The ontogeny of immune responses.

It has been known for some years that a "window of opportunity" exists for transplantation in neonates. Patients who undergo transplantation during the first weeks of life usually have a very quiescent postoperative course. During fetal development a process of building tolerance to self-antigens occurs. It is now known that this "recognition of self" process is restricted by the major histocompatibility complex. Data will be presented that demonstrate that this major histocompatibility complex-restricted self-tolerance is the cause of the allogeneic effect. Thus when a transplantation is performed antigen presenting cells of the donor stimulate the T cells of the recipient to a very high degree. This stimulation is caused by presentation of monomorphic antigens in an inappropriate major histocompatibility complex environment. Loss of these antigen presenting cells can result in a quiescent transplantation course that represents the "window of opportunity" phenomenon. During the immediate postnatal period the infant possesses a naive immune system. One of the characteristics of this naiveté is a lack of class II expressing cells and a failure of appropriate antigen presentation. It will be suggested that this failure could contribute to the ease with which neonatal transplants can be immunosuppressed.