AB0 blood type system: The antigens of the AB0 blood type system
are found on the surface of the red blood cells. They are used to type
human blood for transfusions. Every human being normally forms antibodies
against the A or B blood type antigens if he does not have them on his
own blood cells.
Active immunization: An active immunization
is performed with antigens. In a passive immunization, on the other
hand, the person is given antibodies against a pathogen.
Acquired immunity: The reaction of antigen-specific lymphocytes
to an antigen is called an acquired immune response. It includes the
formation of an immunological memory. Acquired (adaptive) immune responses
come about through the selection of certain lymphocyte clones. The adaptive
immunity is distinct from congenital and non-adaptive immunity, which
are not based on the selection of antigen-specific lymphocyte clones.
Adrenalin: Hormone formed naturally
in the adrenal glands. It is administered in synthetic form as a medication.
Adrenalin causes the blood vessels to contract and raises blood pressure.
Agglutination is the clumping together of single particles,
usually via antibody molecules which bind to antigens on the surface
of the particles. If the particles are red blood cells, it is called
haemagglutination.
AIDS (Acquired Immune Deficiency
Syndrome): The acquired immune deficiency syndrome (AIDS) is
caused by the human immunodeficiency virus (HIV). The symptoms appear
when a patient has lost most of his CD4 T cells. Infections from opportunistic
germs then appear.
Alleles are variants of the same gene.
Allergens are antigens which cause
an hypersensitive or allergic reaction.
Allergic asthma is a contraction of the bronchial tubes as a
consequence of an allergic reaction to a breathed-in antigen.
Allergic reaction: An allergic reaction
is an immune response to harmless environmental antigens or allergens,
caused by already existing antibodies or T cells. Usually an allergen
binds to IgE antibodies on mastocytes, as is the case with asthma, hay
fever and other common allergies.
Allergies are symptomatic reactions to environmental antigens.
They result from the interaction between the antigen and antibodies
or T cells which formed during a previous contact with the same antigen.
Anaphylactic shock is an allergic reaction
to antigens which causes a collapse of the circulatory system along
with a swelling of the trachea and the danger of suffocation. It is
triggered by the binding of antigens to IgE antibodies on mastocytes
in connective tissues in many body regions and leads to numerous inflammation
sites throughout the body.
Antibiotics are growth-inhibiting substances produced by bacteria
and fungi and other microorganisms. Penicillin, derived from the mould
Penicillium notatum, is the best-known antibiotic.
Antibodies
are plasma proteins which bind specifically to antigens. They are the
key molecules of the humoral immune response. They come about as a reaction
to an immunization with the appropriate antigens and bind selectively
to foreign organisms, neutralizing them or preparing them for destruction
by phagocytes. Every antibody molecule has a unique structure which
enables it to recognize a specific antigen. However, the overall structure
is the same for all antibodies. For this reason, they are also all grouped
together under the term immunoglobulins.
Antigens
are molecules which react with antibodies. Their name comes from their
ability to trigger the formation of antibodies.
Antigen-antibody complexes are groups of antigen and
antibody molecules which are bound to each other. They range in size
from small, soluble complexes to large, insoluble ones.
Antigen binding point: The antigen binding point of an antibody
consists of six hypervariable loops; three of them belong to the variable
region of the light chains, three to the variable region of the heavy
chains.
Antigen display means the displaying of antigens
in the form of peptide fragments which are bound to MHC molecules on
the cell surface. T cells recognize antigens only in this form.
Antigen displaying cells are highly
specialized. They can break down protein antigens and display the peptide
fragments, together with molecules required for the activation of lymphocytes,
on their surface. The most important cells which display antigens to
the T cells are dendritic cells, macrophages and B cells. The most important
antigen displaying cells for B cells, on the other hand, are the follicular
dendritic cells.
Antigen drift is a situation in which isolated
mutations lead to slight changes in the structure of the viral surface
antigens. Influenza viruses change their antigen profile from year to
year with the help of this mechanism. See also Antigenshift.
Antigen shift refers to the exchange
of individual genome segments between two viruses. In this way, influenza
viruses can completely change their surface antigens from time to time.
The immune system does not recognize the new virus strain, even if the
person was previous immune to influenza. This is why worldwide flu epidemics,
some of them very serious, occur at certain intervals.
Apoptosis (programmed cellular death) is the situation
in which the cell activates an internal destruction program. It is characterized
by the breakdown of the nuclear DNA and the phagocytosis of cell residues.
Apoptosis is frequently a natural part of the development of profiling
cells. This is especially the case for lymphocytes. They frequently
die before maturity or as part of immune responses. Apoptosis is distinct
from necrosis, which is caused by external circumstances such as toxins.
Autoantibodies are directed against endogenous
antigens.
Autoimmune diseases are cause by an immune reaction against
endogenous antigens.
Autoimmune reaction: An acquired immune response against endogenous
antigens is called an autoimmune reaction. Correspondingly, acquired
immunity to endogenous antigens is known as autoimmunity.
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