Impact of Heat Stress on Immune System
The immune system is foremost important component of body for the defense mechanism against the invading pathogens. The invading pathogens are acted upon by the innate and adaptive immune systems.
The equilibrium between the innate and the adaptive immune systems is regulated by Hypothalamic Pituitary Adrenal (HPA) and Sympathetic-Adreno-Medullar (SAM) axis. In addition, toll-like receptors (TLRs), HSPs, NOD-like receptors (NLRs), C-type lectin receptors (CLRs), AIM2-like receptors (ALRs), RIG-I like receptors (RLRs) and cytokines are also involved in the maintenance of immune system. Innate immunity is non-specific which is the first line of defense; whereas, adaptive immune system is induced by the entry of the pathogen which is more specific to act on the invading antigen. The cell-mediated immunity (CMI) and humoral mediated immunity (HMI) are the components of adaptive immune system. The T-lymphocytes are responsible for CMI, and B-lymphocytes constitute HMI. The mast cells, natural killer cells, eosinophils, basophils and phagocytic cells are fundamentals of the immune system. The balance between CMI and HMI is highly essential for effective functioning due to their interdependent actions. The heat stress impairs or depresses both humoral and cellular immunity by reducing generation of primary and secondary immunoglobulin, and production of inflammatory and anti-inflammatory cytokines.The elevated level of cortisol depresses the production of L-selectin expression on the surface of neutrophil that controls recruitment of neutrophil and restricts movement of neutrophils with reduced phagocytic activity. The induction of HSPs during heat stress enhances and facilitates the innate immune cells to act against the invading pathogen. The movement of circulating white blood or immune cells is reduced into the mammary glands during heat stress and results more incidents of mastitis in dairy animals.
Moreover, the production of IgG and IgM decreases during summer. Heat stress also impairs the digestive process, thereby leading to reduced absorption of IgG in calves. HSPs play an important role in antigen processing and presentation in addition to their chaperonic activities, thereby activating adaptive and innate immune response during heat stress. During heat stress, the expression of TLR genes was also observed to be altered which could be indicative of the adaptive strategy adopted by animals to combat the deleterious impact of heat stress on their immune system. Further, the animal maintains the balance between Th1, which is responsible the CMI, andTh2 which favours the HMI under normal conditions. The increased levels of glucocorticoids during heat stress disrupt the Th1-Th2 balance, shifting from Th1 to Th2 which enhances the HMI. Thus, prolonged heat stress negatively affects the immune system by modifying balance between innate and adaptive immune system and also making the animals more prone to infectious diseases.
28.8