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Adaptive Immunity

Antigen-Antibody Binding

Antigen-antibody binding produces protective outcomes such as cross-linking, neutralization, opsonization, complement activation, immobilization, and cellular cytotoxicity.

Antibodies play many different roles during the course of an immune response and can have direct effects themselves on stopping an infection or can act as a signal to other immune cells. The major function of antibodies is to move throughout the body and bind to target antigens or antigen-expressing cells that have protein pieces presented on their external cell surfaces. Binding of antibodies to an antigen-expressing cell can signal other immune cells to come and kill a target cell. Alternately, antibodies can neutralize an antigen via binding without the need for the involvement of immune cells. Antibodies that are able to directly neutralize an antigen are referred to as neutralizing antibodies. A specific kind of neutralizing antibody is known as an antitoxin, which is an antibody that can neutralize a specific toxin, such as anthrax, diphtheria, or tetanus.

Binding of IgG and IgM antibodies to their antigens can also induce a pathway, called a complement. Proteins in the complement pathway can recognize antibodies that are bound to a pathogen, and these complement proteins pile onto the antibody scaffold. Binding of complement proteins to pathogens such as bacteria can lead to two results: complement protein complexes can poke holes in the bacterial membrane, leading to bacterial cell death and/or the bacteria become opsonized for killing by white blood cells.

The binding of antibodies to a pathogen or to antigen-expressing cells is opsonization, the coating the surface of an antigen with opsonins to enhance the interaction between phagocytic cells and the antigen. Opsonins are any antibodies or substances that bind to an antigen and enhance phagocytosis. A specific downstream effect of opsonization is known as antibody-dependent cellular cytotoxicity. In antibody-dependent cellular cytotoxicity, opsonization leads most commonly to natural killer cells—lymphocytes that can recognize a wide range of proteins, including pathogens, cancerous cells, and antibodies—being recruited to kill the target cell.

Antibodies can also engage in cross-linking, where multiple antibodies are linked to each other through binding to the same foreign molecule. Cross-linking is especially common in allergic responses. Cross-linking of IgE antibodies triggers the common symptoms of allergic responses, including swelling, runny nose, and watery eyes.