Which of the following statements about allograft rejection are true?

B. Allograft rejection may be mediated by antibodies or by cells.

DISCUSSION: In the absence of immunosuppression, allografts from randomly selected donors are always rejected, and the rate of rejection is rapid as compared with the rate of development of most immune responses. Although allograft rejection in naive recipients is mediated predominantly by cells, antidonor antibodies can cause very severe types of rejection, including hyperacute and acute vascular rejection. Antidonor antibodies or cellular responses may contribute to the development of chronic rejection, which is now the most common cause of graft loss. Recent studies demonstrate that helper T cells may differentiate along one of two pathways. The Th1 pathway leads to secretion of interferon-gamma and other cytokines and is associated with delayed-type hypersensitivity and allograft rejection. The Th2 pathway is associated with secretion of interleukin-10 (IL-10) and IL-4 and may actually inhibit alloimmune responses. The development of Th2 responses may thus contribute to tolerance. Like allograft rejection, tolerance is highly specific. Thus, a person who is tolerant to one antigen or one individual is still able to mount an immune response against other antigens and other individuals.