This book provides an updated overview of agammaglobulinemia, a rare form of primary immunodeficiency which is considered the prototype of the congenital humoral defects, and which is characterized by the absence of peripheral B cells and very low serum immunoglobulin levels. The book opens by discussing the highly orchestrated early B cell development in the bone marrow and the genes involved based on both human and animal models. The pathogenesis and clinical presentation of X-linked agammaglobulinemia, caused by mutations in the BTK (Brutonâ€{u3834}yrosine kinase) gene, are then presented in detail, followed by descriptions of the clinical manifestations and molecular basis of the less frequent autosomal recessive and autosomal dominant forms of agammaglobulinemia.ÂСtientsâ€{u0B61}nagement in terms of respiratory complications, gammaglobulin replacement therapy and the potential value of novel experimental therapeutic strategies areÂ{u0929}scussed. The bookâ€{u3823}losing chapters offer a comprehensive and updated description of mutations in the BTK gene, and the expression and function of BTK in cells other than B cells.