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agretope 26 AIDS belt
exposure to cytotoxic drugs such as nitrogen mustard or follow
administration of the antibiotic chloramphenicol.
agretope
The region of a protein antigen that combines with a major
histocompatibility complex (MHC) class II molecule during
antigen presentation. This is then recognized by the T cell
receptor MHC class II complex. Amino acid sequences differ
in their reactivity with MHC class II molecules.
agrin
An aggregating protein crucial for formation of the neuromuscular
junction. It is also expressed in lymphocytes and is
important in reorganization of membrane lipid microdomains
in setting the threshold for T cell signaling. T cell activation
depends on a primary signal delivered through the T
cell receptor and a secondary costimulatory signal mediated
by coreceptors. Costimulation is believed to act through the
specific redistribution and clustering of membrane and intracellular
kinase-ridge lipid raft microdomains at the contact sites
between T cells and antigen-presenting cells. This site is known
as the immunological synapse. Endogenous mediators of raft
clustering in lymphocytes are essential for T cell activation.
Agrin induces the aggregation of signaling proteins and the
creation of signaling domains in both immune and nervous
systems through a common lipid raft pathway.
AH50
An assay to measure the functional activity of the alternative
complement pathway. This alternative pathway (AP) hemolytic
assay (AH50) employs complement-mediated lysis of rabbit
red blood cells in Mg 2+- EGTA buffer. The EGTA chelates the
Ca 2+ needed for complement C1 macromolecular assembly,
thereby blocking classical complement pathway activity. The
AH50 assay is valuable to screen for homozygous deficiencies
of AP constituents including C3, factor I, factor B, properdin,
factor H, and factor D. Deficiencies of the fluid phase regulatory
proteins of the AP, factor I, or factor H induce a secondary
deficiency of C3, properdin, and factor B as a result of uncontrolled
consumption of these proteins. The lack of hemolysis in
both the AH50 and CH50 assays strongly suggests a deficiency
in a complement terminal pathway component (C5–C9).
When the AH50 values are normal and there is no lysis in
CH50, a classical pathway component deficiency (C1, C2, or
C4) occurs. A normal CH50 and a lack of hemolysis in AH50
firmly suggest an AP component deficiency. A lack of lysis
strongly points to homozygous deficiency. Low levels of lysis
in AH50 or CH50 assays suggest either heterozygous deficiency
or complement activation. Hemolytic functional assays
for individual complement components facilitate the detection
of homozygous and some heterozygous deficiencies in addition
to mutations that lead to inactive complement protein. Usually,
AP deficiency is associated with recurrent bacterial infections.
Homozygous factor H deficiency has been observed in
patients with glomerulonephritis, recurrent pyogenic infection,
hemolytic–uremic syndrome, or systemic lupus erythematosus
and in healthy subjects. Factor D deficiency has been linked to
recurrent upper respiratory infections with Neisseria species.
AICD
Abbreviation for activation-induced cell death.
AIDS (acquired immune deficiency syndrome)
A disease induced by the human immunodeficiency retrovirus
(designated HIV-1). The two genetically different but
antigenically similar HIV viruses are designated HIV-1 and
HIV-2. HIV-1 is more common in the United States, Europe
and central Africa, whereas HIV-2 produces a similar
disease mainly in west Africa. Although first observed
in homosexual men, the disease affects both males and
females equally in central Africa and is now affecting a
greater number of heterosexuals, with cases reported in
both males and females in western countries, including
North America and Europe. Viral entry into a cell requires
CD4 and coreceptors that are chemokine receptors. This
involves binding of viral gp120 and fusion with the cell via
viral gp41 protein. The principal cellular targets include
CD4 + helper T cells, macrophages, and dendritic cells.
The provirus genome integrates into host cell DNA. Virus
expression is initiated by stimuli that activate infected cells.
Acute infection is associated with viremia with dissemination
of the virus. There is latent infection of lymphoid tissue
cells with continuing viral replication and progressive loss
of CD4 + T cells. HIV is transmitted by blood and body fluids
but is not transmitted through casual contact or through
air, food, insect bites, or other means. Besides homosexual
and bisexual males, others at high risk include intravenous
drug abusers, recipients of blood transfusions, and hemophiliacs
(rare since 1985), the offspring of HIV infected
mothers, and sexual partners of HIV-infected individuals
in the above groups. The two most important laboratory
parameters in following an HIV infected patient include
the absolute CD4 lymphocyte count, which gives information
concerning the patient’s immune status at that time,
and viral load, i.e., the number of viral transcripts, which
indicates disease progression. Clinically, HIV infection is
marked by various opportunistic infections, neoplasms, and
CNS involvement. Clinically there are three phases including:
(1) early phase; (2) middle chronic phase; (3) final crisis
phase. The acute phase is marked by nonspecific symptoms
such as sore throat, myalgia, fever, rash and, occasionally,
aseptic meningitis. There are high levels of virus production
during this phase, which induces a virus-specific
immune response associated with seroconversion within 3
to 17 weeks of exposure and development of virus-specific
CD8 + cytotoxic T lymphocytes. During the middle chronic
phase, which may last for several years, there is continued
HIV replication but relative containment of the virus. Virus
replication continues in the lymphoid tissues during this
phase; CD4 + T cells continue to be lost; and HIV levels
increase. Without treatment, most HIV infected patients
develop AIDS after a chronic phase ranging from 7 to 10
years. The final crisis phase is characterized by a profound
breach of host defenses characterized by increased viremia
and clinical illness. Patients often present with fever,
fatigue, weight loss and diarrhea, and diminished CD4
T cell counts ≤200/μl and opportunistic infections. Great
strides have been made in HIV therapy including cocktails
that may be altered to account for antigenic variation of the
virus. Atripla represents a remarkable improvement in
reducing the previously numerous medications required for
HIV therapy to one dose of medication per day. Attempts to
develop an HIV vaccine remain elusive, however.
AIDS belt
A geographic area across central Africa that describes a
region where multiple cases of heterosexual AIDS related to
sexual promiscuity have been reported. Nations in this belt
include Burundi, Kenya, Central African Republic, Rwanda,
the Congo, Malawi, Zambia, Tanzania, and Uganda.