Severe Combined Immune Deficiency

Disease severity can range from mild to fatal, depending upon what part of the immune system is affected. Immunodeficiency can originate in normal individuals as a consequence of chemotherapy, viral infections (such as AIDS, which is caused by the HIV virus), or as the result of other processes that prevent immune system function. When immunodeficiency occurs in this manner, it is called acquired.

In contrast, immunodeficiency can also be inherited as a genetic mutation that prevents the normal development and function of the immune system. This is called primary immunodeficiency, of which there are three subtypes: mutations that prevent the function of B-lymphocytes (antibody production), those that prevent the function of T-lymphocytes ("invader" recognition), and those that affect both B-and T-lymphocyte production. The last group is called severe combined immunodeficiency disease (SCID). These patients make none or very few T-lymphocytes, have nonfunctional B-lymphocytes, and may or may not have a type of immune cell called natural killer cells. This combination results in the absence of a functioning immune system from the moment of birth.