Severe combined immunodeficiency disease (SCID, pronounced “skid”) is a rare, potentially fatal syndrome of diverse genetic cause in which there is combined absence of T-lymphocyte and B-lymphocyte function (and in many cases also natural killer, or NK lymphocyte function). These defects lead to extreme susceptibility to serious infections. There are currently thirteen known genetic causes of SCID. Although they vary with respect to the specific defect that causes the immunodeficiency, some of their laboratory findings and their pattern of inheritance, these all have severe deficiencies in both T-cell and B-cell function.
Our son, Taylor, has the most common form of SCID called X-linked, affecting nearly 45% of all cases. The mutation is in a gene on the X chromosome that encodes a component (or chain) shared by the T-cell growth factor receptors. Mutations in this gene result in very low T-lymphocyte and NK-lymphocyte counts, but the B-lymphocyte count is high. Despite the high number of B-lymphocytes, there is no B-lymphocyte function since the T-cells are not able to “help” the B-cells function normally. This deficiency is inherited as an X-linked recessive trait. Only males have this type of SCID, but females may carry the gene and have a 1 in 2 chance (50%) of passing it on to each son.
source: Immune Deficiency Foundation “Patient & Family Handbook.”