A genetically heterogeneous group of rare diseases associated with significantly increased risk of mortality and marked by hypogammaglobulinemia, a failure to produce antibodies in response to antigens, and repeated bacterial infections. Patients have poor response to antibiotics, illnesses undeterred by vaccinations, and recurrent complicating conditions such as bronchitis, bronchiectasis, sinusitis, bacterial pneumonia, otitis, and pulmonary damage. Diagnosis typically occurs in the first or third decade of life.

Some patients may develop an autoimmune disorder, most commonly either idiopathic thrombocytopenia purpura or autoimmune hemolytic anemia. Lymphadenopathy, splenomegaly, and cancers such as lymphoma and gastric cancer may occur. Most cases are idiopathic; a minority are genetic.