Infections in infants and children under five years of age are a public health in México and are one of the major causes of death. Methods In this study, lymphocyte immunophenotyping for CD3+ (T-cells), CD3+CD4+, CD3+CD8+,
CD3+CD19+, CD3+CD16/56+, CD45RA+,
CD45RO+, CD62L- and CD28- were determined in the whole blood of gastrointestinal and respiratory bacterial infected children, using a four-color flow cytometry technique. Results: Our data showed that the percentages and the absolute numbers of monocytes and granulocytes are increased in infected children, when compared to the control group. Similarly, we observed increases in the percentages of B lymphocytes, CD8+ cells, memory T cells
(CD4+CD45RO+ and CD8+CD45RO+) and effector lymphocytes (CD4+CD62L? and CD8+CD28?) in infected children compared with the control group. In contrast, naive T cells were decreased in the bacterial infected children relative to the control group. Additionally, we used ELISA assays to identify the pathogen agent in gastrointestinal and respiratory infection. Comparing different types of infection, we found that the children with respiratory bacterial infections had higher percentages of B lymphocytes, and cytotoxic lymphocytes (CD8+CD28-); and the children with gastrointestinal infections had higher percentages of CD3+ lymphocytes and effector cells (CD4+CD62L-). Conclusions The increase in B lymphocytes and CD8+CD28- cells in the children with respiratory infections and the increase of T lymphocytes and CD4+CD62L- cells in the children with gastrointestinal bacterial infections indicate that both cellular and humoral responses coincide, and both responses are necessary for eliminating the pathogen.