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T cell receptor a chain (TCRa) 686 T cell system
αβ TCR
β α
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γ (Cγ1) δ
T cell receptor chain (TCR)
One of the two polypeptide chains comprising the predominant
type of T cell receptor.
T cell receptor chain (TCRβ )
One of the two polypeptide chains comprising the predominant
type of T cell receptor.
T cell receptor (TCR) complex
The combination of T cell receptor α and β chains and the
invariant signaling proteins CD3 γ, δ, and ε and the ζ chain.
T cell receptor γδ (TCR γδ)
A far less common receptor than the αβ TCR. It is composed
of γ and δ chains and occurs on the surfaces of early
thymocytes and less than 1% of peripheral blood lymphocytes.
The γδ TCR appears on double-negative CD4 – /CD8 –
cells. Thus, the γδ heterodimer resembles its αβ counterpart
in possessing V and C regions but has less diversity. TCR
specificity and diversity are attributable to the multiplicity
of germline V gene segments subjected to somatic recombination
in T cell ontogeny, leading to a complete TCR gene.
Cells bearing the γδ receptor often manifest target cell
killing that is not major histocompatibility complex (MHC)
restricted. Monoclonal antibodies to specific TCR V regions
are undergoing investigation for possible use in treating
autoimmune diseases. γδ T cells are sometimes found associated
with selected epithelial surfaces, especially in the
gut. The TCR complex is composed of the antigen-binding
chains associated at cell level with the signal transduction
molecules CD3, ζ, and η.
T cell receptor (TCR) genes
Four separate sets of genes encode the antigen–MHC binding
region of the T cell receptor. Approximately 95% of
peripheral T lymphocytes express α and β gene sets. Only
about 5% of circulating peripheral blood T cells and a subset
of T lymphocytes in the thymus express γ and δ genes.
The αβ chains or γδ chains encoded by their respective
genes form intact T cell receptors and are associated with
γ, δ, ε, ζ, and ή chains that comprise the CD3 molecular
complex. The arrangement of TCR genes resembles that
of genes that encode immunoglobulin heavy chains. The
TCR δ genes are located in the centers of the α genes. V, D,
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γδ TCRs
T cell receptor showing αβ and γδ receptors.
γ (Cγ2 2 ) δ
γ (Cγ2 3 ) δ
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and J segment recombination permits TCR gene diversity.
Rearrangement of a V α segment to a J α segment yields an
intact variable region. Two sets of D, J, and C genes exist at
the β locus. During joining, marked diversity is achieved by
V–J, V–D–D–J, and V–D–J rearrangements. Humans have
eight V γ, three J γ, and an initial C γ gene. Before reaching
Cγ2, there are two more J γ genes. The δ locus contains five
V δ, two D δ, and six J δ genes. TCR gene recombination
occurs via mechanisms that resemble those of B cell genes.
B and T lymphocytes have essentially the same rearrangement
enzymes. TCR genes do not undergo somatic mutation,
which is essential to immunoglobulin diversity.
T cell replacing factor (TRF)
An earlier term for B cell differentiation factor derived
from CD4 + helper T lymphocytes that permits B lymphocytes
to synthesize antibody without the presence of
T lymphocytes.
T cell rosette
E rosette.
T cells
T lymphocytes that mature in the thymus and are responsible
for cell-mediated immunity. Their cell surface
receptor for antigen is designated T cell receptor (TCR).
Cells derived from hematopoietic precursors that migrate
to the thymus, where they undergo differentiation that
continues to completion in the various lymphoid tissues or
during their circulation to and from these sites. T cells are
primarily involved in the control of the immune responses
by providing specific cells capable of helping or suppressing
such responses. They also fulfill other functions related to
cell-mediated immune phenomena. Refer to T lymphocyte.
T cell specificity
Refer to MHC restriction.
T cell system
Mediates self–nonself recognition. Appeared earlier in
evolution than did antibodies that are essentially confined to
vertebrates. Vertebrate T and B lymphocyte responses are
clearly defined. Separate developmental sites have been identified.
The appearance of a true thymus in bony fishes (teleosts),
amphibians, reptiles, birds, and mammals was accompanied
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