Induction of hyporesponsiveness to particulate antigen by feeding: the sequence of immunologic response to fed antigen.

The sequence of immunologic response to feeding with particulate antigen (sheep red blood cell, SRVC) was studied in inbred Dark Agouti (DA) rats. Feeding of SRBC to rats led to systemic sensitization as demonstrated by the presence of agglutinating and hemolytic antibodies to SRBC in sera. These humoral responses as well as the number of specific plaque-forming cells (PFCs) to SRBC in the spleen and mesenteric lymph nodes of these animals could be boostered by the intravenous administration of SRBC during the early stages of feeding. With prolongation of feeding both hemolytic and agglutinating antibodies in sera dropped below presensitization levels. In addition, upon parenteral challenge with SRBC the number of PFCs to SRBC and the humoral antibody responses elicited in SRBC-fed rats were found to be significantly lower than those of controls. Antigen-reactive cells, however, were not completely abolished even after 10 wk of feeding. The results demonstrate that a diminished responsiveness to particulate antigen achieved by prolonged feeding is preceded by a state of immunity and is not absolute. The mechanism underlying the phenomenon remains to be elucidated.