The immune system is a defense system that protects invertebrate and vertebrate animals from microorganisms and substances recognized as foreign and potentially harmful. It evolved through a gradual process of increasing diversification that led to the enormous complexity of recognition and elimination mechanisms present in higher vertebrates.
These recognition and elimination mechanisms operate and cooperate within the two components of the immune system—that is, the innate immune system and the adaptive immune system, thus leading to immune responses that protect the body from infectious diseases and cancer. Recognition mechanisms allow the detection of foreign organisms and molecules as well as cancer cells through binding by receptors expressed on the cell surface of certain cells of the immune system.
The specific portion of the foreign invader that is detected by the recognition mechanisms is called antigen. Elimination mechanisms are responsible for the destruction of foreign invaders and cancer through the action of specialized cell types. The complement system, a set of plasma proteins, is also involved in destroying foreign invaders. Innate and adaptive immune responses rely on the activity of white blood cells, or leukocytes. Innate immune responses represent the first line of defenses, exist prior to exposure to a pathogen, are nonspecific, occur in a few hours or days, and involve the participation of two major types of leukocytes—neutrophils and macrophages—which are phagocytic cells able to take up and destroy foreign invaders.
Adaptive immune responses develop several days after encounter with the pathogen, are specific, include the development of immunological memory, and require the participation of two types of lymphocytes: B and T lymphocytes. B lymphocytes produce antibodies, which activate the complement system, neutralize pathogens, or prepare them for uptake and degradation by the phagocytic cells of the innate immune system. T lymphocytes exist in two subsets, helper T lymphocytes and cytotoxic T lymphocytes. Helper T lymphocytes help B lymphocytes and cytotoxic T lymphocytes to become activated for their respective functions.
Cytotoxic T lymphocytes kill cancer cells and infected cells by opening up pores in the surface of these cells and inserting molecules that cause cell death. Adaptive immune responses also require the participation of antigen-presenting cells, which prepare and present the antigen in a form recognizable by T lymphocytes. All cells of the immune system originate in the bone marrow. B and T lymphocytes undergo a maturation process that includes the assembly of their antigen receptors. B lymphocytes mature in the bone marrow, whereas T lymphocytes develop in the thymus. All cells then migrate to the bloodstream, circulating in the blood and in the lymphatic system, a specialized system of vessels that collects extracellular fluid from the tissues and returns it to the blood.
Adaptive immune responses develop in lymph nodes, which are located at the points of convergence of vessels of the lymphatic system. Although the immune system is generally capable of protecting us from foreign invaders, there are several instances in which it may fail. Immune responses are sometime elicited by antigens not associated with infectious agents and, in conjunction with genetic and environmental factors, may result in serious autoimmune diseases such as multiple sclerosis and systemic lupus erythematosus. Responses to environmental antigens cause allergic diseases and other hypersensitivity reactions. In immunodeficiency diseases, a defective gene causes the elimination of one or more components of the immune system leading, in the most severe cases, to overwhelming infection. The acquired immune deficiency syndrome, or AIDS, is a form of immunodeficiency. It is caused by a virus, the human immunodeficiency virus or HIV, and is characterized by the destruction of several cell types of the immune system, especially helper T lymphocytes.
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