**1. Introduction**

Primary immunodeficiencies (PID) are a heterogeneous group of genetic disorders which manifest clinically as recurrent infections, autoimmune and/or autoinflammatory diseases or malignancies. In PIDs, the immune system is affected quantitatively or qualitatively and includes more than 400 different entities, with an incidence of approximately 1:2000 live newborns. The Expert Committee of the International Union of Immunological Societies has classified PIDs into nine groups based on the clinical manifestations and laboratory immunological abnormalities [1]. Prompt identification of PID patients reduces complications and is associated with a more favorable prognosis [2]. In addition to a complete medical history, physical examination, and general laboratory tests, the initial evaluation protocol when PID is suspected includes analysis of serum immunoglobulin levels and extended immunophenotyping in peripheral blood. Immunophenotype abnormalities can range from a complete absence of a specific cell population to

more subtle variations in the differentiated states of specific subpopulations [3]. For basic and advanced lymphocyte phenotype studies, the technical option is classical flow cytometry, although new approaches mainly used in research like mass cytometry or CyTOF, have emerged [4]. The following step in the diagnosis of patients with PID is the biochemical and functional characterization of the altered molecules, for example, perforin, CD40 ligand, etc. The advantage of using flow cytometry over other techniques such as western-blot in the case of protein expression studies and functional evaluation of different subpopulations is the rapid and sensitive result.
