The condition of being immune. Immunity can be innate (for example, humans are innately immune to canine distemper) or conferred by a previous infection or immunization.
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1. The status or quality of being immune (1). 2. Protection against infectious disease. SYN: insusceptibility. [L. immunitas (see immune)]
- acquired i. resistance resulting from previous exposure of the individual in question to an infectious agent or antigen; it may be active and specific, as a result of naturally acquired (apparent or inapparent) infection or intentional vaccination (artificial active i.); or it may be passive, being acquired from transfer of antibodies from another person or from an animal, either naturally, as from mother to fetus, or by intentional inoculation (artificial passive i.), and, with respect to the particular antibodies transferred, it is specific. Passive, cell-mediated i. produced by the transfer of living lymphoid cells from an immune (allergic or sensitive) animal to a normal one is sometimes referred to as adoptive i..
- active i. acquired i..
- adoptive i. acquired i..
- antiviral i. i. resulting from virus infection, either naturally acquired or produced by intentional vaccination; compared to some bacterial immunities, it is of relatively long duration, but this may be the result of infection-i. rather than being peculiar to virus infection per se, since it occurs also in bacterial i. after infections such as typhoid fever.
- artificial active i. acquired i..
- artificial passive i. acquired i..
- bacteriophage i. the state induced in a bacterium by lysogenization, the lysogenic bacterium being insusceptible to further lysogenization or to a lytic cycle by a superinfecting bacteriophage, in contradistinction to bacteriophage resistance.
- cell-mediated i. (CMI), cellular i. immune responses that are initiated by an antigen-presenting cell interacting with and mediated by T lymphocytes ( e.g., graft rejection, delayed-type hypersensitivity). SYN: delayed hypersensitivity (1).
- concomitant i. SYN: infection i..
- general i. i. associated with widely diffused mechanisms that tend to protect the body as a whole, as compared with local i..
- group i. SYN: herd i..
- herd i. the resistance to invasion and spread of an infectious agent in a group or community, based on the resistance to infection of a high proportion of individual members of the group; resistance is a product of the number susceptible and the probability that susceptibles will come into contact with an infected person. SYN: group i..
- humoral i. i. associated with circulating antibodies, in contradistinction to cellular i..
- infection i. the paradoxical immune status in which resistance to reinfection coincides with the persistence of the original infection. SYN: concomitant i..
- innate i. resistance manifested by a species (or by races, families, and individuals in a species) that has not been immunized (sensitized, allergized) by previous infection or vaccination; much of it results from body mechanisms that are poorly understood, but are different from those responsible for the altered reactivity associated with the specific nature of acquired i.; in general, innate i. is nonspecific and is not stimulated by specific antigens. SEE ALSO: self. SYN: natural i., nonspecific i..
- local i. a natural or acquired i. to certain infectious agents, as manifested by an organ or a tissue, as a whole or in part.
- maternal i. i. acquired by a fetus because of the presence of maternal IgG that passes through the placenta.
- natural i., nonspecific i. SYN: innate i..
- passive i. acquired i..
- relative i. a modified, not completely effective resistance that results when there is a sort of “fluctuating equilibrium” between the defense mechanisms of the host and the infective agent.
- specific i. the immune status in which there is an altered reactivity directed solely against the antigenic determinants (infectious agent or other) that stimulated it. See acquired i..
- specific active i. acquired i..

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im·mu·ni·ty im-'yü-nət-ē n, pl -ties the quality or state of being immune esp a condition of being able to resist a particular disease esp. through preventing development of a pathogenic microorganism or by counteracting the effects of its products see ACQUIRED IMMUNITY, ACTIVE IMMUNITY, NATURAL IMMUNITY, PASSIVE IMMUNITY

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the body's ability to resist infection, afforded by the presence of circulating antibody and white blood cells. Healthy individuals protect themselves by means of physical barriers, phagocytic cells, natural killer cell, and various blood-borne molecules. All of these mechanisms are present prior to exposure to infectious agents and are part of natural (or innate) immunity. Antibodies are manufactured specifically to deal with the antigens associated with different diseases as they are encountered. Active immunity arises when the body's own cells produce, and remain able to produce, appropriate antibodies following an attack of a disease or deliberate stimulation (see immunization). Passive immunity, which is only short-lived, is provided by injecting ready-made antibodies in antiserum taken from another person or an animal already immune. Babies have passive immunity, conferred by antibodies from the maternal blood and colostrum, to common diseases for several weeks after birth. See also immune response.

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im·mu·ni·ty (ĭ-muґnĭ-te) [L. immunitas] the condition of being immune; the protection against infectious disease conferred either by the immune response generated by immunization or previous infection or by other nonimmunologic factors.

Medical dictionary. 2011.

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