lymphoid nodules covered by stratified squamous keratinized [27].

v) The Tubal Tonsil
This tonsil is located on both lateral walls of the nasopharynx, close to the opening of the auditory tube ( Figure 2). The lymphoid nodules of this tonsil are diffused and arranged within the epithelial folds of nasopharynx, resembling that of the pharyngeal tonsil [27].

vi) The Paraepiglotic Tonsil
This tonsil is located on both sides at the base of the epiglottis, forming circular clustered cryptic lymphoid nodules ( Figure 2; Figure 5). Histologically, this tonsil has similar features to the oropharynx by forming smaller aggregates of lymphoid nodules molded together, constituting a very compact tonsil [27].

b) Peyer's Patches (PPs)
Peyer's patches, also known as intestinal tonsils, are subepithelial aggregations of lymphoid tissue located along the antimesenteric side of the small intestine. In young dromedary camels, the PPs are cup-shaped masses which elevate for about 1 cm above the luminal surface [31]. Each of these masses is formed of lymphoid ules ranges from 500 -900 µm [31]. In the adult dromedary camel, the number and size of the PPs are less than that of young camels. Similar to other species, the mucosa which is occupied by the PPs lacks any intestinal villi [32].
c) The Lymph Node (Figure 1(C)) The lymph node is encapsulated in lymphoid tissue and is designed to filter lymph. Zidan and Pabst (2012) demonstrated that the capsule of the lymph node is formed of two layers, an outer, thicker layer of connective tissue, and an inner thinner layer consisting mainly of smooth muscles [33]. From the capsule, the trabeculae extend toward the centre of the lymph node, dividing the parenchyma characteristically into incomplete lobules [33]. In this respect, two types of trabeculae have been recorded in the dromedary camel: type I is formed of two layers of connective tissue and muscle in which branches of blood and lymphatic vessels are located; type II is formed of smooth muscles only [34]. Moreover, the lymph node receives one or two large afferent lymphatics, which penetrate the capsule and may extend to the connective tissue seta, and are drained by four or five efferent lymphatics [34] [35].
d) The Spleen (Figure 1(D)) The spleen is the largest lymphatic organ in domestic animals [36]. In the Open Journal of Veterinary Medicine dromedary camel, the spleen is crescent shaped and is located at the dorsocaudal aspect of the omentum [21]. The spleen is surrounded by thick connective tissue capsule and is covered externally by mesothelial cells [36]. The capsule can be easily subdivided into outer and inner layers: the outer layer is composed of connective tissue including collagen, elastic and fibroblast, in addition to a few smooth muscle cells, and the inner layer is formed of smooth muscles supported by connective tissues [36] [37]. The trabeculae, which are either vascular or avascular, extend from the capsule to the parenchyma [37]. The vascular trabeculae contain nerve fibers and arteries without veins, while the avascular can be further subdivided into primary and secondary trabeculae. The primary trabeculae are composed mainly of smooth muscle cells supported by reticular, collagen and elastic fibers [38]. The secondary trabeculae are formed of smooth muscle with reticular fibers [38].
The parenchyma of the spleen in the dromedary camel is made of white and red pulps. The white pulp is comprised of the periarterial lymphatic sheath (PALS) and lymphoid follicles [38]. The lymphoid follicles are spherical and sometimes indented on one side where the PALS is located [37] [38]. They are mainly occupied by B-lymphocytes; hence they represent the B-dependent zone in the spleen, whereas the PALS is considered the T-dependent zone [39].

