What are antigens?

Antigens are structures or substances on the surface of cells in the human body. They are mostly proteins, but can also be fats, carbohydrates or even completely different compositions. Either they are the body’s own structures, which under normal circumstances are always present in the human body, or they are foreign structures or substances that have entered the body but do not actually belong there.

These foreign antigens are usually recognised by the B- or T-lymphocytes of the defence system and are bound and rendered harmless by specific antibodies which have previously been produced by the B-lymphocytes. Right from the start, the immune system learns to distinguish between the body’s own structures and foreign ones, so that under healthy circumstances only foreign antigens are fought. However, if the immune system mistakenly recognizes the body’s own harmless structures as foreign antigens and also fights them, this pathological process is called an autoimmune reaction, from which autoimmune diseases can develop.

Function of antibodies

The main function of antibodies is to recognize, bind and destroy pathogens or foreign substances or materials that have entered the body. The protein molecules produced by the B-lymphocytes (a certain subspecies of white blood cells) can be divided into different classes of antibodies, each of which has different tasks and properties and some of which have their main site of action in different parts of the body. If the pathogen or the foreign molecule (antigen) in the body is recognised by the defence system, the B cells immediately begin to produce the appropriate antibody, which then docks with its one connection point to the structure to be fought and with its other connection point to other defence cells of the body (e.g. macrophages = scavenger cells). These are then activated and take up the antibody-antigen complexes, thus rendering the foreign substances or pathogens harmless.

Antibody screening test

The antibody screening test (AKS for short) is a test in laboratory medicine in which the patient’s blood serum is screened for certain antibodies directed against specific structures (antigens) on the membrane of the red blood cells (erythrocytes). A distinction is made between regular and irregular antibodies against the red blood cells: the regular ones are the so-called anti-A and anti-B antibodies, whereby the anti-A antibody is present in patients with blood group B, the anti-B antibody correspondingly in patients with blood group A. The irregular antibodies include the anti-D antibody, which is directed against the rhesus factor D. In order to find the regular and irregular antibodies in the patient’s blood serum, the patient serum is mixed with the corresponding antigens after the blood sample has been taken, so that if antibodies are present, a clumping reaction of the blood occurs: the test is then evaluated as positive. The antibody screening test is primarily performed in preparation for blood transfusions and as part of pregnancy screening. In everyday clinical practice, the term “antibody screening test” is also generally used for the determination of antibodies in the context of e.g. infectious or autoimmune diseases, but should not be confused with the actual meaning as described above.