Phagocyte: A cell that eats something. The
phagocyte engulfs its
food, which may be another cell - a dead cell in the body, a
bacterium, a cancer cell - or particles that it encounters in the
body.
Complement system: A system of proteins - about 20 - that interacts in the response to invasion of the body by bacteria and fungi. These proteins normally circulate in an inactive state, but are activated by invasion. The complement proteins do -
Inflammation: A series of events that destroys invaders and restores tissues and intracellular conditions to normal, including -
Histamine: A substance released from the
granules of mast cells and
basophils. Histamine delates fine blood vessels, slowing down the
flow of blood in them. It also increases permeability of the vessel
walls, promoting seepage of fluid containing complement proteins
into the surrounding tissue.
Vertebrate immune system: The immune system has
2 functional
divisions - the innate immune system and the adaptive immune system.
Innate immunity acts as a first line of defence against infectious
agents and most potential pathogens are checked before they
establish an overt infection. If these first defences are
insufficient, the adaptive immune system is activated and produces a
specific reaction to each infectious agent. The infectious agent is
normally wiped out by the specific reaction. The adaptive immune
system also remembers the infectious agent and can prevent it
causing disease later. Thus, the vertebrate immune system is
characterized by specificity and memory.
Barriers to invasion: Various physical, chemical, and ecological barriers to the invasion of the body by foreign organisms-
Cell-mediated immune response: The specific
response carried out by
T cells.
Antibody-mediated response: The specific
response carried out by B
cells.
Macrophages: Phagocytic cells that engulf
invading microbes. In the
process of engulfing the invader, the macrophage moves the antigen
from the invader to its own surface. The presentation of the foreign
antigen with the macrophage's own MHC marker triggers helper T cells
to stiumlate B cells to initiate their part of the immune response.
Killer T cells: White blood cells which can
recognize cell surface
changes that occur on some virally infected cells and some tumor
cells. They attach to these target cells and kill them.
Chemotaxis: The process by which phagocytes are
attracted to sites
of inflammation. The phagocytes will actively migrate up a
concentration gradient of certain molecules. A fragment (C5a) of one
of the complement components is particularly active.
Phagocytosis: The process by which phagocytes
engulf particles.
After the phagocyte attaches to the particle, it extends pseudopodia
around the particle. The pseudopodia fuse and the particle in
surrounded and internalized in a phagosome. Lysosomes fuse with the
phagosome and destroy the trapped particle.
Once phagocytes arrive at the site of inflammation, they have to
attach (=recognize) the infectious agent before engulfing it. They
have receptors on their surface which allow them to attach
non-specifically to a variety of microorganisms. However, the
attachment is greatly enhanced if the microorganism has been
identified by the deposition of a component (C3b) of complement.
Component activation at the site of infection causes C3b to be
deposited on the infectious agent. Since both neutrophils and
macrophages have receptors which specifically bind to C3b, this
allows the phagocytes to recognize their targets.
Antibody: A class of molecules, in the shape of
a "Y", produced by B
cells. The tail end of the antibody is constant, and binds to the
phagocyte. The constant part also activates complement. The other
end of the antibody binds to only one type of infectious agent.
When the invading microorganism does not match the receptor on a
phagocyte or does not activate complement, the antibody acts as a
flexible adaptor that can attach at one end to the microorganism and
at the other to the phagocyte.
Major histocompatibility complex: Genes that
specify proteins that
project from the surface of cells of the body. Parts of these
projecting proteins act as markers (MHC markers), by which the body
recognizes the cells as belonging to itself.
Lysosome: An intracellular vesicle that
contains powerful digestive
enzymes. The lysosome fuses with the phagosome containing an
engulfed microorganism. This fusion releases the enzymes into the
vicinity of the microorgansim, protecting the body, but destroying
the invader.
Plasma cell: A B cell which has been stimulated
to differentiate
into a cell which mass-produces antibodies.
