Affected individuals are males who have an increased susceptibility to bacterial infections due to severely reduced immunoglobulins of all isotypes. The underlying cause of XLA is an arrest of B-cell differentiation, which leads to an extreme deficiency of mature B lymphocytes (<1% of the normal value) and an absence of plasma cells. The age of onset of infections is generally around 6-12 months, with the average age of diagnosis at 3 years. Infections originating in mucosal surfaces are common in patients with XLA, and they may progress to the blood or other organs if left untreated. Pneumonia, meningitis, septic arthritis, cellulitis and septicemia can be seen in patients with XLA. Affected individuals may have an increased risk of colorectal cancer. Treatment entails immunoglobulin replacement and antibiotics. Female carriers are usually asymptomatic and have skewed X inactivation in B cells corresponding to expansion of only those B cells that use the normal BTK allele.