ICD-10 Specific code Z94.9: Transplanted organ and tissue status, unspecified

Equine anti-thymocyte globulin is composed of purified gamma globulin containing primarily IgG against human thymus lymphocytes. It is formed by inoculating a horse with an antigen (human thymoyctes) which then induces the horse immune system’s B-lymphocytes to produce IgG immunoglobulins specific for that antigen. The result is polyclonal IgG that is then purified from the horse’s serum to produce a usable drug product that can be used for immunosuppression. Although the exact mechanism of action is unknown, equine anti-thymocyte globulin targets a variety of immune system proteins including lymphocyte surface proteins, granulocytes, platelets, bone marrow cells, and other cell types. Equine ATG is currently indicated for the suppression of the immune system to prevent renal transplant rejection and in the treatment of aplastic anemia. Induction of T cell apoptosis and resulting T-cell lymphopenia found in vivo is credited for its therapeutic effect in these conditions.

There are currently various ATG products available, which differ in the source of inoculated animal (rabbit, horse, or pig) and in the type of antigen product used to produce immunoglobulin (thymocytes, peripheral T cells, etc.).