Immunology is a branch of biomedical science that covers the study of immune systems in all organisms. Immunology has applications in numerous disciplines of medicine, particularly in the fields of organ transplantation, oncology, virology, bacteriology, parasitology, psychiatry, and dermatolog.
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The organs of the immune system are positioned throughout the body. They are called lymphoid organs because they are home to lymphocytes, small white blood cells that are the key players in the immune system.
Bone marrow, the soft tissue in the hollow center of bones, is the ultimate source of all blood cells, including white blood cells destined to become immune cells. The thymus is an organ that lies behind the breastbone; lymphocytes known as T lymphocytes, or just “T cells,” mature in the thymus. Lymphocytes can travel throughout the body using the blood vessels. The cells can also travel through a system of lymphatic vessels that closely parallels the body’s veins and arteries. Cells and fluids are exchanged between blood and lymphatic vessels, enabling the lymphatic system to monitor the body for invading microbes. The lymphatic vessels carry lymph, a clear fluid that bathes the body’s tissues. Small, bean-shaped lymph nodes are laced along the lymphatic vessels, with clusters in the neck, armpits, abdomen, and groin. Each lymph node contains specialized compartments where immune cells congregate, and where they can encounter antigens. Immune cells and foreign particles enter the lymph nodes via incoming lymphatic vessels or the lymph nodes’ tiny blood vessels. All lymphocytes exit lymph nodes through outgoing lymphatic vessels. Once in the bloodstream, they are transported to tissues throughout the body. They patrol everywhere for foreign antigens, then gradually drift back into the lymphatic system, to begin the cycle all over again. The spleen is a flattened organ at the upper left of the abdomen. Like the lymph nodes, the spleen contains specialized compartments where immune cells gather and work, and serves as a meeting ground where immune defenses confront antigens. Clumps of lymphoid tissue are found in many parts of the body, especially in the linings of the digestive tract and the airways and lungs—territories that serve as gateways to the body. These tissues include the tonsils, adenoids, and appendix (Figure 1a).
The immune system stockpiles a huge arsenal of cells, not only lymphocytes but also cell-devouring phagocytes and their relatives. Some immune cells take on all comers, while others are trained on highly specific targets. To work effectively, most immune cells need the cooperation of their comrades. Sometimes immune cells communicate by direct physical contact, sometimes by releasing chemical messengers. The immune system stores just a few of each kind of the different cells needed to recognize millions of possible enemies. When an antigen appears, those few matching cells multiply into a full-scale army. After their job is done, they fade away, leaving sentries behind to watch for future attacks. All immune cells begin as immature stem cells in the bone marrow. They respond to different cytokines and other signals to grow into specific immune cell types, such as T cells, B cells, or phagocytes. Because stem cells have not yet committed to a particular future, they are an interesting possibility for treating some immune system disorders.
B cells work chiefly by secreting substances called antibodies into the body’s fluids. Antibodies ambush antigens circulating the bloodstream. They are powerless, however, to penetrate cells. The job of attacking target cells—either cells that have been infected by viruses or cells that have been distorted by cancer—is left to T cells or other immune cells. Unlike B cells, T cells do not recognize free-floating antigens. Rather, their surfaces contain specialized antibody-like receptors that see fragments of antigens on the surfaces of infected or cancerous cells. T cells contribute to immune defenses in two major ways: some direct and regulate immune responses; others directly attack infected or cancerous cells. Natural killer (NK) cells are another kind of lethal white cell, or lymphocyte. Like killer T cells, NK cells are armed with granules filled with potent chemicals. Phagocytes are large white cells that can swallow and digest microbes and other foreign particles. Monocytes are phagocytes that circulate in the blood. When monocytes migrate into tissues, they develop into macrophages. Granulocytes are another kind of immune cell. They contain granules filled with potent chemicals, which allow the granulocytes to destroy microorganisms. Some of these chemicals, such as histamine, also contribute to inflammation and allergy. One type of granulocyte, the neutrophil, is also a phagocyte; it uses its prepackaged chemicals to break down the microbes it ingests. Eosinophils and basophils are granulocytes that “degranulate,” spraying their chemicals onto harmful cells or microbes nearby. The mast cell is a twin of the basophil, except that it is not a blood cell. Rather, it is found in the lungs, skin, tongue, and linings of the nose and intestinal tract, where it is responsible for the symptoms of allergy. A related structure, the blood platelet, is a cell fragment. Platelets, too, contain granules. In addition to promoting blood clotting and wound repair, platelets activate some of the immune defenses (Figure 1b). The more detail about the origin and development of immune cells, see The Hemopoietic System.
Figure 1. Immune organs (a) and immune cells (b).
2. Native immunity and adaptive immunity
Once a pathogen has penetrated into the body, it is greeted by the human immune system. This system is divided into two forces, both of which get to work straight away: one responds quickly in a non-specific manner and the other occurs slowly and is specific to infecting organisms. These are the innate and adaptive immune systems, respectively (Table 1 and Table 2).
Table 1. Essential differences between the innate and adaptive immune systems
|Innate immune system||Adaptive immune system|
Provides a rapid response
It is not antigen specific
The response does not improve
with repeated exposure
The response takes time to develop, because:
It is specific for each different antigen
Initial exposure to an antigen leaves memory
cells; subsequent infections with the same
antigen are therefore dealt with more quickly
Table 2. Components of the innate and adaptive immune systems
|Innate system||Adaptive system|
Natural killer cells
B cells/plasma cells
Acute phase proteins
The immune system recognize the pathogen through a variety of specific molecules called receptors. Different receptors can recognize different component of the pathogen and transfer the recognition signal back to the immune system to initiate the immune response against those recognized pathogens. Read more about immune recognition, please visit Immune Recognition and Receptors and Receptors of The Innate Immune System.