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terminal deoxynucleotidyl transferase 689 tertiary reaction
terminal deoxynucleotidyl transferase
Terminal deoxynucleotidyl transferase (TdT) is an enzyme
catalyzing the attachment of mononucleotides to the 3′ terminus
of DNA. It thus acts as a DNA polymerase. It is present
in immature B and T lymphocytes but not demonstrable
in mature lymphocytes. TdT is present both in the nuclear
and soluble fractions of thymus and bone marrow. It can also
incorporate ribonucleotides into DNA. In mice, two forms of
TdT can be separated from a preparation of thymocytes and
are designated peak I and peak II. They have similar enzymatic
activities and appear to be serologically related but
display significant differences in biologic properties. Peak
I appears constant in various strains of mice and at various
ages, while peak II varies greatly. In some strains, peak II
remains constant up to 6 to 8 months of age; in others, it
declines immediately after birth. Eighty percent of bone
marrow TdT is associated with a particular fraction of bone
marrow cells separated on a discontinuous bovine serum
albumin (BSA) gradient. This fraction represents 1 to 5%
of the total marrow cells but is O antigen-negative. These
cells become O-positive after treatment with the thymopoietin
thymic hormone, suggesting that they are precursors of
thymocytes. Thymectomy is associated with rapid loss of
peak II and a slower loss of peak I in this bone marrow cell
fraction. TdT is detectable in T cell leukemia, 90% of common
acute lymphoblastic leukemia cases, and half of acute
undifferentiated leukemia cells. Approximately one third
of chronic myeloid leukemia cells in blast crisis and a few
cases of pre-B cell acute lymphoblastic leukemia show cells
that are positive for TdT. This marker is rarely seen in cases
of chronic lymphocytic leukemia. In blast crisis, some cells
may simultaneously express lymphoid and myeloid markers.
Indirect immunofluorescence procedures can demonstrate
TdT in immature B and T lymphocytes. It inserts nontemplated
nucleotides (N nucleotides) into the junctions between
gene segments during T cell receptor and immunoglobulin
heavy-chain gene rearrangement.
terminal transferase
Refer to DNA nucleotidyl exotransferase.
termination of tolerance
The unresponsive states of several forms of tolerance may
be terminated by appropriate experimental manipulation.
(1) Tolerance to heterologous γ globulin can be terminated
by normal thymus cells; however, this is only possible in
adoptive transfer experiments with cells of tolerant animals
81 days after the induction of tolerance and after supplementation
with normal thymus cells. By this time, B cell
tolerance vanishes, and only the T cells remain tolerant.
Similar experiments at an earlier date do not terminate tolerance.
(2) Allogeneic cells injected when B cell tolerance
has vanished or has not yet been induced can also terminate
or prevent tolerance. The mechanism is not specific and
involves the allogeneic effect factor with activation of the
unresponsive T cell population. (3) Lipopolysaccharide
(LPS), a polyclonal B cell activator, can terminate tolerance
if the B cells are competent. LPS can bypass the requirements
for T cells in response to the immunogen by providing
the second (mitogenic) signal required for response. The
termination of tolerance by LPS does not involve T cells.
LPS may also circumvent tolerance to self by a similar
mechanism. (4) Cross-reacting immunogens (a heterologous
protein in aggregated form or a different heterologous
protein) also are capable of terminating tolerance to a soluble
form of a protein. Termination occurs by a mechanism
that bypasses the unresponsive T cells and is obtainable at
time intervals after tolerization when the responsiveness of
B cells is restored. The antibody produced to the crossreacting
antigen also reacts with the tolerogenic protein and
is indistinguishable from the specificity produced by this
protein in the absence of tolerance.
tertiary granule
A structure in the cytoplasm of polymorphonuclear neutrophils
(PMNs) in which complement receptor 3 precursor,
acid hydrolase, and gelatinase are located.
Serum Antibody Level
1st Contact
with antigen
Primary
response
2nd Contact
with antigen
Secondary
response
Tertiary immune response.
3rd Contact
with antigen
Tertiary
response
tertiary immune response
An immune response induced by a third (second booster)
administration of antigen. It closely resembles the secondary
(or booster) response.
tertiary immunization
The immune response following the third injection of the
same immunogen.
tertiary lymphoid tissue
Inflamed peripheral tissues in which postcapillary venules
have been modified to allow ingress of leukocytes including
effector and memory lymphocytes.
tertiary reaction
May result from either primary or secondary interactions of
antibody with antigen including in vivo biological manifestations
of antibody reactivity. Some in vitro secondary interactions
such as cytophilic reactions (adherence of antibody via
its Fc to a cell surface) may, when occurring in vivo, give rise
to tertiary manifestations. Because reactions occur in vivo,
they tend to be very complex and subject to many variables.
In an immune response, antibodies are directed against
specific confirmational areas on antigen molecules known
as antigenic determinants. Antigens are macromolecules
that stimulate antibody production. Antibody populations
directed against these macromolecules are notoriously
heterogenous with respect to their antibody specificity and
affinity because antibodies to different antigenic determinants
may be present simultaneously in the sera. Bivalent
and multivalent antibodies directed against multideterminant
antigens result in the formation of large antibody–antigen
aggregates of type (Ab) x(Ag) y varying in size, complexity,
and solubility. Landsteiner devised a method whereby an
immune response could be directed against small molecules
of known structure and called them haptens. They were too
small to initiate an immune response alone, but were capable
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