Antigen presentation in acquired immunological tolerance.

In acquired tolerance, previous exposure to antigen under certain conditions induces specific unresponsiveness instead of specific immunological memory. It has been studied as an approach to the mechanisms of self-tolerance that operate on immunocompetent T and B lymphocytes once they leave their sites of origin in the thymus and the bone marrow. Possible mechanisms involve induction of specific suppressor cells or inactivation of antigen-specific lymphocytes (clonal anergy) as a consequence of abortive antigen presentation, in which the antigen receptor is effectively engaged but certain poorly defined accessory signals the T lymphocytes require are lacking. We propose that small, resting B lymphocytes, which lack these accessory signals, are the inactivating antigen-presenting cells in acquired tolerance to proteins and to the class II transplantation antigens. B lymphocytes, which can use their antigen receptors to gather and process antigens that are present at very low concentrations, may play a role in self-tolerance. In addition, B lymphocytes and T lymphocytes rendered anergic by encounter with self antigens could persist as self-specific suppressor cells to block an autoimmune response of autoreactive clones that had escaped deletion or anergy.