ADA deficiency, or Adenosine Deaminase Deficiency, is a rare genetic disorder that affects the immune system. It occurs when the body lacks the enzyme adenosine deaminase, which is crucial for breaking down toxic substances in the body. Without this enzyme, toxic levels of adenosine build up, leading to severe immune system problems. Individuals with ADA deficiency often experience recurrent infections and may develop severe combined immunodeficiency (SCID), a condition that leaves them vulnerable to various illnesses. Treatment options include enzyme replacement therapy and gene therapy, which aim to restore the function of the missing enzyme and improve immune function.