Severe combined immunodeficiencies (SCID) are inherited immune system disorders characterized by abnormalities with responses of both T cells and B cells (specific types of white blood cells needed for immune system function). Common signs and symptoms include an increased susceptibility to infections including ear infections; pneumonia or bronchitis; oral thrush; and diarrhea. Due to recurrent infections, children with SCID do not grow and gain weight as expected (failure to thrive). SCID may be caused by genetic changes in any of several genes and can be inherited in an X-linked recessive (most commonly) or autosomal recessive manner. The most common type of SCID is called X-linked Severe combined immunodeficiency (XSCID). Another form of SCID is caused by a deficiency of the enzyme adenosine deaminase (ADA).