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avidin 89 avidity
Oswald T. Avery.
Avian immunity.
Another difference from the mammalian immune system
is the multilobed thymus comprised of six to seven distinct
lobes on one side of the neck. Chicken T cells develop from
precursors that enter the thymus during development. Avian
nonlymphoid hematopoietic cells are well defined as are the
chicken immunoglobulin classes IgM and IgA and an IgGlike
class termed IgY. Chicken β 2-microglobulin and MHC
class I α (BF) and class II (BL) molecules are homologous
to their mammalian counterparts both structurally and
functionally. Chicken T cell surface constituents are similar
to mammalian T cell receptors (TCRs), in addition to CD3,
CD4, CD5, CD6, CD8, CD28, and CD45 antigens and the
interleukin-2 (IL-2) receptor α chain (CD25). Whereas
λ and γ chain genes are linked closely, light and heavy
chain gene complexes are not linked. Chicken TCR αβγ
and δ genes have been cloned and sequenced. The chicken
MHC (B complex) is situated on microchromosome 16,
which bears the nucleolar organizer region (NOR). The
avian immune system is especially susceptible to the avian
leukoses, Marek’s disease, and infectious bursal disease
(IBD). Vaccines against IBD and other diseases of chicks
are available.
avidin
A 68-kDa tetrameric egg-white glycoprotein that has a very
high affinity for and binds biotin, a water-soluble vitamin.
Four identical 128-amino-acid-residue subunits that bind a
single biotin molecule each comprise the avidin molecule.
It also has an N-linked oligosaccharide and one disulfide
bridge. Avidin’s strong affinity for biotin has made it useful
as an indicator molecule in a number of methods. An
enzyme can be linked to avidin, and the complex can be
bound to an antibody linked to biotin. The avidin–biotin–
peroxidase complex (ABC) method is an immunoperoxidase
reaction used extensively in antigen identification in
histopathological specimens, especially in surgical pathological
diagnosis. See also streptavidin.
avidin–biotin–peroxidase complex (ABC) technique
A method useful for the localization of peptide hormones
or other antigens in formalin-fixed tissues. After incubating
the tissue section with primary antibody specific for
the antigen sought, biotin-labeled secondary antibody is
applied. This is followed by avidin-biotinylated horseradish
peroxidase complex that then binds to the biotinylated
secondary antibody. The specific antigenic markers may
be visualized in tissue sections by conventional light
microscopy following incubation in a solution of peroxidase
substrate. The technique makes use of the very high affinity
of the 68-kDa avidin glycoprotein from egg white for
biotin. The ease with which biotin may be covalently linked
to antibody makes the ABC staining system feasible. In
addition to the widespread use of the technique in surgical
pathologic diagnosis, the principle can be applied to in situ
hybridization, gene mapping, double labeling, immunoelectron
microscopy, Southern blotting, radioimmunoassay,
solid-phase ELISA, hybridoma screening, etc.
Antibody
Antigen
Sum total
of all
interactive
forces
Avidity.
avidity
The strength of binding of an antibody and its specific antigen.
The stability of this union is a reflection of the number
of binding sites they share. Avidity is the binding force or
intensity between a multivalent antigen and a multivalent
antibody. Multiple binding sites on both the antigen and
the antibody (e.g., IgM or multiple antibodies interacting
with various epitopes on the antigen) and reactions of high
affinity between each of the antigens and its homologous
antibody all increase the avidity. Nonspecific factors
such as ionic and hydrophobic interactions also increase
avidity. Whereas affinity is described in thermodynamic
terms, avidity is not; it is described according to the assay
procedure employed. The sum of the forces contributing
to the avidity of an antigen and antibody interaction may
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