Before transplantation, the donor and recipient must be confirmed to have compatible blood types. In addition, precautions must be taken to ensure immune compatibility. Recipient serum must be tested against donor lymphocytes to ensure the recipient does not have preformed antibodies to donor proteins. The problematic alloantibodies are most commonly directed against donor major histocompatibility complex (MHC) class I and class II antigens. MHC class I antigens are expressed on most nucleated cells, albeit at variable levels, whereas MHC class II antigens are expressed mainly on antigen presenting cells (B lymphocytes, dendritic cells, and some endothelial cells). Thus recipient serum is tested against donor lymphocytes, which contain both MHC antigens. A positive crossmatch predicts a high likelihood of hyperacute or early rejection.