An antibody-antigen complex is a chemical interaction between antibodies and antigens. This complex is also called an immune complex. The chemical interaction takes place between the epitope on the antigen and the paratope on the antibody. Weak, noncovalent bonds such as electrostatic interactions, Van der Waals forces, hydrophobic interactions and hydrogen bonds are involved in the formation of the antibody-antigen complex. The strength of this complex is characterized by two parameters: affinity and avidity.

Antibody-antigen complex showing antibodies specifically binding to an antigen displayed on the surface of a pathogen.

Figure 1: Antibodies binding to their corresponding antigen on the surface of a pathogen.