Stages of Antigen-Antibody Reactions
The antigen–antibody reaction occurs in two stages: primary and secondary.
Primary stage is the initial interaction between antigen and antibody. It is rapid and reversible, but without any visible effects. The ionic bonds, hydrogen bonds, van der Waals forces, and hydrophobic interactions are the weaker intermo-lecular forces that bind antigen and antibodies together in this primary stage.
Covalent binding, which is a stronger intermolecular force between antigen and antibody, however, does not occur in this stage.
Secondary stage is an irreversible interaction between anti-gen and antibody, with visible effects, such as agglutination, precipitation, neutralization, complement fixation, and immobilization of motile organisms. The binding between antigen and antibody during this stage occurs by covalent binding.
A single antibody is capable of causing different types of antigen–antibody reactions, and a single antigen is capable of inducing production of different classes of immunoglobulins, which differ in their biological properties.
The results of agglutination, precipitation, neutralization, and other tests are usually expressed as a titer. Titer is defined as the highest dilution of serum that gives a positive reaction in test. Higher titer means greater level of antibodies in serum. For example, a serum with a titer of 1/128 contains more antibodies than a serum with a titer of 1/8.