The Camel Antibody Isotypes and Their Tissue Distribution
Antibody is one of the most important multifunctional effector elements of humoral immune responses. Antibody is the secreted receptor of the B-lymphocyte [40]. The molecule is constructed of a pair of light (L) chains (25 KD) and a pair of heavy (H) chains (50 KD) [40]. Two types of L chain-the kappa (κ) and lambda (λ) light chains-exist due to the subtle differences in the amino acid sequences. Both light chains are found in the antibody of almost all camel species. Each antibody light chain either is of κ or λ type, but not both [40]. The specificity of the antibody is based on antibody repertoire, which defines the extent to which the antibody can respond to a wide range of antigens. The antibody repertoire is generated by DNA rearrangements, or what is known as somatic recombination, during B-cells development [40]. The repertoire is expanded further by the process of somatic hypermutation in the developed B-cells.
The genes that encode immunoglobulin are arranged in three loci-the κ and λ genes of the L chain, and the H chain. The isotypes of the immunoglobulin are The diversity of the antibody specificity is generated in four main ways: two are a result of recombination process during B cell development; the third is due to the vast probability of L and H chains combinations (the probability of the H and L chains pairing is one of the major mechanisms that expand the antibody repertoire); and the fourth way is generated by mutation of the V loci of the rearranged DNA in the mature B cells [40]. The diversity of the repertoire can be expanded further by imprecise joining of V, D, and J gene segments and somatic hypermutation.
The diversity of the complementary determining region-3 (CDR3) of the H and L chains is dramatically increased by addition and deletion of nucleotides in the junction between gene segments [40].

The Camel Antibody Isotypes and their Genes
Similar to most vertebrates, the camel antibody is made up of IgG, IgM, IgA and IgE classes. Camel IgG comprises of three subclasses, namely, IgG1, IgG2 and IgG3. IgG1 has the conventional structure-a pair of L and H chains, while IgG2 and IgG3 are only made of H chain and lack L chain and the (CH1) domain. The IgG2 and IgG3 subclasses account for about 75% of the total antibody classes [41] (see Table 1). The camel germline repertoire contains nine γ genes. Five of these γ genes are only functional as they were detected in the rearranged Ig genes [42]. Two of these five genes encode the H chain of the conventional antibodies IgG1a and IgG1b, while the other three genes are allocated for the production of IgG2 and IgG3 (γ2a, γ2c, γ3) [42]. The genes that encode the H chain of the conventional antibody and the H chain of the camel heavy chain-only antibodies (HCAbs) are located in two different germline genes [41]. The camel L chain is also made of κ and λ variants [43]. The two variants of the IgG1 are constructed of different numbers of amino acids in the hinge region. The IgG1a variant has 19 amino acids whereas IgG1b has 12 residues in that region [43] (see Table 1).
A phylogenic analysis of the camel heavy chain reveals that the camel heavy chain framework VH3 is closely similar to that of the human while the VH4 framework similarity is of a lesser extent [42] [43]. It has been suggested that

The HCAbs
The HCAbs, IgG2 and IgG3 differ from each other by their adsorption behavior to A protein and the molecular weight of the H chain [42]. The IgG2 and IgG3 sub isotype (γ2a, γ2c, γ3) encode different hinge regions. They are made of 35 amino acids, 12 amino acids and 15 amino acids respectively [41] [42] (see Table   1). The hinge region of the HCAbs is in direct contact with the variable heavy domains [41]. The V domains of the heavy chains (V H H) of the HCAbs are distinct due to the presence of few highly conserved amino acids [42].
The intrinsic sequence of V H H is more diverse than that of the variable heavy for the short antigen binding site in these antibodies [42]. The long CDR3 enhances its accessibility and increase its paratope diversity [42]. Similar to camels, bovine antibodies also have large CDR3. However, bovine CDR3 is known for its ultralong subset, which ranges from 50 to 60 amino acids [44].  [46]. It has been clearly indicated that aging has a significant impact on the density and distribution of IgA + and IgG + cells [45].

The Tissue Distribution of the Camel IgA + and IgG + Cells
Existing studies on the IgG antibody secretory cells (ASCs) distribution in the small intestine have shown that they are densely scattered in the LP, and some are aggregated around the intestinal glands [45]. The IgG ASCs are evenly distributed in the duodenum, jejunum and ileum. However, the density of the ASCs is highest from the middle segment of the duodenum to the middle segment of the jejunum, and decreases in the ileum [45] [46]. The scattered distribution of the IgG and IgA ASCs in the small intestine is of significant importance for providing full-protection and immune surveillance of the mucosal immune homeostasis and intestinal integrity [45] [46]. The densities of IgA ASCs are great-Open Journal of Veterinary Medicine er than IgG ASCs in the duodenum, jejunum and ileum of young and pubertal camels [46].

