Antibodies are an important component of the immune system used to combat extracellular and intracellular infections. As it is possible to produce an antibody targeting almost any molecule, these proteins can be used to make an effective adaptive immune response.
IgG antibodies are capable of reaching high levels in serum and body tissue (except immune privileged tissues). This allows opsonisation, agglutination, and sequestration of antigens.
Opsonisation marks a pathogen as something a phagocyte should target. This is normally through a chemotactic response to the Fc region.
Agglutination exploits the multiple binding domains of antibody. IgG has 2 variable regions, and IgM forms a pentameric structure. This clumps the pathogen together, making it a more obvious target for other components of the immune system (such as phagocytes).
If an exotoxin is secreted by a pathogen, an antibody could be used in response. This would allow the toxin molecules to be sequestered, preventing them from reaching their host target. Cimzia, a rheumatoid arthritis monoclonal antibody, sequesters tumour necrosis factor-alpha (produced by the autoimmune response), reducing the inflammatory response and subsequent pain experienced by sufferers.
Intracellular pathogens may cause a change in cell-surface proteins, which can be targeted by antibody. This could include viral infection, targeting NK cells through antibody-dependent cell mediated cytotoxicity.
Antibody, specifically IgG, can also target transport proteins. By blocking ion uptake or nutrient transporters, a bacterial cell can be starved to death.