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.