The Antibody Concentration in the Colostrum
The colostrum is a vital resource of maternal immunity and is essential for the passive immunization of newborn camels. The concentration of IgG in the camel colostrum has not been explored in detail. Although the IgG isotype is abundant in the colostrum, the relative levels of IgG1, IgG2 and IgG3 have not been clarified [41]. The epitheliochorial nature of the camel placenta urges the transfer of maternal immunity through the colostrum suckling. The study of the camel colostrum constituents of the antibody isotypes confirmed the presence of IgM, IgG and IgA in the colostrum [41].

The Therapeutic Applications of HCAbs
HCAbs clinical applications have been widely reported in different medical studies; however, none of the HCAbs-derived products developed by several manufacturers have reached the market or passed to phase IV. Without affecting their safety or efficacy, HCAbs in some clinical trials have shown a lower incidence of antidrug antibodies (3%) [47], whereas others, although small in number, have resulted in high immune adverse effects that necessitated terminating the trials prematurely [48].
As diagnostic tools, HCAbs are employed in immunodiagnostic assays of infectious diseases [49], such as Trypanosoma congolense in plasma [50] and Listeria monocytogenes in milk [51], and for protein detection such as enzymes produced by aerobic and anaerobic bacteria [52]. Low molecular weight molecules, such as methotrexate, have also been detected by HCAbs [53]. For in vivo and in vitro imaging diagnosis, HCAbs are one of the strong applications for their simple functionalization with radiolabels or fluorescent probes [54]. The epidermal growth factor receptor (EGFR), which is related to different epithelial cell tumours, is a biomarker used to localize tumours, or could be coupled to medications to treat some types of cancer or used in imaging studies [55].
As therapeutic tools, HCAbs have been used to remove immunoglobulins in patients with high titres of circulating autoantibodies, as in the case of systemic lupus erythematosus [56]. In infectious diseases, HCAbs have been developed as neutralizing antibodies against H5N1 influenza virus to prevent the virus attachment to the host cells. [57] HCAbs were seen effective as anti-Clostridium difficile toxin by preventing the induction of cytotoxicity [58]. Treatment of human respiratory syncytial virus infection through pulmonary delivery was also recorded [59]. HCAbs have been attempted to treat neurodegenerative diseases, such as Alzheimer's [60] and Parkinson's diseases [61]. For toxicity management, a higher neutralizing capacity was found when HCAbs were used against snake venom and scorpion toxins [62]. HCAbs in vivo studies appeared to be efficiently delivered across blood-brain barrier and might be used to carry any cargos into the brain [63]. HCAbs conjugates showed potential efficacy for cancer immunotherapy. Conjugates to EGFR could be used to treat solid tumours [64].
Compared to a renal clearance limit of 60 kDa, HCAbs smaller size of 15 kDa allow them to rapidly clear from the blood when used as parenteral treatment.
To overcome this problem, HCAbs should be ligated to large proteins such as albumin or immunoglobulin by using bivalent formats [65]. Some products derived from HCAbs have succeeded in passing phase I and II trials, although they are still in the pipeline [62]. The accomplishment of HCAbs treatment will be achieved if more patient-friendly administration (topical, oral, or inhalation) products are developed. Nevertheless, the short period of time was found to produce HCAbs owing to their low-cost compared to conventional antibodies; therefore, HCAbs are precious in human medicine, and have much to offer as a research tool in the scientific community.

