Final answer:
When an antigen and antibody bind, they form clumps together in an agglutination reaction, which is different from precipitation, neutralization, or opsonization reactions, all of which are interactions involving antibodies.
Step-by-step explanation:
When the antigen and antibody bind, they clump together in a process called agglutination reaction. This phenomenon occurs when antibodies (immunoglobulins) cross-link antigens to create large aggregates. For example, IgG antibodies have two antigen-binding sites and can bind two separate pathogen cells, leading to their clumping. Furthermore, IgM's pentameric structure allows for even more extensive cross-linking, making it highly effective in agglutination.
Precipitation reactions are also related to the antibody-antigen interaction, where soluble antigens are added to antibodies in a solution, resulting in the formation of visible precipitins. However, this is distinct from agglutination, which involves the clumping of cells or particles. Other processes, such as neutralization and opsonization, are additional actions of antibodies but are not characterized by the clumping of antigens.