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Antibodies

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:

— Fab: F = fragment, ab = antigen-binding; notice that IgG has two Fab's. Paratope binds the eptiope of a specific antigen. — Fc end: c = crystallizable region, but it can also be remembered because the C fragment interacts with Fc Cell surface receptors.

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.