Markers of Camel Lymphocytes and Other Leukocytes and Their Tissue Distribution (zt2)
Comprehensive study of camel cell markers is hindered by absence of specific anti-camel CD markers antibodies. However, in the last decade major efforts have led to the identification of several important markers, especially that of the Open Journal of Veterinary Medicine Analysis of the CD markers on the camel lymphocytes in the peripheral blood of newborn camels indicated a higher expression of γδ cells and B cells than in adult camels, whereas the expression of the CD4 + T-helper cells in camel calves was less than in adult camels [77]. Expressions of the AM, CD11a, CD11b and CD18 on camel calves' leukocytes and lymphocytes were significantly lower than in adult camel cells [77]. A low AM molecule expression in newborn leukocytes could be due to a reduction in the migratory activity of the leukocyte populations [77]. Analysis of different camel lymphocyte phenotypes indicated that 83% are naïve CD4 + CD11a and CD44, whereas 13% of the lymphocytes are effector cells with CD4 + , CD11a and CD44 hi [76]. On the other hand, bovine lymphocytes consist of 23% B cells, 20% CD4 + and 6% WC1 + . The WC1 + camel lymphocytes differ from bovine lymphocytes in their higher expression of AMs CD11b and CD62L [76]. The expression rate of the WC1 + on the lymphocytes of young camels is higher than in adults, whereas the CD4 + expression is higher on the effector adult camel lymphocytes [76].
The expression of the lymphocyte markers in camel mammary glands and related lymph nodes were revealed in studies employing the immunohistochemical technique and using immunoperoxidase [74]. The CD8 + T cells were detected in lactating and non-lactating mammary alveolar tissues and the supramammary both the lactating and non-lactating stage; however, the expression was higher in the non-lactating period [74]. Expressions of CD4 + and CD20 + were not detected at all. In addition, the over expression of WC1 + in the non-lactating mammary tissues reflects the importance of these cells at this stage, in which major immunological changes occur [78]. The over expressions of the mucosal addressin cell-adhesion molecule-1 (MAdCAM-1) and the WC1 + molecules at the late lactation period indicates that these molecules are essential for up-regulating of the immune responses at this period, during which mammary glands are highly susceptible to mastitis.
The phenotypes of camel monocyte, expressing CD172a + , were revealed using anti-CD14 + , CD163 and MHC-II antibodies [79] (see Table 3). According to the rate of the expression of these CD markers, it was shown that the expression of the monocyte phenotypes were as follows, the monocytes-I (Mo-I) phenotype has a high expression of CD14 + and CD163 and a low MHC-II expression, while the Monocyte-II (Mo-II) phenotype was characterized by a high expression of all three molecules. In contrast to the Mo-I phenotype, the Monocyte-III (Mo-III) phenotype has a low expression of CD14 + and CD163 and a high expression of MHC-II [79]. The Mo-II was seen as the prime phenotype for inflammatory responses, and it is present in high levels in adult camels, whereas newborn animals have a lower percentage of the Mo-II phenotype. New-born monocytes indicated high expression of CD172a + and CD163 but fewer CD14 + and MHC-II molecules [79]. In addition to the variation in the expression of the previous CD markers in adult camels, it was also noticed that these phenotypes indicated a variation in the expression of the AM. The Mo-II and Mo-III phenotypes have shown a high expression of CD11a, while Mo-I has indicated a high CD18 + expression. However, the CD11b showed high expression of Mo-I and Mo-II, especially the monocytes that have a high expression of CD14 + and MHC-II [79] (see Table 3). The camel monocyte phenotypes Mo-I, Mo-II and Mo-III could be considered as the counterpart to the bovine classical, intermediate and non-classical monocyte phenotypes respectively. [79].  [80].

Camels with endometritis have indicated the elevation of the monocyte Mo-II
Study of the camel cells markers of mammary glands with mastitis revealed a significant expression of the chemokine receptor CXCR2 and the adhesion molecules CD11a/CD18 and CD62L [81]. CD62L is an important selectin that mediates the naive lymphocytes to the peripheral lymph nodes and other mucosal associated lymphoid tissue [82]. An important finding in the study of camel milk from infected mammary glands detected Peyer's patches adhesion molecule-1 (LPAM-1) [81]. MAdCAM-1 promotes the homing of leukocytes expressing LPAM-1 to mucosal tissue [83]. This selective expression of MAd-CAM-1 in different tissues is known to be a prime factor that mediates the specific migration of lymphocytes. For instance, the expressed MAdCAM-1 in the mesenteric lymph node or Peyer's patches (PP) has a selective binding with L-selectin to the specific glycosylation modification. However, the MAdCAM-1 expression on the high endothelial venules around the mammary glands lobules mediates the selective binding to LPAM-1 but not L-selectin [69]. However, it has been demonstrated that bovine mammary tissues lack distinct expression of MAdCAM-1 due to very low MAdCAM-1 mRNA transcriptional activity [68].
The nature of the CD markers expressed in the intestine of camels naturally infected with Mycobacterium avium subspecies paratuberculosis (MAP) has also been explored [18]. The expression of the markers CD8 + , CD25 + , CD11c + , CD14 + , WC1 + , CX3CR1, and MAdCAM-1 were significantly elevated in the ileum and mesenteric lymph node of older camels in comparison to young camels [18]. The overall results indicated extensive presence of different lymphocyte subsets, dendritic cells and macrophages that play vital role in orchestrating the intestinal homeostasis and the immune responses to MAP infection [18].

