Buridan's ass: one man's view of the immune system.

This essay offers a highly biased and prejudiced interpretation of the immune response. The major points of this interpretation are: B and T lymphocyte diversity is generated by rearrangements within and among existing receptorcoding genes; the T cell receptor is a molecule genetically more closely related to immunoglobulins than to the products of the major histocompatibility complex (MHC); the course of B and T lymphocyte differentiation is largely the same except for a final processing of T lymphocytes in the thymus; the processing is directed by MHC molecules of the thymus epithelium; Ir and Ia genes are identical; and the function of MHC polymorphism is to generate diversity at the population level.