Immunological responses to chronic heat exposure and food restriction in rats

Immunological variables were studied in rats chronically exposed to high environmental temperature (35 degrees C). Responses were compared with those of rats at 25 degrees C both fed ad libitum and pair fed to the decreased intake found in heat-exposed rats. Heat-exposed rats showed slower delayed-type hypersensitivity responses to keyhole limpet hemocyanin. They showed lower counts of peripheral blood total T cells (OX19+) as well as helper T cells (W3/25+) and smaller numbers of splenic T cells. The thymus was decreased in size. Increased levels of serum IgG antitetanus toxoid antibodies were found in heat-exposed rats. [3H]-thymidine incorporation into Concanavalin A (ConA)-stimulated splenic lymphocytes was decreased in pair-fed rats but not significantly altered in heat-exposed rats compared with controls. Heat exposure alters some aspects of both cellular and humoral immune function in a manner different from that induced by comparable food restriction without heat exposure.