Primary antibody deficiency syndromes (PAD) are a group of primary immunodeficiencies (PID) characterized by reduced production of immunoglobulins and recurrent respiratory tract infections. PAD varies from rare but life-threatening agammaglobulinaemias to frequent but often asymptomatic conditions such as selective immunoglobulin(Ig)A deficiency or IgG subclass deficiency. Common variable immunodeficiency is the clinically most important PAD. Hypogammaglobulinaemia may be associated with other PID or may be drug-induced or caused by infectious diseases, haematological malignancies or protein loss.