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In biology, immunity is the balanced state of having adequate biological defenses to fight infection, disease, or other unwanted biological invasion, while having adequate tolerance to avoid allergy, and autoimmune diseases. == Innate and adaptive immunity == It is the capability of the body to resist harmful microorganisms or viruses from entering it. Immunity involves both specific and nonspecific components. The nonspecific components act either as barriers or as eliminators of wide range of pathogens irrespective of antigenic specificity. Other components of the immune system adapt themselves to each new disease encountered and are able to generate pathogen-specific immunity. The basic premise for the division of the immune system into innate and adaptive components comes down to the innate system being composed of primitive bone marrow cells that are programmed to recognise ''foreign'' substances and ''react'', versus the adaptive system being composed of more advanced lymphatic cells that are programmed to recognise ''self'' substances and ''don't react''. The reaction to foreign substances is etymologically described as inflammation, meaning ''to set on fire'', while the non-reaction to self substances is etymologically described as immunity, meaning ''to exempt''. The interaction of these two components of the immune system creates a dynamic biological environment where "Health" can be seen as an active physical state where what is self is immunologically spared, and what is foreign is inflammatorily and immunologically eliminated. Extending this concept, "Disease" then can arise when what is foreign cannot be eliminated, or what is self cannot be spared. Innate immunity, or nonspecific immunity, is the natural resistances with which a person is born. It provides resistances through several physical, chemical and cellular approaches. Microbes first encounter the epithelial layers, physical barriers that line skin and mucous membranes. Subsequent general defences include secreted chemical signals (cytokines), antimicrobial substances, fever, and phagocytic activity associated with the inflammatory responses. The phagocytes express cell surface receptors that can bind and respond to common molecular patterns expressed on the surface of invading microbes. Through these approaches, innate immunity can prevent the colonization, entry and spread of microbes. Adaptive immunity is often sub-divided into two major types depending on how the immunity was introduced. 'Naturally acquired immunity' occurs through contact with a disease causing agent, when the contact was not deliberate, whereas 'artificially acquired immunity' develops only through deliberate actions such as vaccination. Both naturally and artificially acquired immunity can be further subdivided depending on whether immunity is induced in the host or passively transferred from an immune host. 'Passive immunity' is acquired through transfer of antibodies or activated T-cells from an immune host, and is short lived—usually lasting only a few months—whereas 'active immunity' is induced in the host itself by antigen and lasts much longer, sometimes lifelong. The diagram below summarizes these divisions of immunity. A further subdivision of adaptive immunity is characterized by the cells involved; humoral immunity is the aspect of immunity that is mediated by secreted antibodies, whereas the protection provided by cell mediated immunity involves T-lymphocytes alone. Humoral immunity is active when the organism generates its own antibodies, and passive when antibodies are transferred between individuals. Similarly, cell mediated immunity is active when the organisms’ own T-cells are stimulated and passive when T cells come from another organism. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Immunity (medical)」の詳細全文を読む スポンサード リンク
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