Immunological responses to chronic heat exposure and food restriction in rats

Immunological variables were studied in rats chronically exposed to high environmental temperature (35°C). Responses were compared with those of rats at 25°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 hemocyasis. 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.