**1. Introduction**

Primary immunodeficiency (PID) is a large group of rare diseases attributed to inborn genetic errors that impair different components of adaptive and innate immune system, resulting in chronic, serious infections, or other complications. The diseases are often accompanied by a predisposition to autoimmune disorders, autoinflammation, atopy, and malignancy [1–4]. Unlike secondary immunodeficiency acquired from other diseases or conditions such as malnutrition, immunosuppression, or HIV infections, PIDs are triggered by genetic defects. Based on the abnormality of one or more components of human immunity, PIDs can be divided into antibody deficiencies, combined T- and B-cell deficiencies, deficiencies in the phagocytic or complement system, and immune dysregulation [1]. Diagnosis of these disorders requires good clinical awareness and specialized laboratory testing. Flow cytometry and genetic testing are essential to identify the phenotypic and genetic defects of the diseases and to confirm the diagnosis. Accurate diagnosis and efficient management are important for reducing morbidity and mortality in

patients with PID [2]. The chapter provides an overview of the classification and manifestation as well as the diagnosis and management of these disorders.
