From OMIM
X-linked agammaglobulinemia is an immunodeficiency characterized by failure to produce mature B lymphocytes and associated with a failure of Ig heavy chain rearrangement. The defect in this disorder resides in BTK, also known as BPK or ATK, a key regulator in B-cell development (Rawlings and Witte, 1994). The X-linked form accounts for approximately 85 to 90% of cases of the disorder. Also see 300310. The remaining 15% of cases constitute a heterogeneous group of autosomal disorders (Lopez Granados et al., 2002; Ferrari et al., 2007). Genetic Heterogeneity of Agammaglobulinemia/Hypogammaglobulinemia A form of X-linked hypogammaglobulinemia (IMD61; 300310) is caused by mutation in the SH3KBP1 gene (300374) on chromosome Xq22. See agammaglobulinemia-1 (AGM1; 601495) for a discussion of genetic heterogeneity of autosomal forms of agammaglobulinemia.  http://www.omim.org/entry/300755

From MedlinePlus Genetics
X-linked agammaglobulinemia (XLA) is a condition that affects the immune system and occurs almost exclusively in males. It is part of a group of disorders called primary immunodeficiencies (or inborn errors of immunity), in which part of the immune system does not function as it should. People with XLA have very few B cells, which are specialized white blood cells that help protect the body against infection. B cells can mature into the cells that produce special proteins called antibodies or immunoglobulins. Antibodies attach to specific foreign particles and germs, marking them for destruction. Individuals with XLA are more susceptible to infections because their body makes very few antibodies.

Children with XLA are usually healthy for the first 1 or 2 months of life because they are protected by antibodies acquired before birth from their mother. After this time, the maternal antibodies are cleared from the body, and the affected child begins to develop recurrent infections. .cf0{font-style:italic;font-family:Segoe UI;font-size:9pt;}Children with XLA generally take longer to recover from infections, and infections often occur again, even in children who are taking antibiotic medications.

The most common bacterial infections that occur in people with XLA are lung infections (pneumonia and bronchitis), ear infections (otitis), pink eye (conjunctivitis), and sinus infections (sinusitis). Infections that cause chronic diarrhea are also common. Recurrent infections can lead to organ damage. .cf0{font-style:italic;font-family:Segoe UI;font-size:9pt;}Treatments that replace antibodies can help prevent infections, improving the quality of life for people with XLA.  https://medlineplus.gov/genetics/condition/x-linked-agammaglobulinemia