| allergy, or allergenic disease, or
hypersensitivity
antigen, foreign substance that, when introduced into the body, is
capable of adhering to lymphocytes, which are the body's
infection-fighting white blood cells. An antigen that induces an immune
response--i.e., stimulates the lymphocytes to produce antibody or
to attack the antigen directly--is called an
immunogen. Virtually any large foreign molecule can act as an antigen,
including those contained in bacteria, viruses, protozoa, helminths,
foods, snake venoms, egg white, serum components, red blood cells, and
other cells and tissues of various species, including humans. On the
surface of the antigens are molecules, called antigenic determinants, that
fit and bind to receptor molecules of complementary structure on the
surface of the lymphocytes. The binding of the lymphocytes' receptors to
the antigens' surface molecules stimulates the lymphocytes to multiply and
to initiate an immune response--including the production of
antibody, the activation of cytotoxic cells, or both--against the
antigen (see
immunity ). The
amount of antibody formed in response to stimulation depends on the kind
and amount of antigen involved, the route of entry to the body, and
individual characteristics of the host. See also
antibody ;
lymphocyte. (see also Index:
antigen-antibody reaction)
Copyright 1994-1999 Encyclopedia Britannica
allergen, substance that in some persons induces the hypersensitive
state of allergy and stimulates the formation of
reaginic
antibodies. Allergens may be naturally occurring or of synthetic
origin and include pollen, mold spores, dust, animal dander, insect
debris, foods, blood serum, and drugs. Identification of allergens is made
by studying both the site of symptoms (e.g., inhalants such as
molds, pollens, and dander usually affect the eyes, nose, and bronchi;
cosmetics often affect the skin of the face and hands) and the time that
symptoms appear (e.g., seasonal allergy to pollen). See also
antigen ;
reagin .
Related Internet Links:
Welcome to BoDD--the Botanical
Dermatology Database
Removing House Dust and Other Allergic Irritants from Your Home
Copyright 1994-1999 Encyclopedia Britannica
lymphocyte, type of leukocyte (white blood cell) that is of
fundamental importance in the immune system. In humans, lymphocytes make
up 20-25 percent of the total number of leukocytes.
The two primary types of lymphocytes are
B
lymphocytes and
T
lymphocytes, or B cells and T cells. Both originate from stem cells in
the bone marrow and travel in the blood to lymphoid tissues, such as the
spleen, tonsils, and lymph nodes. Most T cells are long-lived, their life
span averaging 2 to 4 years, while most B cells are short-lived, with an
average life span of a week to a few months.
Through receptor molecules on their surfaces, lymphocytes are able to
bind to
antigens (foreign substances or microorganisms that the host
recognizes as "nonself") and help remove them from the body. Each
lymphocyte bears receptors that bind to a specific antigen. The ability to
respond to virtually any antigen comes from the fact that the body
contains millions of lymphocytes, with many carrying unique receptors.
Once stimulated by binding to an antigen, such as a bacterium or a
virus, a B cell multiplies into a clone of identical cells. Some of these
cloned B cells differentiate into
plasma cells that produce
antibody molecules. These antibodies are closely modeled after the
receptors of the precursor B cell, and, once released into the blood and
lymph, they bind to the target antigen and initiate its neutralization or
destruction (see
antibody ).
Antibody production continues for several days, until the antigen has been
overcome. Other B cells, called memory cells, are stimulated to multiply
but do not differentiate into plasma cells; they provide the immune system
with long-lasting memory and a heightened sensitivity to a particular type
of antigen should it return to the host.
Before arriving in the lymphoid tissues, T cells mature in the thymus,
where they multiply and differentiate into helper, suppressor, and
cytotoxic T cells. Once stimulated by the appropriate antigen,
helper T cells secrete lymphokines, which stimulate the
differentiation of B cells into plasma cells, thereby promoting antibody
production.
Suppressor T cells act to diminish this reaction, hence their name.
Cytotoxic T cells, which are activated by the leukokine interleukin 2,
bind to and kill infected cells (especially those infected by viruses).
See also
leukocyte .
Copyright 1994-1999 Encyclopedia Britannica
T cell, or T lymphocyte, or thymus-derived cell,
or thymus-derived lymphocyte (cytology)
B cell, or B lymphocyte, or thymus-independent
lymphocyte (biol.)
antigen-antibody reaction, or immune reaction, or
immunological response (biol.)
IgE, or immunoglobulin E (biochem.)
mast cell, type of cell of the immune system of vertebrate animals.
Mast cells mediate inflammatory responses such as hypersensitivity and
allergic reactions. They are scattered throughout the connective tissues
of the body, especially beneath the surface of the skin, near blood
vessels and lymphatic vessels, within nerves, throughout the respiratory
system, and in the digestive and urinary tracts. Mast cells store a number
of different chemical mediators--including histamine, interleukins (IL-4
and IL-5), proteoglycans (e.g., heparin), and various enzymes--in
coarse granules found throughout the cytoplasm of the cell. Upon
stimulation by an allergen, the mast cells release the contents of their
granules (called
degranulation) into the surrounding tissues. The chemical mediators
produce local responses characteristic of an allergic reaction, such as
increased permeability of blood vessels (i.e., inflammation and
swelling), contraction of smooth muscles (e.g., bronchial muscles),
and increased mucus production.
