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antigen-binding site 52 antigen presentation
B-cell antigen
receptor
(membrane bound)
Heavy chain
Light chain
Plasma
membrane
Antigen-binding site.
Antibody
(secreted)
temperature, the unbound cells are washed out, and the
bound cells are collected following melting of the gelatin
layer at 37°C. The harvested cells are washed, counted, and
used for various other assays.
antigen-binding site
The location on an antibody molecule where an antigenic
determinant or epitope combines with it. The antigenbinding
site is located in a cleft bordered by the N
terminal variable regions of heavy and light chain parts
of the Fab region. Also called paratope. The term also
refers to that part of a T cell receptor that binds antigen
specifically.
antigen capture assay
A method to identify minute quantities of antigen in blood
sera or supernatants. Antibodies of high titer are linked to
an insoluble solid support, and the specimen containing
the antigen to be evaluated is passed over the solid phase.
This will bind or capture the antigen, making it available
for reaction with a separate enzyme-labeled antibody that
reacts with and reveals the captured antigen.
antigen clearance
Exogenous clearance is a principal immune system function.
Antigen removal is by phagocytosis, cytolysis, or
complement-mediated elimination. Fixed mononuclear
phagocytic system cells are the principal mechanisms for
eliminating antigen. The mechanism of removal depends
on the biological and physical chemical properties of the
antigen, its mode of presentation, and its capacity to induce
a specific humoral or cellular immune response.
antigen excess
The interaction of soluble antigen and antibody in the
precipitin reaction leads to occupation of all of the binding
sites of the antibody molecules and leaves additional
antigenic determinants free to combine with more antibody
molecules if excess antigen is added to the mixture. This
leads to the formation of soluble antigen–antibody complexes
in vitro (i.e., the postzone in the precipitin reaction).
A similar phenomenon may take place in vivo when
immune complexes form in the presence of excess antigen.
These are of clinical significance in that soluble immune
complexes may induce tissue injury, leading to immunopathologic
sequelae.
antigen masking
The ability of some parasites (e.g., Schistosoma mansoni) to
become coated with host proteins, theoretically rendering
them “invisible” to the host’s immune system.
antigen presentation
The display of peptide antigens on the cell surface together
with either MHC class I or class II molecules that permits
T cells to recognize antigen on a target cell or antigen-
Immunogen
Endosome MHCII
Lysosome
Antigen-presenting cell
TCR
T H cell
presenting cell surface. T lymphocytes recognize antigens
only in the context of self-MHC molecules on the surfaces
of antigen-presenting cells. During processing, intact protein
antigens are degraded into peptide fragments. Most epitopes
that T cells recognize are peptide chain fragments. B cells
and T cells often recognize different epitopes of an antigen
leading to both antibody and cell-mediated immune
responses. Before antigen can bind to MHC molecules, it
must be processed into peptides in the intracellular organelles.
CD4 + helper T lymphocytes recognize antigens in the
context of class II MHC molecules, a process known as class
II MHC restriction. By contrast, CD8 + cytotoxic T lymphocytes
recognize antigens in the context of class I molecules;
this is known as class I MHC restriction. Following the
generation of peptides by proteolytic degradation in antigenpresenting
cells, peptide–MHC complexes are presented on
the surface of antigen-presenting cells where they may be
recognized by T lymphocytes. Antigens derived from either
intracellular or extracellular proteins may be processed to
produce peptides from either self or foreign proteins that are
presented by surface MHC molecules to T cells. In the class
II MHC processing pathway, professional antigen-presenting
cells, such as macrophages, dendritic cells, or B lymphocytes
incorporate extracellular proteins into endosomes where they
are processed. Enzymes within the vesicles of the endosomal
pathway cleave proteins in the acidic environment.
Class II MHC heterodimeric molecules united with
invariant chains are shifted to endosomal vesicles from the
CD4
Capture, processing, and presentation of antigen by an antigen-presenting
cell.
Antigen presentation.