what is ADA deficiency??

ADA stands for adenosine deaminase. Adenosine deaminase is a type of enzyme which deficiency causes a disease known as ADA deficiency.  It is one form of SCID (severe combined immunodeficiency- a type of disorder that affects the immune system). The disease is caused by a mutation in a gene on chromosome 20. The gene codes for the enzyme adenosine deaminase (ADA). Without this enzyme, the body is unable to break down a toxic substance called deoxyadenosine. The toxin builds up and destroys infection-fighting immune cells called T and B lymphocytes.