<p>The mechanism of action by which polyclonal antilymphocyte preparations suppress immune responses is not fully understood. Possible mechanisms by which Anti-Thymocyte Globulin may induce immunosuppression in vivo include: T-cell clearance from the circulation and modulation of T-cell activation, homing, and cytotoxic activities. Anti-Thymocyte Globulin includes antibodies against T-cell markers such as CD2, CD3, CD4, CD8, CD11a, CD18, CD25, CD44, CD45, HLA-DR, HLA Class I heavy chains, and &szlig;2 micro-globulin. In vitro, Anti-Thymocyte Globulin (concentrations &gt;0.1 mg/mL) mediates T-cell suppressive effects via inhibition of proliferative responses to several mitogens. In patients, T-cell depletion is usually observed within a day after initiating Anti-Thymocyte Globulin therapy. Anti-Thymocyte Globulin has not been shown to be effective for treating antibody-mediated (humoral) rejections.</p>