Background: Selective IgA deficiency (SIgAD) is the most prevalent primary immunodeficiency with almost unknown etiology. This study aimed to investigate the clinical diagnostic and prognostic values of lymphocytes subsets and function in symptomatic SIgAD patients. Methods: A total of 30 available SIgAD patients from the Iranian registry and 30 age-sex-matched healthy controls were included in the present study. We analyzed B and T cell peripheral subsets and T cell proliferation assay by flow cytometry in SIgAD patients with mild and severe clinical phenotypes. Results: Our results indicated a significant increase in naïve and transitional B cells and a strong decrease in marginal zone-like and switched memory B-cells in SIgAD patients. We found that naïve and central memory CD4+ T cell subsets, as well as Th1, Th2 and regulatory T cells have significantly decreased. On the other hand, there was a significant reduction in central and effector memory CD8+ T cell subsets, whereas proportions of both (CD4+ and CD8+) terminally differentiated effector memory T cells (TEMRA) were significantly elevated in our patients. Although some of T cell subsets in severe SIgAD were similar, decrease in marginal-zone and switched memory B cells and increase in CD21low B cell of severe SIgAD patients were slightly prominent. Moreover, the proliferation activity of CD4+ T cells was strongly impaired in SIgAD patients with a severe phenotype. Conclusion: SIgAD patients have varied cellular and humoral deficiencies. Therefore, T cell and B cell assessment might help in better understanding the heterogeneous pathogenesis and prognosis estimation of the disease. Keywords: Primary immunodeficiency, Selective IgA deficiency, B cell subsets, T cell subsets, flow cytometry, proliferation assay