Structure of the antibodies
The structure of each antibody is usually the same and consists of four different amino acid chains (amino acids are the smallest building blocks of proteins), two of which are called heavy chains and two are called light chains. The two light and the two heavy chains are completely identical and are connected to each other by molecular bridges (disulfide bridges) and are brought into the characteristic Y-silon form of an antibody. The light and heavy chains consist of constant amino acid sections that are the same for all different antibody classes and of variable sections that differ from antibody to antibody (IgG therefore has a different variable section than IgE).
The variable domains of the light and heavy chains together form the respective specific binding site for the antigens (any structure or substance in the body) matching the antibodies. In the region of the constant part, there is a second binding site (Fc-part) for each individual antibody. However, this is not intended for an antigen, but is a binding site with which they can bind to certain cells of the immune system and activate their function.
Functions of antibodies
With the two short, upper arms they can bind the foreign cells. Either they use both or only one arm. If they only use one arm, they can bind to another antibody with the other arm.
If this happens with multiple antibodies, they clump together and can be eaten by macrophages. The macrophages then break down these clusters and destroy the foreign cells. If they use both upper arms, they can use their lower arm to bind directly to other cells of the immune system, such as T-helper cells.
The T-helper cells then take up the antibodies, degrade them and incorporate the foreign cell components into their own membrane. In this way, they mediate as information cells for other immune cells. Roughly speaking, antibodies help to recognise foreign cells and allow them to be destroyed by other cells. They thus serve as a kind of link between the immune cells.
Antibodies in blood
When a pathogen or other foreign substance (antigen) enters the human body (e.g. via the skin or mucous membranes), it is first recognised and bound by the “superficial” defence cells of the immune system (so-called dendritic cells) and then migrates to the deeper lymph nodes. There the dendritic cells show the antigen to the so-called T-lymphocytes, a class of white blood cells. These are then awakened to become “helper cells” and in turn activate the B-lymphocytes, which immediately begin producing antibodies that are a perfect match for the respective antigen to be rendered harmless.
These antibodies are released into the circulating blood once they have been fully formed, so that they can reach all parts of the body with the physiological bloodstream. Another possibility of B-cell activation is the direct contact of a B-cell floating in the blood with the pathogen or the foreign substance, without prior activation by a T-cell. The antibodies released into the blood (also called immunoglobulins) can generally be divided into different classes (IgG, IgM, IgA, IgD and IgE) and can be determined by taking a blood sample and subsequent laboratory tests.