The LPAM-I and MAdCAM-1 Expression
LPAM-1 integrin is constructed from subunits α4 and β7 and expressed on the lymphocytes, monocytes, eosinophils and basophils, but not on neutrophils. press MAdCAM-1, as in mice at PP, gut lamina properia, and the mesenteric lymph node; however, in ruminants, MAdCAM-1 expression is missing in these tissues [68] [70]. According to this finding, it was therefore clearly demonstrated that lymphocytes that home to MAdCAM-1 expressing tissues are of mucosal origin, as in monogastric animals. Cells that home to tissues that lack MAd-CAM-1 expression are of a peripheral nature, as in most ruminant animals. The origin of the cells' trafficking plays a major role in dictating the route of vaccination and the pathogenesis of different mammary gland infections [84].
Immunostaining and flowcytometry studies have proven beyond doubt the expression of MAdCAM-1 and LPMA-1 in healthy camels, emphasizing that the camel is closer to monogastric animals in this respect [74] [81]. Hence, the possible mucosal nature of cell trafficking to camel mammary glands will merit important consideration in the treatment of mastitis and approaches to vaccinations for camel mastitis. In addition, the possible mucosal nature of camel mammary glands could provide a convenient model to exploit the immune responses that are essential for designing effective therapeutic approaches for humans that share the mucosal nature of cell trafficking with camels [74].

The CX3CR1 Expression
Analysis of the CD markers of the camel intestine and the related lymph nodes naturally infected with MAP revealed a peculiar expression of the chemokine receptor CX3CR1, which is expressed by special lineage of intestinal macrophages [85]. The CX3CR1 macrophages play a central role in the regulation of intestinal immune homeostasis, mucosal defence and maintaining the barrier structure. The peculiar activity of the CX3CR1 macrophages in the human intestine lies in reducing the lumen microbial load and acting as an antigen sampler by bidirectionally shuttling through the intestinal barriers. The cells are also important for excluding the pathogens that are traversed to the intestinal epithelium [85]. The CX3CR1 macrophages are crucial for regulating intestinal tolerance due to their extreme susceptibility to any changes in the gut environment [85]. In cattle, however, studies on CX3CR1 macrophage phenotype are scarce. The expression of the CX3CR1 in the camel intestine could be considered extraordinary and in need of further investigation. Therefore, the high expression of the CX3CR1 macrophages and the expression of the LPMA-1 and MAdCAM-1 in different healthy and diseased camel tissues is novel and unprecedented. The CX3CR1 and MAdCAM-1 expression could play a certain role in the camel immunopathological responses to MAP infection in the intestines, a subject meriting extensive exploration [18].

Antibody Immune Responses
As vertebrate, camels produce the conventional immunoglobulin classes, IgM IgD, IgG, IgA, and IgE [41]. IgA plays a key role in the camel's mucosal immunity by modulating immune exclusion, inhibiting inflammation and allergic re-Open Journal of Veterinary Medicine actions [86]. For instance, IgA was predominantly detected at the local mucosal sites of helminth infestation [87]. On the other hand, IgG is predominant in the serum of infected camels. IgE, however, is produced in low concentrations in mucosa and in serum. IgE is involved in immediate hypersensitivity reactions and parasitic infestation in camels [87] [88]. The predominance of IgG in the colostrum is crucial for the transfer of maternal immunity to newborn camels during the first 24 hours post parturition [89]. Maternal antibody transfer to camel calves is seen as critical for protecting against rotavirus infections in the endemic areas [90].