Suppressor T cells: Cells that secrete a
substance to reduce the
immune response, when exposed antigens decrease - meaning that the
infection is being brought under control.
Retrovirus: A virus that contains its genetic
information in RNA.
Once inside a host, the viral RNA is used as a template for making
DNA, which is then inserted into a host chromosome, taking over
control of the host cell.
CD4 antigen: An MHC marker found primarily on
helper T cells. It is
this antigen to which HIV binds when it infects the helper T cells.
AIDS: Acquired immune deficiency syndrome. The
immune system is
weakened, which leaves the body open to all sorts of infections,
some of which are extremely rare in noninfected individuals.
Monoclonal antibody: An antibody that binds
only to a single,
specific molecular target. That target may be a sequence of amino
acids on the surface of a cell - the target cell. These antibodies
have been derived from a single clone of cells, and so are
homogeneous.
Syncytium: A cytoplasm containing more than one
nucleus. The various
nuclei are not separated from each other by cell membranes, but
coexist together within the same cell membrane. Syncytia form when
HIV-infected T cells fuse with one another.
Lymphocytes: White blood cells including T
cells, B cells,
macrophages, neutrophils, eosinophils, basophils.
HIV: Human immunodeficiency virus. The virus
infects helper T cells,
destroying a critical trigger of the human immune system. The immune
system loses much of its ability to respond to infection.
Glycoprotein: A protein with attached sugar
chains. The markers on
the surface of HIV are glycoproteins.
Fate of integrated HIV: Once the HIV has entered the T cell and its genetic information incorporated into the DNA of the T cell, several results may occur -
Epitope: A molecular shape recognized by
antibodies and molecules of
the adaptive immune system. In other words, an epitope is that part
of the antigen that stimulates the immune response. An antigen may
be relatively large, containing several different antigenic
sequences (epitopes) or the epitope may be repeated numerous times
on the antigen.
GP120: A glycoprotein in the envelope of the
HIV particle, that
binds to the CD4 molecule on the surface of host cells. It is
anchored beneath to GP41.
GP41: A glycoprotein embedded in the lipid
membrane of the virus
particle, attaching above to the GP120 knobs. It is speculated that
GP41 may be uncovered when GP120 binds to the CD4 marker on the T
cell, then facilitating the fusion of the virus particle with the T
cell.
Lymphokines: Polypeptides secreted by both T and B cells that are involved in signalling between cells of the immune system. Lymphokines include both interleukins and interferons. They
Cytokines: A generic term for soluble molecules
which mediate
interactions between cells.
Antigen-presenting cells: A variety of cell
types which carry
antigen in a form that can stimulate lymphocytes. One example is
macrophage.
Antigen: A chemical marker on the surface of a
cell that triggers a
reaction by the immune system. When the immune system detects an
antigen (a marker that identifies the cell as "foreign" or
"not-self"), the body reacts to eliminate the foreign cell.
Primary immune response: The actions taken by
the immune system the
first time a particular antigen is encountered. Full mobilization of
the body's defenses usually takes 5-6 days. Some of the T and B
cells involved in this response continue to circulate in the body as
memory cells.
Secondary immune response: Memory cells
recognize an antigen that
has previously triggered the immune system. These cells are
stimulated to divide at once. A full mobilization of the body's
defenses occurs in 2-3 days, and is greater and of longer duration
than the primary immune response.
Allergy: A secondary immune response to a
normally harmless
substance, such as pollen or dust. The sensitized individual
produces IgE antibodies that become atttached to cells that release
histamine and prostaglandin. The result includes a drippy nose,
sneezing, congestion, and labored breathing.
Autoimmune response: The immune system becomes
sensitized against
its own cells, eventually destroying these cells. An example is
rheumatoid arthritis, in which the joints are destroyed.
AZT: Azidothymidine, an analog of thymidine.
When reverse
transcriptase attempts to reconstitute the HIV DNA from its RNA
genetic code, the AZT blocks further linkage of nucleotides,
inhibiting virus production.
