The second line of defence is enforced when the first line of defence has failed. It involves non-specific responses to pathogens that have invaded the internal environment of the organism. Fever The mechanism of fever appears to be a defensive reaction by the body against infectious disease. When bacteria or viruses invade the body and cause tissue injury, one of the immune system’s responses is to produce pyrogens. These chemicals are carried by the blood to the brain, where they disturb the functioning of the hypothalamus, the part of the brain that regulates body temperature. The pyrogens inhibit heat-sensing neurons and excite cold-sensing ones, and the altering of these temperature sensors deceives the hypothalamus into thinking the body is cooler than it actually is. In response, the hypothalamus raises the body’s temperature above the normal range, thereby causing a fever. The above-normal temperatures are thought to help defend against microbial invasion because they stimulate the motion, activity, and multiplication of white blood cells and increase the production of antibodies. At the same time, elevated heat levels may directly kill or inhibit the growth of some bacteria and viruses that can tolerate only a narrow temperature range. Exogenous pyrogen. A type of pyrogen that is of microbial origin, such as the lipopolysaccharide in the cell wall of phagocytosed gram-negative bacteria. Supplement. Pyrogens are substances that induce fever. Endogenous pyrogen is a low-molecular-weight protein released from phagocytic leukocytes in response to several substances of diverse nature. ... In its monomer form, endogenous pyrogen is a potent fever-producing substance and mediates fever by its action on the thermoregulatory center. #FeverNaturalBodyResponse #WhatIsFever #ImmuneSystem