Cellular Immune Responses
Major histocompatibility complex (MHC) genes play a pivotal role in the regulation of cellular immune responses, most critically during host-pathogen interactions [91]. The MHC locus is located on chromosome 20 in Camelus dromedarius [92]. Sequencing studies on the camels' MHC genes of the three breeds of old-world camels have indicated that they are of low genetic diversity [92] [93].
However, the question is whether the low diversity of the camel genomes could be explained by the low MHC diversity [92]. Recent study has shown significant low genetic diversity of both MHC class I and class II genes [93]. Nevertheless, the question remains as to whether the low MHC diversity might be linked to the low diversity of the camel genomes [92].
The major leukocytes that play a central role in camel cellular immunity include lymphocytes, monocytes, neutrophils, dendritic cells, eosinophils and basophils.
Lymphocytes are the most abundant leukocyte subpopulation in camel blood, followed by neutrophils [17]. T and B lymphocytes, together with the antigen processing cells (APC), are critical for the regulation of the adaptive immune response [92]. Subsets of the B-lymphocytes B-1 or B-2 have been reported in camel blood [94]. However, there is scant information about the camel T-lymphocyte subsets. Regarding the T cell receptor (TCR), two types of TCR-αβ and γδ have been reported [88] [92].
γδ T-lymphocytes are likely to predominate both epithelial and mucosal sites [87] [92]. γδ T-lymphocytes have the characteristics of an innate-like immune response as well as adaptive responses. γδ T-lymphocytes play a major role during the innate immunity and the early phase of the cell-mediated response. They also act as professional antigen processing cells (APCs) by processing different types of antigens, such as responding to unconventional antigens like lipid in an MHC-independent manner [76]. They show cytotoxic activity by eliminating the infected macrophages and are considered a major source of IFN-γ. [92] [95].
Camels' αβ T-lymphocyte, however, recognizes peptide antigens presented on MHC molecules. The majority of T-cells in camel blood are αβ T cells. Monitoring the T-cells in camel blood at different ages has revealed that effector CD4 + T-cell percentages were elevated in old camels compared to young camels [76]. infestation [87]. CD8 + lymphocytes mediate the killing of the intracellular pathogens or damaged cells through MHC-restriction approach [87]. Regulatory T-cells (Treg) activity was reported in camels [96], as well as T-helper 17 (Th17) activity, mainly due to the stimulation of the toll-like receptor (TLR)-ligands [87].
Camel monocytes are innate immune cells playing an essential role in maintaining innate immunity [80] by opsonisation, either by Ig and/or complement [88] [97]. Monocytes are important for initiating the adaptive immune response by processing microbial antigens and activating T-cells [88].
Dendritic cells (DCs), on the other hand, are innate immunity cells which are well known as APC [98]. DCs can be detected in most of the body tissues, particularly the skin, mucosa and in the secondary lymphoid tissues [88] [99]. DCs express TLR for recognition of the pathogen associated molecular patterns (PAMPs) of invading parasites, and can prime specific immunity by antigen recognition associated with MHC-II restriction [100] [101]. DCs play a central role in antiviral responses and are able to regulate certain autoimmune diseases [88].
Camel neutrophils exhibit an oxidative respiratory burst against Staphylococcus aureus similar to the neutrophils of other species [103]. CD14 markers expressed on neutrophils mediate the TNF-α secretion in response to lipopolysaccharide (LPS) in the mammary glands [104].
There is evidence that camel eosinophils might express MHC antigen after stimulation by granulocyte-macrophage colony stimulation (GM-CSF) [103].
Camel eosinophils are also effective killers of parasites by their prominent cytoplasmic granules, which contain toxic molecules and enzymes that are particularly active against helminths and other parasites. It is not yet clear how these structural differences render the camel with such an apparent efficiency for killing parasites [87] [88] [103].

Camel Immune Responses to Certain Infectious Diseases
The research history of camel infectious diseases has shown that camels are important sources of zoonotic diseases [105]. Camels have expressed significant resistance to certain infectious diseases [91], whereas they are susceptible to infections of viral, bacterial and protozoan pathogens [106]. Here we discuss selected viral, bacterial and parasitic diseases in camels to elaborate the most effective immune responses.

