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natural antiviral immunity 524 natural killer (NK) cells
blood sera are (1) specific, antigen-induced antibodies, the
synthesis of which depends on external antigenic stimuli
and corresponds to acquired specificities; and (2) a type
that expresses broad specificity, is genetically determined,
and does not depend on a specific antigenic stimulus.
Both kinds of natural antibodies of the IgM, IgG, and IgA
isotypes specific for many antigens are present in normal
sera of humans and other animals. Natural antibodies have
a variety of biological functions ranging from physiological
to pathological effects. They comprise a component of
innate immunity.
natural antiviral immunity
Occurs when virus-infected host cells synthesize type I
interferon. This blocks virus replication. Natural killer
(NK) cells that are not MHC-restricted provide early antiviral
effects following infection. Type I interferon accentuates
their action. Both complement and phagocytosis play
significant roles in removal of extracellular viruses.
natural autoantibodies
Polyreactive antibodies of low affinity synthesized by
CD5 + B cells that compose 10 to 25% of circulating B
lymphocytes in normal individuals, 27 to 52% in those with
rheumatoid arthritis, and less than 25% in patients with
systemic lupus erythematosus (SLE). Natural autoantibodies
may appear in first-degree relatives of autoimmune
disease patients and in older individuals. They may be
predictive of disease in healthy subjects. They are often
present in patients with bacterial, viral, or parasitic infections
and may have a protective effect. In contrast to natural
antibodies, autoantibodies may increase in disease and lead
to tissue injury. The blood group isohemagglutins are also
considered natural antibodies even though they are believed
to be of heterogenetic immune origin as a consequence of
stimulation by microbial antigens.
natural cytotoxicity
NK cell cytotoxicity of target cells mediated by perforin
and granzyme.
natural cytotoxicity receptors (NCRs)
Activating NK cell receptors that belong to the immunoglobulin
family.
natural fluorescence
Autofluorescence.
natural immunity
Innate immune mechanisms that do not depend upon previous
exposure to an antigen. Among the factors that contribute
to natural resistance are the skin, mucous membranes,
and other barriers to infection; lysozyme in tears and other
antibacterial molecules; and natural killer (NK) cells.
natural interferon-producing cells (NIPCs)
Refer to interferon-producing cells.
natural killer (NK) cells
Lymphoid cells that recognize nonself molecular configurations
with broad specificity. They attack and destroy tumor
cells and certain virus-infected cells. They constitute an
important part of the natural immune system, do not require
prior contact with antigen, and are not major histocompatibility
complex (MHC)-restricted by MHC antigens. NK cells
are lymphoid cells of the natural immune system that express
cytotoxicity against various nucleated cells including tumor
cells and virus-infected cells. NK cells, killer (K) cells, and
antibody-dependent, cell-mediated cytotoxicity (ADCC) cells
induce lysis through the action of antibody. Immunologic
9–12 µm
Natural killer (NK) cell schematic representation.
Natural killer (NK) cell schematic representation, transmission and scanning
electron microscopy (SEM) of human large granular lymphocyte.
NK cells
K562 Target cell
NK cells.
memory is not involved, as previous contact with the antigen
is not necessary for NK cell activity. The NK cell is approximately
15 μm in diameter and has a kidney-shaped nucleus
with several, often three, large cytoplasmic granules. The
cells are also called large granular lymphocytes (LGLs).
In addition to their ability to kill selected tumor cells and
some virus-infected cells, they also participate in ADCC by