Antibodies are produced in the body to serve 3 major purposes in the immune system: neutralization, opsonization, and compliment protein activation. Each of these functions helps to protect the body from potentially harmful pathogens.
Neutralization occurs when the antibodies produced specifically bind a pathogen in such a way that prevents its ability to infect cells. This can be done by either blocking the active site of a protein necessary for infection, or when several antibodies bind enough sites of the pathogen that it is unable to bind with high enough affinity to a cell surface to facilitate infection. Antibodies that can neutralize active proteins are called functional antibodies, while antibodies that can neutralize other antibodies are called anti-idiotypic antibodies.
Opsonization describes the process of antibodies ‘flagging’ an antigen for phagocytosis and degradation. Typically, an antibody would recognize a pathogen, a phagocyte would bind to the constant domain of the antibody, and the pathogen would be phagocytosed. This is one of the most common methods in which pathogens are targeted for degradation, and where the adaptive immune system helps to bolster the innate.
Complement Protein Activation
Complement proteins also play a part in the targeting of pathogens. In addition to their own ability to serve as opsonins, some complement proteins can also bind the constant domain of antibodies, increasing the size of the target pathogen, and increasing the chance that an immune cell will target it for degradation or apoptosis.