Copyright 1994-1999 Encyclopedia Britannica
anaphylaxis, in immunology, a severe, immediate, often fatal bodily
reaction to contact with a foreign substance, or antigen, to which the
individual has become sensitized. The condition has been extensively
studied in
guinea pigs, a species very liable to anaphylactic shock. The first
injection of an antigen, such as egg albumin, into the body of this animal
is generally harmless. A similar injection two or three weeks later has
quite a different result. Within a few minutes or even seconds after
reinjection, the animal sneezes, scratches its nose, becomes restless, and
has great difficulty in breathing. The guinea pig may then die from
asphyxia owing to its inability, in spite of violent effort, to ventilate
its lungs effectively.
Anaphylaxis occurs, with varying severity, in many animals. Rats are
resistant, rabbits moderately so. In humans it occurs rarely, after such
events as injection of antiserum or antibiotic or after bee or wasp
stings. Symptoms in humans include itching of the scalp and tongue, skin
flush of the whole body, difficulty in breathing because of swelling or
spasm of the bronchi, an abrupt fall in blood pressure, and
unconsciousness. In milder cases, hives may spread over the whole body,
and often there is a severe headache. Treatment, which must begin within a
few minutes of attack, involves the injection of
epinephrine (adrenaline), followed by the administration of
antihistamines, cortisone, or similar drugs.
The mechanism of anaphylaxis is not fully understood but results from
antigen-antibody reactions taking place on the surface of mast cells,
thus damaging them and causing them to suddenly release a combination of
chemicals including histamine and leukotrienes. These in turn produce the
symptoms of anaphylactic shock. Anaphylaxis may occur after contact with
extremely small amounts of antigen and is more common in persons with a
history of atopic dermatitis.
Related Internet Links:
Anaphylaxis
Health Hazards of Imported Fire Ant Stings
desensitization, also called HYPOSENSITIZATION, treatment that
attempts to eliminate
allergic reactions, as of hay fever or bronchial asthma, by a series
of injections in graded strengths of the substance to which the person is
sensitive (e.g., pollen, house dust). Extracts of the material to
be injected are purified and put into an alkaline buffer solution, to
which
epinephrine (adrenaline) is often added to minimize local inflammatory
reactions. The first injections contain little antigen, but, as
desensitization progresses, more and more is added. A three-month program
of injections often suffices for
hay-fever sufferers, but
asthmatics may require a longer program.
Desensitization is successful in about 80 percent of hay-fever
sufferers and up to 90 percent of asthmatics; treatment is more effective
in persons with a few, well-defined allergies than in those allergic to
many substances. The success of desensitization is attributed to special
antibodies, called blocking antibodies, that appear in the serum after
treatment and combine preferentially with allergen. This prevents the
reaction of allergen with allergic antibodies in the skin and precludes an
allergic reaction. Desensitization can also be required when a
penicillin-sensitive person contracts a disease such as bacterial
endocarditis, which is best treated with penicillin. See also
allergy ;
anaphylaxis ;
antibody ;
antigen .
Copyright 1994-1999 Encyclopedia Britannica
complement, in blood chemistry, a complex system of at least 20
proteins, known as complement components, found in the normal blood serum
of humans and other vertebrates. The complement system mediates many
immunological phenomena, including the
lyses
(bursting) of foreign and infected cells, the
phagocytosis (ingestion) of foreign particles and cell debris, and the
inflammation of surrounding tissue.
In the late 19th century complement was identified as one of two
substances in human serum responsible for killing bacteria, the other
substance being
antibody. Complement was so named because it "complemented" the action
of the antibody, meaning it was necessary for cell lyses to occur.
Each complement component is a protein, made up of polypeptides (chains
of amino acids held together by peptide bonds). The immunologic activity
of complement begins when one component binds to an antigen-antibody
complex and begins a chemical chain reaction with other components. In one
process, called complement fixation, activated components pierce the outer
membrane of the bacterium or cell to which the antibody is attached,
effectively lying and killing it. In another process, fragments of
complement released during the reaction attract macrophages, which engulf
and digest the particle. Such fragments can also cause the release of
histamine and attract a flood of phagocytic granulocytes, which contribute
to the swelling and discoloration common to inflammation.
The components of complement differ in life spans and periods of
activity; this factor, as well as the presence of inhibitors in the blood,
helps explain why complement is activated only at a site of microbial
invasion and not throughout the whole body.
Copyright 1994-1999 Encyclopedia Britannica
delayed allergic reaction
type IV hypersensitivity, or delayed allergic reaction,
or delayed hypersensitivity (med.)
Copyright 1994-1999 Encyclopedia Britannica
allergies
As explained earlier, the immune system must be able to recognize and
respond to almost any foreign molecule, since it cannot foretell which
molecules will be characteristic of potentially infective agents and which
will not. Consequently, an immune response can be induced by materials
that have nothing to do with infection, and the mechanisms brought into
play, though beneficial for eliminating microbes, are not necessarily
beneficial when otherwise innocuous substances are concerned. Furthermore,
even initially protective mechanisms can cause secondary disorders when
they operate on too great a scale or for longer than necessary, thereby
damaging tissues remote from the infection. The terms allergy and
hypersensitivity are commonly used to describe inappropriate immune
responses that occur when a subject is reexposed to substances that were
harmless when encountered for the first time. (see also Index:
drug
allergy)
The manifestations of a particular allergic reaction depend upon which
of the immune mechanisms predominates in the response. Based on this
criterion, immunologists recognize four types of hypersensitivity
reactions. Types I, II, and III involve antibody-mediated mechanisms and
are of rapid onset. Type IV, on the other hand, stems from cell-mediated
mechanisms and has a delayed onset. Although this categorization is
useful, it should be noted that an allergy can involve a combination of
hypersensitivity reactions.
Copyright 1994-1999 Encyclopedia Britannica
|