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α helix 2 αβ TCR checkpoint
mucosa and the mesenteric lymph nodes synthesize α
chains alone, usually α-1, with no production of light
chains. Even though the end-terminal sequences are intact,
a sequence stretching from the V region through much of
the Ca-1 domain is deleted. Thus, there is no cysteine residue
to crosslink light chains. α heavy chain disease is more
frequent than the γ type. It has been described in North
Africa, in the Near East and Mediterranean areas, and in
some regions of southern Europe. Rare cases have been
reported in the United States. The condition may prove
fatal, even though remissions may follow antibiotic therapy.
helix
A spiral or coiled structure present in many proteins and
polypeptides. It is defined by intrachain hydrogen bonds
between –CO and –NH groups that hold the polypeptide
chain together in a manner that results in 3.6 amino acid
residues per helical turn. There is a 1.5 Å rise for each
residue. The helix has a pitch of 5.4 Å. The helical backbone
is formed by a peptide group and the α carbon. Hydrogen
bonds link each –CO group to the –NH group of the fourth
residue forward in the chain. The α helix may be left or
right handed. Right-handed α helices are the ones found in
proteins.
2 macroglobulin ( 2M)
A 725-kDa plasma glycoprotein that plays a major role in
inhibition of proteolytic activity generated during various
extracellular processes. α 2M is synthesized in the liver and
reticuloendothelial system. α 2M is produced by lymphocytes
and is found associated with the surface membranes
of a subpopulation of B cells. It has the unique property of
binding all active endopeptidases. Other enzymes or even
the inactive forms of proteinases are not bound. Complexes
of α 2M with proteinase are rapidly cleared from circulation
(in minutes), in contrast to the turnover of α 2M that requires
several days. Some of the roles of α 2M include: (1) regulation
of the extracellular proteolytic activity resulting from
clotting, fibrinolysis, and proteinases of inflammation; and
(2) specific activity against some proteinases of fungal or
bacterial origin. It is elevated significantly in nephrotic syndrome.
Increased levels have also been reported in atopic
dermatitis and ataxia telangiectasia.
1-microglobulin
A 30-kDa protein that belongs to the lipocalin family and
possesses hydrophobic prosthetic groups. It is synthesized
in the liver and is present in the urine and serum. α 1M may
be complexed with monomeric IgA and may be increased
in IgA nephropathy. Elevated serum α 1M in patients with
acquired immune deficiency syndrome (AIDS) may signify
renal pathology. α 1M blocks antigen stimulation and migration
of granulocytes, plays a role in immunoregulation, and
functions as a mitogen.
2-plasmin inhibitor–plasmin complex ( 2PIPC)
Complex formed by the combination of α 2-PI or
α 2-macroglobulin with plasmin, the active principle in
fibrinolysis. These complexes are found in elevated quantities
in plasma of systemic lupus erythematosus (SLE)
patients with vasculitis compared to plasma of SLE patients
without vasculitis.
AA amyloid
A nonimmunoglobulin amyloid fibril of the type seen following
chronic inflammatory diseases such as tuberculosis
and osteomyelitis or, more recently, chronic noninfectious
inflammatory disorders. Kidneys, liver, and spleen are the
most significant areas of amyloid-associated (AA) deposition.
The precursor for AA protein is apo-SAA (serum
amyloid associated), which has a monomer mol wt of 12.5
kDa and is found in the circulation as a 220- to 235-kDa
molecular complex because it is linked to high density lipoproteins.
Interleukin-6 stimulates its synthesis. AA deposition
is either associated with an amyloidogenic isotypical
form of SAA or results from the inability to completely
degrade SAA. Amyloid consists of nonbranching fibrils
7.5 to 10 nm in width and of indefinite length. Chemically,
amyloid occurs in two classes. The AL (amyloid light) chain
type consists of immunoglobulin light chains or parts of
them. The AA type is derived from the SAA protein in
the serum. SAA acts like an acute phase reactant, increasing
greatly during inflammation. Thus, AA protein is the
principal type of amyloid deposited in chronic inflammatory
diseases. AL amyloid consists of either whole immunoglobulin
light chains or their N-terminal fragments, or a
combination of the two. The λ light chain especially gives
rise to AL. AL amyloid protein is often deposited following
or during B cell disorders. Other biochemical forms of amyloid
include transthyretin, β 2 microglobulin, and β 2 amyloid
protein, among others. Amyloid filaments stained with
Congo red exhibit green birefringence with polarized light.
AB blood group
Refer to ABO blood group system.
ABC method
A unique immunoperoxidase procedure for localizing a
variety of histologically significant antigens and other
markers. The procedure employs biotinylated antibody and
a preformed avidin–biotinylated enzyme complex and has
been termed the “ABC” technique. Because avidin has such
an extraordinarily high affinity for biotin, the binding of
avidin to biotin is essentially irreversible. In addition, avidin
has four binding sites for biotin, and most proteins including
enzymes can be conjugated with several molecules of biotin.
These properties allow macromolecular complexes (ABCs)
to be formed between avidin and biotinylated enzymes.
T cell receptor ( TCR)
The structure on both CD4 + and CD8 + T lymphocytes that
recognizes peptide antigen presented in the context of a
major histocompatibility complex (MHC) molecule. The
variable (V) regions of both α and β chains comprise the
antigen binding site. The αβ TCR also contains constant
(C) regions. There is structural homology between TCR V
and C regions and the corresponding regions of an immunoglobulin
molecule. The αβ TCR is the most common form
of T cell receptor, accounting for approximately 95% of T
cells in human blood, with γδ TCR comprising the remaining
5%.
T cells
T lymphocytes that express αβ chain heterodimers on their
surfaces. The vast majority of T cells are of the αβ variety.
Their antigen receptor is comprised of α and β polypeptide
chains. This population, to which most T cells belong,
includes all those that recognize peptide antigen presented
by major histocompatibility complex (MHC) class I and
class II molecules.
TCR checkpoint
The second principal checkpoint in development of T cells.
Positive selection of DP thymocytes that manifest a T cell