How the Immune System Works
Innate and Acquired Immunity
There are two principal types of immune response, innate and adaptive (or acquired) immunity, which are distinguished from one another by both their speed and specificity. The innate immune system, so called because it is present from birth, involves nonspecific responses that are the first line of defense against common infectious agents, including bacteria and viruses. This system is generally able to recognize foreign organisms but is unable to distinguish between particular invaders. Thus, an innate response does not require stimulation by sophisticated celltocell interactions to remove bacteria or other foreign material and degrade it.
In contrast to the innate immune system, the more specific adaptive (acquired) immune system must be triggered by a specific virus, bacterium, or other foreign material, which stimulates lymphocytes (see below) to produce antibodies that can combat the foreign substance. At the next exposure, the preformed antibodies will allow the person to respond with an even stronger, more specific response. This is called immunological memory.
Cells of the Immune System
The immune system consists of white blood cells (leukocytes), which are produced in the bone marrow and mature there or in the thymus and other lymphoid organs. Leukocytes circulate in the blood along with oxygencarrying red blood cells. Under normal conditions, leukocytes leave the circulation and migrate into organs, including the skin, lungs, intestine, and reproductive tract, as these are places where germs can appear. There, they can wait for infectious agents, or they can migrate back through the circulation to other organs. There are three major types of leukocytes.
Neutrophils are the most plentiful of the white blood cells in humans. They are the immune system’s first line of defense, as they contain an arsenal of preformed chemicals known as enzymes, which are capable of destroying bacteria. In addition, they are phagocytic, meaning that they can engulf viruses, bacteria, or other foreign material, protecting the host from further damage. Neutrophils are very shortlived and are often destroyed during the process of fighting infection.
Monocytes are leukocytes that, after migrating to tissues, mature into macrophages. Like neutrophils, macrophages are phagocytic and can remove foreign material and parts of dead cells from the tissues. They too contain enzymes that can destroy infectious material but live longer than neutrophils and do not tend to selfdestruct as easily. The tissue macrophage in the liver is called the Kupffer cell.
Lymphocytes, the most selective cells of the immune system, are specialized white blood cells that can combat specific infectious agents. There are two types of lymphocyte: B cells and T cells. B cells, which are responsible for humoral immunity (socalled because it takes place in the body fluids, classically known as the humors), release specialized, soluble proteins known as antibodies into the blood and other body fluids. The antibodies recognize and bind to the surface of foreign substances (i.e., pathogens), immobilizing them and further labeling them as foreign so that they can be more readily taken up by phagocytic cells.
T cells, in contrast, act directly on other cells rather than manufacturing antibodies to combat infectious agents. Because of this direct interaction with other cells, T cells are responsible for cellular immunity. They can be further divided into helper T cells, which recognize foreign invaders and stimulate immune responses from other cells; and cytotoxic T cells, which destroy infected cells. Whereas some of these cells survive only briefly, others are extremely longlived, including “memory cells,” which are capable of remembering certain features on the foreign molecules so that, if the organism encounters that foreign molecule in the future, it can quickly stimulate its response team.
Communication Between Immune Cells
One form of communication between immune cells is direct celltocell contact, which can occur either as a loose, transient association or as a tighter, more longlasting encounter. Either way, cells must make physical contact with one another.
In the second form of contact, cells release small proteins called cytokines, which bind to specific receptors on the surface of target cells. This enables cytokines to interact only with the appropriate target cell with no effect on surrounding cells. Although many of the effects of cytokines are local, they have been called the hormones of the immune system, because like hormones, they are transported by the circulating blood.
Cytokines can affect the same cell that produced them, a neighboring cell, or a cell far away. They stimulate or dampen cell proliferation (replication), production of other cytokines, killing of damaged cells or tumor cells (cytotoxicity), and cell migration (chemotaxis). The latter response is controlled by a subset of cytokines called chemokines. Just as there are cells that can stimulate or inhibit immune response, cytokines produced by those cells can regulate a variety of cell functions either positively or negatively.
— Elizabeth J. Kovacs and Kelly A.N. Messingham