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ALL 29 allergic conjunctivitis
of single polypeptide chains containing 500 amino
acid residues and are membrane bound. Calf intestine
alkaline phosphatase is used in immunology in ELISA
assays, immunoblotting, and molecular cloning. AP
deletes 5′-phosphate from linear DNA terminals. It is
also used in DNA recombinants to inhibit recircularization
that diminishes background for DNA. It is also
used to treat genomic DNA fragments to prevent two or
more fragments from ligating to each other. It may be
employed to prepare 5′-32P-N-labeled DNA.
ALL
Abbreviation for acute lymphoblastic leukemia.
allele
One of the alternative forms of a gene at a single locus on
a chromosome that encodes the phenotypic features of a
certain inherited characteristic. The presence of multiple
alleles, such as at the MHC locus, leads to polymorphism.
allelic dropout
In the amplification of a DNA segment by the polymerase
chain reaction, one of the alleles may not be amplified,
leading to the false impression that the allele is absent. The
phenomenon takes place at 82 to 90°C in the thermocycler.
VH D JH V H
Vκ
D JH Vκ Jκ
Jκ
Allelic exclusion.
allelic exclusion
Only one of two genes for which the animal is heterozygous
is expressed, whereas the remaining gene is not.
Immunoglobulin genes manifest this phenomenon. Allelic
exclusion accounts for the ability of a B cell to express only
one immunoglobulin or the capacity of a T cell to express
a T cell receptor of a single specificity. Investigations of
allotypes in rabbits established that individual immunoglobulin
molecules have identical heavy chains and light
chains. Immunoglobulin-synthesizing cells produce only
a single class of heavy chain and one type of light chain at
a time. Thus, by allelic exclusion, a cell that is synthesizing
antibody expresses only one of two alleles encoding an
immunoglobulin chain at a particular locus. The synthesis
of a functional μ heavy chain from the IgH locus on one
chromosome blocks more V(D)J recombination and μ chain
synthesis from the other IgH allele.
allelic exclusion (TCR locus)
The synthesis of a functional TCR β chain from one chromosome’s
TCR β locus prevents more V(D)J recombination and
synthesis of TCR β chains from the other TCR β allele and
V(D)J recombination in the TCR γ locus on both chromosomes.
allelic variant
It explains human leukocyte antigen (HLA) associations with
rheumatoid arthritis, type I diabetes, multiple sclerosis, and
κ
H
H
κ
celiac disease. A minimum of 6 α and 8 β genes occur in distinct
clusters, termed HLA-DR, DQ, and DP, within the HLA
class II genes. DO and DN class II genes are related but map
outside DR, DQ, and DP regions. The two types of dimers
along the HLA cell-surface HLA-DR class II molecules are
made up of either DRα-polypeptide associated with DRβ 1polypeptide
or DR with DRβ 2-polypeptide. Structural variation
in class II gene products is linked to functional features
of immune recognition leading to individual variations in
histocompatibility, immune recognition, and susceptibility to
disease. The two types of structural variations appear among
DP, DQ, and DR products in primary amino acid sequence by
as much as 35% and individual variations attributable to different
alleleic forms of class II genes. The class II polypeptide
chain possesses domains that are specific structural subunits
containing variable sequences that distinguish among class II
α genes or class II β genes. It has been suggested that these
allelic variation sites form epitopes that represent individual
structural differences in immune recognition.
allergen
An antigen that induces an allergic or hypersensitivity
response in contrast to a classic immune response produced
by the recipient host in response to most immunogens.
Allergens include such environmental substances as pollens
(i.e., their globular proteins) from trees, grasses, and
ragweed, certain food substances, animal danders, and insect
venom. Selected subjects are predisposed to synthesize IgE
antibodies in response to allergens and are said to be atopic.
The crosslinking of IgE molecules anchored to the surfaces
of mast cells or basophils through their Fc regions results in
the release of histamine and other pharmacological mediators
of immediate hypersensitivity from mast cells or basophils.
allergen immunotherapy
Desensitization treatment. Refer to desensitization.
allergenic extracts
Substances derived individually from various biological
sources containing antigens that demonstrate immunologic
activity. They are classified based on standardization and
dose form. Standardization systems include: (1) biological
activity expressed in allergenic units (AUs); (2) weight-tovolume
(w/v); and (3) protein nitrogen units (PNUs). Dose
forms include aqueous, glycerinated, and alum-precipitated.
The mechanism of action is ill-defined. Specific IgG
appears in the serum after inoculation of allergenic extracts.
IgG competes with specific IgE for a specific antigen.
After binding to receptors on mast cell membranes, IgE
produces an allergenic reaction by releasing histamine and
other pharmacological mediators upon uniting with an
antigen. Serum IgE levels diminish over time. Decreased
leukocyte sensitivity to allergens and increased numbers
of T-suppressor cells for IgE-producing plasma cells are
also observed. Hyposensitization may diminish the release
of pharmacological mediators such as histamine in the
responses of basophils to a specific allergen.
allergic alveolitis
Refer to farmer’s lung.
allergic asthma
Bronchial constriction that is a consequence of an allergic
reaction to an inhaled antigen.
allergic conjunctivitis
A hypersensitivity reaction in the conjunctiva of the eye,
often induced by airborne antigens.
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