Inflammation is redness, swelling, heat, and pain in a tissue due to chemical or physical injury, or to infection.
When body tissues are damaged, specialized mast cells release a chemical called histamine (other substances are also involved in the inflammatory response, but histamine is believed to be responsible for most of the effects). Histamine increases blood flow to the damaged tissue, which causes the redness and heat. It also makes the blood capillaries more leaky, resulting in fluid oozing out of them and into the tissues, which causes localized swelling. The pain of inflammation is due to stimulation of nerve endings by the inflammatory chemicals.
Inflammation is usually accompanied by an accumulation of white blood cells, which are attracted by the inflammatory chemicals. These white blood cells help destroy invading microorganisms and are involved in repairing the damaged tissue. Thus, inflammation is an essential part of the body's response to injury and infection.
If inflammation is inappropriate (as in rheumatoid arthritis and some other autoimmune disorders), it may be suppressed by immunosuppressant drugs such as corticosteroids or by non-steroidal anti-inflammatory drugs.