Antibody Structure:
Because IgG is a the most abundant antibody, we'll use it as an example of representative antibody structure; however, be aware that IgM and IgA look quite different.
Antibodies, also called immunoglobulins*, are glycoproteins that comprise heavy and light chains.
- They can be separated into fragments and regions:
- Variable and constant regions; the variable regions differ across antibodies, while the constant region is constant.
Key Functions
Antibodies do not directly kill pathogens.
- Neutralize microbes by blocking their extracellular receptors and inhibiting their attachment to host cells; antibody binding can also inhibit viral replication, stopping the spread of infection.
- Antibodies can opsonize microbes, which means they bind to them and make it easier for phagocytes to recognize and engulf them.
- Activate the complement cascade, leading to formation of membrane attack complexes (MAC) and, consequently, microbe lysis.
- Agglutination occurs when antibodies bind multiple cell-bound antigens simultaneously, causing them to clump; precipitation, which we haven't shown, occurs when antibodies simultaneously bind multiple soluble antigens. In both cases, binding makes antigen capture and phagocytosis easier.
- Finally, show that antibody-dependent cellular cytotoxicity occurs when antibody-coated cells are targeted by natural killer cells.