Passive immunity: Immunity to a disease
conferred by antibodies
produced by another body and obtained by injection or through the
placenta.
Clonal selection theory: When antigen combines
with the receptor of
a lymphocyte, the lymphocyte multiplies rapidly producing a clone of
cells that are specific to that antigen only - the antigen that
"selected" them.
Interferons: A group of small proteins released
by cells following a
viral attack. The interferon binds to the plasma membrane of other
cells in the body and induces resistance to a wide range of viruses.
Immunoglobulins: Another name of antibody. They occur in several classes -
Diabetes: A disease resulting from the absence
of insulin in the
body. Cells do not remove glucose from the blood - glucose accumu-
lates in the blood at the same time that the cells are starved.
Excess glucose in the blood is removed by the kidneys, resulting in
large quantities of sweet urine. The individual becomes excessively
thirsty and hungry. Diabetes mellitus (insulin-dependent or Type 1)
is caused by an autoimmune response in which the beta cells of the
pancreas become inflamed and are destroyed by the individual's own
immune system.
Insulin: A hormone produced by the beta cells
of the pancreas. This
hormone promotes cellular uptake of glucose from the blood, helping
to provide energy for the cells.
Molecular mimicry: A situation in which a
chemical substance from
outside the body (an antigen that may have come from a virus or a
bacterium) is so similar to one of the body's proteins that the
immune system cannot tell them apart. Thus, when the immune system
becomes sensitized to the foreign antigen and destroys it,
antibodies and killer cells also destroy those cells in the body
with the similar protein. An example is insulin-dependent diabetes.
Immunosuppresive agent: Drugs, such as
cyclosporine, steroids, and
azathioprine, which suppress the action of the immune system. Such
drugs may reduce the overall population of T cells or inhibit their
activiation. A potentially dangerous side effect of these drugs is
that the body becomes more susceptible to the risk of infection and
cancer.
Animal model: An animal used in scientific
tests to take the place
of human beings. Examples are the BB rat and the NOD (nonobese
diabetic) mouse used in the search for understanding and cure for
insulin-dependent diabetes. Another is the SKID mouse, which does
not produce an immune reaction to human blood. The SKID mouse is
being used to find a cure for AIDS.
Agammaglobulinemia: A failure of cells that
make antibodies to
differentiate. The disease results from a mutation and is recessive.
Innate immune system: The part of the immune
system that, at birth,
recognizes and destroys invading microbes.
Self: The cells of your own body. On the
surface of each of your
cells are molecules that uniquely identify them as part of your
body. The cells of your immune system "learn" to recognize self and
do not destroy your cells.
Interleukin: A substance produced by one cell
that stimulates a
response in another cell. In particular, interleukins released by
cells of the immune system help coordinate the attack on invaders.
Mannose: A sugar that is very common on some
bacteria.
Adaptive immunity: An immune response that
develops after exposure
to an antigen, involving both B cells and T cells. The response is
specific and more rapid in subsequent exposures to the same antigen.
Hypermutations: Slight variations in antibody
structure that result
from the development of the B cells responding to antigens.
Tolerance: During their passage through the
thymus gland, T cells
that react to self are destroyed. Thus, the surviving cells of the
immune system tolerate the cells of the body.
Transporter molecules: MHC molecules that take
a peptide to the
surface membrane of a cell and then display it.
Inflammatory T cell: A T cell that activates a
macrophage.
Helper T cell: A T cell that activates a B cell
to produce antibody.
Kinase: An enzyme that adds a phosphate group
to a substrate and, in so doing, converts the binding of antigen and cell
membrane into activating the cell's DNA to respond.
Ligand: A molecule that binds to another molecule,
often a small molecule that binds to a specific target molecule. Examples are
an antigen binding to an antibody or a hormone, like insulin, binding to a
cell surface receptor.
Last updated on February 17, 2004
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