The gene that encodes this protein, Btk , is localized in the 21.3-22 region of the long arm of the X chromosome, and the locus encompasses about 38 kb in humans. BTK is located in all hematopoietic cells, but the protein is selectively down-regulated in T cells and plasma cells and its proper function only seems to be required in B cells. The PH region, on the other hand, has proven useful because monoclonal antibodies have been produced that are specific against this pleckstrin domain. Researchers have hypothesized that there may be a cascade of signaling tyrosine kinases, of which BTK is an important one, that is necessary for B lymphocytes to develop. On the other hand, occasionally B cells mature despite the defects in XLA, suggesting that while the signals transmitted by BTK promote the rearrangement of light-chain genes, they are not absolutely required for this rearrangement to occur.
Point mutations are the most commonly occuring type or mutation but are by no means the only kind.
The clinical manifestations and immune system consequences of X-linked agammaglobulinemia are numerous. Serum levels of IgG, IgM, and IgA antibodies are consistently low, normally at least 2 SD below the normal for the age of the patient. In many cases IgD and IgE values are similarly low. Haemophilus influenzae within the first five years of their life. Diagnosis of the disease can usually be made early in life, during the first 2 to 3 years.