Nonspecific Immunity
Nonspecific (innate) immunity is present at birth. Nonspecific immunity is so named because its components treat all foreign substances in much the same way.
The white blood cells involved in nonspecific immunity are monocytes (which develop into macrophages), neutrophils, eosinophils, basophils, and natural killer cells. Each type has a slightly different function. The complement system and cytokines also participate in nonspecific immunity.
Macrophages
Macrophages develop from a type of white blood cell called monocytes after monocytes move from the bloodstream to the tissues. When infection occurs, monocytes leave the bloodstream and move into the tissues. There, over a period of about 8 hours, monocytes enlarge greatly and produce granules within themselves. The granules are filled with enzymes and other substances that help digest bacteria and other foreign cells. Monocytes that have enlarged and contain granules are macrophages. Macrophages stay in the tissues. They ingest bacteria, foreign cells, and damaged and dead cells. (The process of a cell ingesting a microorganism, another cell, or cell fragments is called phagocytosis, and cells that ingest are called phagocytes.)
Neutrophils
Neutrophils ingest bacteria and other foreign cells. Neutrophils contain granules that release enzymes to help kill and digest these cells. Neutrophils circulate in the bloodstream and must be signaled to leave the bloodstream and enter tissues. The signal often comes from the bacteria themselves, from complement proteins, or from macrophages, all of which produce substances that attract neutrophils to a trouble spot. (The process of attracting cells is called chemotaxis.)
Eosinophils
Eosinophils can ingest bacteria and other foreign cells, contain granules filled with enzymes to digest the ingested bacteria and cells, and circulate in the bloodstream. However, they are less active against bacteria than are neutrophils and macrophages. Their main function may be to attach to and thus help immobilize and kill parasites. Eosinophils also participate in allergic reactions (such as asthma (see Section 16, Chapter 185)).
Basophils
Basophils do not ingest foreign cells. They contain granules that release histamine, a substance involved in allergic reactions. Basophils also produce substances that attract neutrophils and eosinophils to a trouble spot.
Natural Killer Cells
Natural killer cells are lymphocytes, a type of white blood cell. Natural killer cells are called "natural" killers because they are ready to kill as soon as they are formed. Natural killer cells attach to foreign cells and release enzymes and other substances that damage the outer membranes of the foreign cells. Natural killer cells kill certain microorganisms, cancer cells, and cells infected by viruses. Thus, natural killer cells are often the body's first line of defense against viral infections. Also, natural killer cells produce cytokines that regulate some of the functions of T lymphocytes, B lymphocytes, and macrophages.
Complement System
The complement system consists of more than 30 proteins that act in a sequence: One protein activates another and so on. This sequence is called the complement cascade. Complement proteins can kill bacteria directly or help destroy bacteria by attaching to them, thus making the bacteria easier for neutrophils and macrophages to identify and ingest. Other functions include attracting macrophages and neutrophils to a trouble spot, causing bacteria to clump together, and neutralizing viruses. The complement system also participates in specific immunity.
Cytokines
Cytokines are the messengers of the immune system. White blood cells and certain other cells of the immune system produce cytokines when an antigen is detected. There are many different cytokines, which affect different parts of the immune system. Some stimulate activity. They stimulate certain white blood cells to become more effective killers and to attract other white blood cells to a trouble spot. Other cytokines inhibit activity, helping end an immune response. Some cytokines, called interferons, interfere with the reproduction (replication) of viruses. Cytokines also participate in specific immunity.
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