One of the body's lines of defense (immune system) involves white blood cells (leukocytes) that travel through the bloodstream and into tissues, searching for and attacking microorganisms and other invaders. (See also Overview of the Immune System.)
This defense has two parts:
- Innate immunity
- Acquired immunity
Innate (natural) immunity is so named because it is present at birth and does not have to be learned through exposure to an invader. It thus provides an immediate response to foreign invaders. However, its components treat all foreign invaders in much the same way. They recognize only a limited number of identifying substances (antigens) on foreign invaders. However, these antigens are present on many different invaders. Innate immunity, unlike acquired immunity, has no memory of the encounters, does not remember specific foreign antigens, and does not provide any ongoing protection against future infection.
The white blood cells involved in innate immunity are
- Monocytes (which develop into macrophages)
- Natural killer cells
Each type has a different function.
Other participants in innate immunity are
- Mast cells
- The complement system
Monocytes and Macrophages
Macrophages develop from a type of white blood cell called monocytes. Monocytes become macrophages when they move from the bloodstream to the tissues.
Monocytes move to the tissues when infection occurs. There, over a period of about 8 hours, monocytes enlarge greatly and produce granules within themselves, becoming macrophages. The granules are filled with enzymes and other substances that help kill and digest bacteria and other foreign cells.
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.)
Macrophages secrete substances that attract other white blood cells to the site of the infection. They also help T cells recognize invaders and thus also participate in acquired immunity.
Neutrophils, the most common type of white blood cell in the bloodstream, are among the first immune cells to defend against infection. They are phagocytes, which 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 damaged tissue, all of which produce substances that attract neutrophils to a trouble spot. (The process of using substances to attract cells to a particular site is called chemotaxis.)
Neutrophils also release substances that produce fibers in the surrounding tissue. These fibers may trap bacteria, thus keeping them from spreading and making them easier to destroy.
Eosinophils can ingest bacteria, but they also target foreign cells that are too large to ingest. Eosinophils contain granules that release enzymes and other toxic substances when foreign cells are encountered. These substances make holes in the target cell’s membranes.
Eosinophils circulate in the bloodstream. However, they are less active against bacteria than are neutrophils and macrophages. One of their main functions is to attach to and thus help immobilize and kill parasites.
Eosinophils may help destroy cancer cells. They also produce substances involved in inflammation and allergic reactions. People with allergies, parasitic infections, or asthma often have more eosinophils in the bloodstream than people without these disorders.
Basophils do not ingest foreign cells. They contain granules filled with histamine, a substance involved in allergic reactions. When basophils encounter allergens (antigens that cause allergic reactions), they release histamine. Histamine increases blood flow to damaged tissues, resulting in swelling and inflammation.
Basophils also produce substances that attract neutrophils and eosinophils to a trouble spot.
Natural Killer Cells
Natural killer cells are called “natural” killers because they are ready to kill as soon as they are formed. Natural killer cells recognize and attach to infected cells or cancer cells, then release enzymes and other substances that damage the outer membranes of these cells. Natural killer cells are important in the initial defense against viral infections.
Also, natural killer cells produce cytokines that regulate some of the functions of T cells, B cells, and macrophages.
Mast cells are present in the tissues. Their function resembles that of basophils in the blood. When they encounter an allergen, they release histamine and other substances involved in inflammatory and allergic reactions.
The complement system consists of more than 30 proteins that act in a sequence: One protein activates another, which activates another, and so on to defend against infection. This sequence is called the complement cascade.
Complement proteins have many functions in acquired immunity as well as innate:
- Killing bacteria directly
- Helping destroy bacteria by attaching to them and thus making the bacteria easier for neutrophils and macrophages to identify and ingest
- Attracting macrophages and neutrophils to a trouble spot
- Neutralizing viruses
- Helping immune cells remember specific invaders
- Promoting antibody formation
- Enhancing the effectiveness of antibodies
- Helping the body eliminate dead cells and immune complexes (which consist of an antibody attached to an antigen)
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 cytokines 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 acquired immunity.