Primary immunodeficiencies are disorders in which part of the body’s immune system is missing or does not function properly. Immunodeficiency syndromes may be caused by disorders of the bone marrow (site of immunocyte production). However, many primary immunodeficiency syndromes are caused by mutations that result in abnormalities in either the maturation of immunocytes or their function and are thus not considered bone marrow disorders per se. To be considered a primary immunodeficiency, the cause of the immune deficiency must not be secondary in nature (i.e., caused by other disease, drug treatment, or environmental exposure to toxins, e.g.). In primary immunodeficiency, there might be either cellular involvement (T-cells), humoral involvement (B-cells), or both cellular and humoral involvement.