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viral immunity 743 vitamin A and immunity
viral immunity
Congenital immunodeficiency patients have demonstrated
the relative significance of various constituents of the
immune system. Subjects with isolated defects of cellmediated
immunity contract severe and often fatal viral
infections that include measles and chickenpox. By contrast,
individuals with isolated immunoglobulin deficiencies
usually recover normally from most viral infections except
enteroviruses that may lead to chronic infection of the
central nervous system. Certain generalizations may be
reached concerning viral immunity: antibodies act mainly
by neutralizing virions, rendering them noninfectious. By
contrast, cell-mediated immunity is against virus antigens
present in infected cells. Antibody prevents primary infection
and reinfection through neutralization of viruses on
mucosal surfaces and limiting their spread in body fluids;
cell-mediated immunity eliminates intracellular infection
and limits reactivation of persistent viruses. The immune
system may be considered in three phases: immediate (96 hours). It may
also be divided into humoral and cell-mediated components
that include both specific and nonspecific mechanisms.
viral interference
Resistance of cells infected with one virus to infection by a
second virus.
Viramune ®
A non-nucleoside reverse transcriptase inhibitor approved
by the FDA to treat human immunodeficiency virus (HIV).
Nevirapine, the active component, binds directly to reverse
transcriptase and blocks the RNA- and DNA-dependent
polymerase activities by causing disruption of the catalytic
site of the enzyme. Nevirapine has no inhibitory effect on
HIV-2 reverse transcriptase.
virgin B cells
Virgin B cells have never interacted with antigen and
must have two separate types of signals to proliferate and
differentiate. The antigen provides the first signal through
interaction with surface membrane immunoglobulin (Ig)
molecules on specific B lymphocytes. Helper T cells and
their lymphokines provide the required second type of
signal. Polysaccharides and lipids, as nonprotein antigens,
induce IgM antibody responses without antigen-specific
T-cell help, while protein antigens that are helper T celldependent
lead to the production of immunoglobulin of
more than one isotype and of high affinity in addition to
immunologic memory.
virion
Complete virus particle.
viroid
A 100-kDa, 300-bp subviral infectious RNA particle composed
of a circular single-stranded RNA segment that induces
disease in certain plants. Viroids may be escaped introns.
viropathic
Host tissue injury resulting from infection by a pathogenic
virus.
virosome
A vaccine comprised of a spherical artificial virus
employed to direct vaccine antigens directly into a host cell.
It consists of a liposome containing the vaccine antigen and
hemagglutinin and neuraminidase proteins of influenza. It
has membrane fusion properties, conformational stability,
and ability to invade host cells of the native virus. The
antigen is present in the virosome lumen or chemically
cross linked to its surface.
virulence
The pathogenicity of a microorganism to invade host tissues
as indicated by the severity of the disease it causes.
virulence genes
Genes that govern the expression of multiple other genes with
changing environmental conditions such as temperature, pH,
osmolarity, etc. An example is the toxR gene of Vibrio cholerae
that coordinates 14 other genes of this microorganism.
virus
An infectious agent that ranges from 10 6 D for the smallest
viruses to 200 × 10 6 for larger ones such as the pox viruses.
Viruses contain single- or double-stranded DNA or RNA
that may be circular or open and linear. The nucleic acid is
enclosed by a protein coat (capsid) composed of a few characteristic
proteins. Most viruses are helical or icosahedral
and may include a lipid envelope containing viral proteins.
Viruses may be incubated with cells in culture, where they
produce characteristic cytopathic effects. Inclusion bodies
may be produced in cells infected by viruses, which infect
host cells through specific receptors. Examples of this specificity
include cytomegalovirus linking to β 2 microglobulin,
Epstein–Barr virus (EBV) linking to C3d receptor (CR2),
and HIV-1 binding to CD4. These submicroscopic acellular
pathogens are composed of either a DNA or RNA nucleic
acid genome enclosed in a protein coat. To replicate, the
virus must invade a host cell and use its protein synthesis
machinery that a virus lacks but is required for independent
life. Also called a virion.
virus-associated hemophagocytic syndrome
An aggressive hemophagocytic state occurring in both
immunocompromised and nonimmunocompromised
individuals, usually those with herpetic infections such as
cytomegalovirus (CMV), Epstein–Barr virus (EBV), and
herpesvirus. It may also occur in adenovirus and rubella
infections, brucellosis, candidiasis, leishmaniasis, tuberculosis,
and salmonellosis. Lymphadenopathy, hepatosplenomegaly,
pulmonary infiltration, skin rash, and pancytopenia
are present. The disease is sometimes confused with
malignant histiocytosis, lymphoma, sinus histiocytosis, and
lymphomatoid granulomatosis.
virus neutralization test
A test based on the ability of a specific antibody to neutralize
virus infectivity. It may be employed to measure the titer of
antiviral antibody. It may be performed in vivo using susceptible
animals or chick embryos or in vitro in tissue culture.
virus neutralizing capacity
The ability of serum to prevent virus infectivity. Neutralizing
antibody is usually of the IgM, IgG, or IgA class.
viscosity
The physical consistency of a fluid such as serum based
on the size, shape, and conformation of its molecules.
Molecular charge, sensitivity to temperature, and hydrostatic
state affect viscosity.
vitamin A
Vitamin A may serve as an adjuvant to elevate antibody
responses to soluble protein antigens in mice. The adjuvant
effect is produced whether administered orally or parenterally.
vitamin A and immunity
A deficiency of vitamin A compromises acquired, adaptive,
antigen-specific immunity. The deficiency has been linked
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