Inflammation is the general response of the body to infection by a pathogen. It is a protective tissue response to injury. Inflammation removes or blocks off the invader in the tissue and begins the process of healing. The cardinal signs of inflammation are pain, heat, redness, and swelling, and loss of function.
In inflammation, resident immune cells recognize generic molecules shared by pathogens. Resident mast cells release histamine and other cells release mediators that cause the signs of inflammation. Histamine causes capillaries to dilate (redness) and become more permeable (swelling). Vasodilation increases blood flow, resulting in a rise in temperature (heat). Swelling and bradykinin released during inflammation stimulate free nerve endings causing pain.
Resident macrophages reside in tissues and respond immediately by release of cytokines. Cytokines attract neutrophils and monocytes, which become macrophages in the tissues. Neutrophils and monocytes pass through blood vessel walls (diapedesis) to wander about (migration) within the tissues, patrolling for pathogens.
Pyrogens released by macrophages increase the temperature set-point in the hypothalamus. This leads to fever. The systemic response is of feeling cold and shivering to increase heat production is due to the body temperature (even though it is elevated) is not as high as the set-point.
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