"The human immune system is an extremely complex network of interacting cells and biological molecules. An immune response to infection develops in three overlapping stages, the first of which is triggered when body cells are damaged. When tissues are injured, the damaged cells release chemicals that trigger the sequence of events described as inflammation. It occurs in response to any type of injury, such as a blow or a cut, an insect bite, or damage caused by pathogens multiplying in body tissues. Inflammation has four characteristic effects at the site of an injury: swelling, redness, heat and pain. The local blood vessels dilate, increasing blood flow into the injury site, so it looks red and feels warmer than the surrounding tissue. The walls of the blood vessels near the injury become leaky, allowing fluid, defensive proteins and immune system cells to flood into the area, which becomes swollen as a result. One of the proteins released during the inflammatory response also makes the area more sensitive to painful stimuli, so inflamed tissue is sore to touch. Flooding the area with fluid dilutes pathogens that are already present, and the local concentration of immune system cells and defensive proteins enables an immune response to begin more quickly. Once the barriers to infection have been breached and inflammation has begun, the active agents of the immune system, the leukocytes, get to work. Leukocytes are often described as white blood cells, but this is misleading because they roam throughout body tissues and only spend part of their lives in the bloodstream. They spend more time in the lymphatic system, the network of fine tubules that collect tissue fluid from all over the body and return it to the bloodstream. The lymphatic system includes specialized organs and tissues where leukocytes develop. During an immune response to pathogens, lymph nodes in the neck, armpits or groin may become swollen as the leukocytes they contain multiply near a site of infection. Leukocytes can distinguish between self, the cells and proteins generated by the organism whose body they patrol, and non-self, or foreign, material such as pathogens. Leukocytes are self-tolerant: they do not normally attack the host’s own cells or body proteins, but direct their actions against non-self material that may pose a threat."