Viral Infections: Middle East Respiratory Syndrome Coronavirus (MERS-CoV)
The camel was identified as a source of the new viral disease spillover which emerged in 2012 in Saudi Arabia. The Middle East respiratory syndrome coronavirus (MERS-CoV) was caused by a novel coronavirus infection which was suspected to have begun in camels [107]. The infection causes severe acute respiratory disease [105] and may demonstrate nasal discharge and transient respiratory symptoms [108]. Notably, a very high seropositive rate, exceeding 90% in the camel population in the Middle East and in sub-Saharan African countries [109]. The dipeptidyl peptidase 4 (DPP4) (CD26) in the upper respiratory tract epithelium was identified as the prime receptor for the attachment of MERS-CoV [108]. This may help explain the restriction of the MERS-CoV replication to the upper respiratory tract [108]. Seropositive MERS-CoV dromedary camels may carry MERS-CoV viral RNA in their nasal excretions [108].
The major brucellosis symptoms in camels are orchitis, epididymitis, metritis, reduced fertility, and abortion [105]. Brucellosis in infected camels could be asymptomatic, behaving as a carrier without revealing detectable immune responses [112].
Clinical brucellosis in camels is associated with proinflammatory cytokines [113]. Inflammatory cytokines could be associated with endotoxemia. Camel infection is associated with the upregulation of the cytokines IL-6, IFN-γ, interleukin-10 (IL-10), interleukin-1α and β (IL-1α and β) and the TNF-α [113] [114]. A study of camels infected with both Brucella abortus and Brucella melitensis revealed a significant increase of IL-1β and IL-10 and a significant decrease of TNFα, IFN-γ and IL-1α. The modulation of the cytokines in the infected camels was seen as an important brucella mechanism to evade an immune response [115].

Parasitic Infection: Trypanosomiasis
Camel trypanosomiasis is one of the main causes of abortions in camels in the Middle East and Africa [116]. Trypanosoma evansi (T. evansi) is the most pathogenic protozoa of camels and causes severe disease known as surra [117].
T. evansi is transmitted mechanically by hematophagous flies such as horseflies (Tabanus) and stable flies (Stomoxys) [117]. T. evansi are extracellular parasites that survive, multiply and differentiate in the extracellular fluids of the mammalian host [117]. The whole parasite surface is covered with a glycoprotein coat called the variant surface glycoprotein (VSG) [118].
The symptoms are associated with significant reductions in the number of red blood cells (RBC), haemoglobin and packed cell volume (PCV) [116]. Outbreak of trypanosomiasis can lead to significant mortalities or abortions.
The most significant immune response to this protozoon is the activation of the classical complement pathway to control parasitaemia in camels. Immunity against T. evansi is also associated with activated macrophages by producing an enhanced amount of TNF-α, IL-6, IL-12, and IL-10 [118]. Nevertheless, the infection could be associated with immunosuppression by decreasing the level of complement haemolytic activity by C3 activation induced by antigen-antibody complexes [103] [119] [120]. Furthermore, various studies have shown that T. evansi could be involved in the immunosuppression mechanism by polyclonal B-cell activation and generation of suppressor T-cells and altered antigen handling and presentation as well as production of large amounts of nonspecific IgM [121] [122].
The host's skin indicates an increased CMI response of CD4 + , CD5 + , CD8 + and MHC-restricted lymphocytes and B cells [121]. Antigenic variation in the exposed membrane-surface glycoprotein of trypanosome is responsible for pro-Open Journal of Veterinary Medicine duction of evading antibodies [117]. Hence, recurring waves of parasitaemia are eminent because several days are required to develop destructive specific antibodies to VSG of T. evansi [118].

Immunodiagnosis of Camel Diseases
Various diagnostic methods can be used, either alone or in combination with other methods, to reach an appropriate and accurate diagnosis. Although reliable diagnosis can be achieved by direct isolation and detection of the causative agent, this approach is laborious and associated with a high contamination and impracticality for regular screening on a large scale [123] [124].
Serology, or antigen-antibody interaction, is one of the most applied approaches to detect active infection. Serological techniques can be applied for the evaluation of specific antibodies (IgG and IgM) in animals, which they play a key role in rapid and appropriate diagnosis [125].

