**1. Introduction**

Primary immunodeficiencies (PIDs) are rare and heterogenous genetic diseases of the immune system. According to updated IUIS (International Union of Immunological Societies) classification in 2019, there is a large spectrum of PIDs including 403 different diseases caused by mutations in 430 genes categorized 10 different subclasses with these topics: Severe combined immunodeficiencies (SCIDs), combined immunodeficiencies (CIDs) less profound than SCID, CIDs with associated or syndromic features and predominantly antibody deficiencies including common variable immunodeficiency (CVID), immune dysregulation, phagocyte system defects, innate immune defects, auto-inflammation, complement deficiencies, bone marrow abnormalities and phenocopies of PIDs. Each disease has unique laboratory and clinical manifestations. Decreased or increased immune cell counts, unbalanced immune cell plasticity, decreased or increased immunoglobulin levels and complement factors, dysregulated functions of immune cells due to

abrogated intracellular molecular functions cause developing clinical manifestations of PIDs [1]. Use of flow cytometry in these laboratory investigations is a significant approach that offers a quantitative, reliable and rapid results. Evaluation of these laboratory findings helps to clinicians for proper diagnose of PIDs [2, 3].
