The body has an arsenal of defenses that it uses to ward off foreign invaders. Healthy immune systems recognize as foreign many things that are not self. Most of the foreign invaders that confront the human immune system are microscopic. Fungi, parasites, bacteria, and viruses populate our planet in far greater numbers than any other living organisms. Some are beneficial. We coexist quite happily with certain types of bacteria that live in our gastrointestinal and genital tracts. These bacteria help prevent infection by harmful organisms. We are even dependent on some microorganisms that inhabit our gastrointestinal tracts for their help in digesting food. Whether beneficial or harmful, however, all foreign microbes in the human body display special markers on antigens called antigenic determinants or epitopes. It is these antigens and their determinants that the immune system may recognize as harmful and identify for destruction.
Organs of the Immune System
The immune system is a complex of organs—highly specialized cells and even a circulatory system separate from blood vessels—all of which work together to clear infection from the body.
The organs of the immune system, positioned throughout the body, are called lymphoid organs. The word “lymph” in Greek means a pure, clear stream—-an appropriate description considering its appearance and the purpose of our immune systems.
Lymphatic vessels and lymph nodes are the parts of the special circulatory system that carries lymph, a transparent fluid containing white blood cells, chiefly lymphocytes. Lymph bathes the tissues of the body, and the lymphatic vessels collect and move it eventually back into the blood circulation. Lymph nodes dot the network of lymphatic vessels and provide meeting grounds for the immune system cells that defend against invaders. The spleen, at the upper left of the abdomen, is also a staging ground and a place where immune system cells confront foreign microbes.
Pockets of lymphoid tissue are in many other locations throughout the body, such as the bone marrow and thymus. Tonsils, adenoids, Peyer’s patches, and the appendix are also lymphoid tissues.
Both immune cells and foreign molecules enter the lymph nodes via blood vessels or lymphatic vessels. All immune cells exit the lymphatic system and eventually return to the bloodstream. Once in the bloodstream, lymphocytes are transported to tissues throughout the body, where they act as sentries on the lookout for foreign antigens.
How the Immune System Works
Cells that will grow into the many types of more specialized cells that circulate throughout the immune system are produced in the bone marrow. This nutrient-rich, spongy tissue is found in the center shafts of certain long, flat bones of the body, such as the bones of the pelvis. The cells most relevant for understanding vaccines are the lymphocytes, numbering close to one trillion.
The two major classes of lymphocytes are B cells, which grow to maturity in the bone marrow, and T cells, which mature in the thymus, high in the chest behind the breastbone.
B cells produce antibodies that circulate in the blood and lymph streams and attach to foreign antigens to mark them for destruction by other immune cells. B cells are part of what is known as antibody-mediated or humoral immunity, so called because the antibodies circulate in blood and lymph, which the ancient Greeks called, the body’s “humors.” Certain T cells, which also patrol the blood and lymph for foreign invaders, can do more than mark the antigens; they attack and destroy diseased cells they recognize as foreign. T cell lymphocytes are responsible for cell-mediated immunity (or cellular immunity). T cells also orchestrate, regulate and coordinate the overall immune response. T cells depend on unique cell surface molecules called the major histocompatibility complex (MHC) to help them recognize antigen fragments.
The antibodies that B cells produce are basic templates with a special region that is highly specific to target a given antigen. Much like a car coming off a production line, the antibody’s frame remains constant, but through chemical and cellular messages, the immune system selects a green sedan, a red convertible or a white truck to combat this particular invader. However, in contrast to cars, the variety of antibodies is very large. Different antibodies are destined for different purposes. Some coat the foreign invaders to make them attractive to the circulating scavenger cells, phagocytes, that will engulf an unwelcome microbe. When some antibodies combine with antigens, they activate a cascade of nine proteins, known as complement, that have been circulating in the inactive form in the blood. Complement forms a partnership with antibodies, once they have reacted with antigen, to help destroy foreign invaders and remove them from the body. Still other types of antibodies block viruses from entering cells.
T cells have two major roles in immune defense. Regulatory T cells are essential for orchestrating the response of an elaborate system of different types of immune cells. Helper T cells, for example, also known as CD4 positive T cells (CD4+ T cells), alert B cells to start making antibodies; they also can activate other T cells and immune system scavenger cells called macrophages and influence which type of antibody is produced. Certain T cells, called CD8 positive T cells (CD8+ T cells), can become killer cells that attack and destroy infected cells. The killer T cells are also called cytotoxic T cells or CTLs (cytotoxic lymphocytes).