Primary immunodeficiency (PI) diseases are a genetically heterogeneous group of disorders that affect distinct components of the innate and adaptive immune system. These include neutrophils, macrophages, dendritic cells, complement proteins, natural killer cells, and T and B lymphocytes. These disorders of the immune system cause increased susceptibility to infection, autoimmune disease, and malignancy.Most PIs are due to genetic defects that affect cell maturation or function at different levels during hematopoiesis. Over 120 distinct genes have been identified, whose abnormalities account for the different forms of PI disease. Genetic defects during B cell maturation lead to conditions like severe combined immunodeficiency, X-linked/autosomal recessive agammaglobulinaemia and HIGM syndrome. Severe combined immunodeficiency is also the outcome of genetic defects in the NK cell maturation pathway as also the T cell maturation pathway. Other genetic defects that affect the T cell maturation pathway include the MHC class I/II deficiency, CD8, LCK and ZAP70 deficiencies. Cellular immunodeficiencies comprise 20% of all PIs while disorders of humoral immunity account for 70% of all PIs.