**5. Conclusions**

Flow cytometry is an accessible technique in clinical laboratories; the implementation of this technology has progressed together with the significant improvements in instrumentation and the availability of a wide variety of monoclonal reagents. While the main clinical indications for immunophenotyping typically have been the quantification of CD4 T lymphocytes in HIV infection, the study of the extended immunophenotype and functional studies by flow cytometry offers nowadays advanced diagnostic approaches. Previously used techniques for the study of cell function (3 H-thymidine incorporation, 51Cr-release assay, etc.) using radioactivity have been gradually replaced by flow cytometry. Flow cytometry can provide rapid and accurate identification of expanded lymphocyte subpopulations, which play an important role in the evaluation and understanding of complex disorders such as primary immunodeficiencies and other immunomediated diseases.

The application of flow cytometry techniques in the evaluation of PID assesses lymphocyte proliferation, intracellular changes associated with activation, cytokine production, and biological effects associated with immune defects and functional immune abnormalities.
