antibody (Synonym: immunoglobulin) are plasma proteins from the group of globular proteins that are formed in response to the presence of antigens from the immune system, specifically the B lymphocytes.
The antibodies to already known pathogens swim freely in the bloodstream. If antigens in the human body bind antibodies to their surface structure. At the same time, the immune system promotes the differentiation of further binding-specific antibodies, i. they can after the Key-lock principle only attach to very specific surface structures. This is enormously important because otherwise there would be a danger that the body's own proteins would be recognized by the antibodies as foreign cells. In this case, one speaks of an autoimmune disease.
If an antibody encounters its binding-specific antigen, depending on the class of antibody, the following may occur to render the antigen harmless:
1. The antibody binds to the antigen, altering its structure so that it can not dock on body cells, nor is its detrimental function able to exercise.
2. The antibody binds to the antigen, marking it for endogenous immune cells. Natural killer cells recognize the label and cause cell death by apoptosis in the target cell.
3. The antibody has multiple binding sites for antigens, allowing an antibody to cause the clumping of several antigens (agglutination). As a result, the clumped cells are no longer 'capable of action'.
4. The antibody binds to the antigen and changes its surface structure so that macrophages recognize the cell as foreign and phagocytize (opsonization).

Structure of an antibody:

An antibody consists of two identical ones light chains (light chains, marked in yellow) and two identical ones heavy chains (heavy chains, red + blue marked). The latter are about one disulfide bridge connected to a ypsilonartigen form. The binding sites for the antigens are located within the variable domain at the end of the chains. Variable because the shape of the antibody changes as soon as an antigen docks there. This docking point is also called Fab fragment designated. In contrast, the constant domain remains unchanged in its form, at least in relation to the antigen-antibody reaction. Macrophages or NK cells recognize the altered structures, bind to the specific chain for them and begin with the degradation of the immune complex or induce cell death.