The Microscopic Agglutination Test (MAT)
This test is widely used for the diagnosis of Leptospirosis, a zoonotic disease that infects camels and other animals, including humans. Using the MAT, leptospiral identification varied from 20% to 32.4% and 6.33% to 22.85% [105] using polymerase chain reaction (PCR) [127]. However, in Saudi Arabia diagnosis of camel leptospirosis was carried out by the Plate Agglutination Test (PAT) with a sensitivity of 6.7% [128]. Niloofa et al. (2015) stated that both IgM-ELISA and Leptocheck-western blotting (WB) have identical sensitivity [129]. They also found that IgM-ELISA may be superior to the MAT during the acute phase and suitable for early diagnosis of leptospirosis.

The Modified Agglutination Test (MAT)
The MAT is highly recommended for the diagnosis of Toxoplasmosis in camels and humans because of its easy procedure and low price. In addition, it has expressed the highest sensitivity compared to all the other serological tests, at 96% [130] [131].

The Complement Fixation Test (CFT)
The CFT is one of the most reliable serological techniques for detecting antigen-antibody reactions using complement system as an indicator. The CFT is used for the detection of foot and mouth disease (FMD) infection in camels. It may also be considered standard for virus diagnosis [132]. In camel brucellosis, the CFT indicated high seroposivity (98.98%) in detection of the disease [133].
Khan et al. (2020) found that the CFT expressed a significant positive rate for detection of anti-Brucella antibodies compared to the ELISA test [134].

Western Blotting (WB)
WB can be used for the detection of low antibody concentration [135]. WB is also the test of choice for the identification of immunospecific proteins. Due to its high sensitivity and specificity, WB is considered a preferable choice as a decision-making tool in the field [136].

Enzyme-Linked Immunosorbent Assays (ELISA)
The most challenging requirement for the sereological test is its sensitivity and specificity to differentiate between infected from vaccinated cases [137]. ELISA was developed to achieve the upmost sensitivity and specificity in detection of antibody-antigen reactions. ELISA is an assay designed in different antigens or antibody combinations to increase the specificity of the test to measure the concentration or presence of antigens or antibodies. Based on different combinations of antigens and antibodies, there are three types of ELISA: indirect, sandwich and competitive ELISA [138].
ELISA is widely applied in the diagnosis of camel diseases. For instance, the indirect ELISA technique was used for the detection of adenovirus type 3 in camels [139]. The overall seroprevalence rate was about 90%. Chlamydiosis is one the diseases that causes foetal death and abortion in camels. However, the indirect ELISA detection rate was >40% in detection of anti-Chlamydia abortus antibodies [140]. The ELISA technique has proved its versatility to differentiate between orthopox and parapoxvirus infections in camels [141]. Moreover, a seropositivity rate of 70.8% of MERS-CoV antibodies was shown in a survey of seroprevalence of MERS-CoV in camels in slaughterhouses [142]. The early detection of the Mycobacterium avium subspecies paratuberculosis (MAP), which causes John's disease in camels, has faced challenges using conventional approaches. However, ELISA has shown reliability and sensitivity in the detection of the MAP infection in camel herds. Nevertheless, detection of the infection in the early stages needs to be enhanced by combining ELISA with PCR [13].

Conclusions
Camels form an integral part of the culture and agriculture of many people around the world. The camel's economy is of considerable importance par- In conclusion, research in the field of the camel immune system is slow and has attracted slow academic attention. However, so far important findings have been made. The major setbacks that hinder the research are, first is the unavailability or poor production of reagents that would enable detailed study of the camel's immune system. Research in camel immune system to be boosted considerably requires allocation of sufficient budgets to meet the need of explicit and extensive research as well as founding major international scientific communities and societies that encourage the research in this field and act as forecasting bodies that contribute to the research perspective in this field.

Data Availability Statement
This review article is based on the published available literature. Otherwise, published were authorized by the corresponding authors.

Ethics Statement
The review was carried out according to International Guidelines for research involving animals (Directive 2010/63/EU).