When antibodies recognize antigens, they bind to them in a reaction called antigen-antibody through Van der Waals bonds, hydrophobic or ionic forces. There are several types of antigen - antibody reactions:
- Precipitation. The antigens are macromolecules with various antigenic determinants (polyvalent antigen) dissolved in body fluids. When they bind to antibodies, they form insoluble antigen-antibody complexes, which precipitate, allowing phagocytes to destroy them.
- Agglutination. The antigens (agglutinogens) are on the surface of bacteria or other cells, and when coupled with the antibodies originate bridges between them, forming aggregates (caking), facilitating its destruction. This is the reaction that occurs when transfusions are made between incompatible blood groups.
- Neutralization. The antibody binds to the antigen, eliminating the negative effects it has on the invaded organism.
- Opsonization. Microorganisms or antigenic particles are more rapidly phagocytosed by phagocytes if they have antibodies on their surface. Antibody coated microorganisms are said to be opsonized.
Antibody-mediated opsonization consists of the binding of the antibody to antigens present on the surface of the pathogen.
The opsonins are molecules that line the microorganisms facilitating phagocytosis, but are not always specific molecules such as antibodies, but may also be non - specific molecules such as proteins the complement.
Complement proteins are so named because they help or complement antibodies. This system is made up of about 20 plasma proteins of the globulin type, which, unlike antibodies, are always present in plasma. These proteins are involved in the opsonization of cells outside the body and in the lysis of other pathogens. When a protein of the complement system binds to the antigen-antibody complex, the activation of the rest of the proteins of the complement system begins, forming an enzyme that breaks the membrane of the invading cell, causing its lysis.