The endogenous function of antibodies is to protect the host from pathogens.  The central mechanisms are neutralization, complement-dependent cytotoxicity (CDC), opsonization/phagocytosis (also known as antibody-dependent cellular phagocytosis (ADCP)), and antibody-dependent cellular cytotoxicity (ADCC).

Functions of Antibodies:

• Neutralization does not require additional effector molecules (such as complement) or cells (such as neutrophils) and refers to the ability of antibodies to disrupt a discrete function or mechanism.  Neutralizing antibodies can suppress the infectivity or pathogenesis of viruses, bacteria, parasites, and fungi by inducing pathogen aggregation, impairing mobility, blocking pathogen-host adhesion, or inhibiting key processes normal to a pathogen’s lifecycle, for example.
• CDC is triggered by clustered antibodies bound on the surface of cells, which activate the classical complement pathway initially by interaction with C1q, ultimately leading to formation of a membrane attack complex (MAC) and target cell lysis.
• Opsonization targets antibody-bound particles (including microorganisms) for phagocytosis. This requires engagement of the Fc region of particle-bound antibody by the phagocyte’s (monocyte, macrophage, neutrophil, dendritic cell, mast cell) Fc receptors (FcR).  The interaction of antibody Fc with cell-expressed FcR provides a critical link to cell-mediated immune responses also necessary for ADCC.
• ADCC is triggered by engagement of FcR on the surface of effector cells (natural killer (NK) cells, macrophages, monocytes, eosinophils) with antibody Fc, which is bound on the surface of target cells.  FcR signaling leads to secretion of cytotoxic enzymes, perforin, granzymes, tumor necrosis factor (TNF), which lyse the target cell.

There are specific FcR for the five classes of Ig (G,A,E,M,D).  Furthermore, there are eight subclasses of FcR for IgG:  CD64 (FcγRI), CD32 (FcγRIIa_H131, FcγRIIa_R131, FcγRIIb/c), CD16 (FcγRIIIa_F158, FcγRIIIa_V158, FcγRIIIb), and FcRn.  The Fcγ receptors have different binding affinities for the four different IgG isotypes (IgG1, IgG2, IgG3, IgG4).  FcR expression varies across leukocyte subsets.  CDC and ADCC effector functions also vary across IgG isotypes.  In general, ADCC effector function is high for human IgG1 and IgG3, and low for IgG2 and IgG4.  In comparison, CDC is high for IgG3 and IgG1, and low for IgG2 and IgG4.

Scientists have subsequently commandeered the properties and functions of antibodies for therapeutic purposes. A variety of approaches have been engaged:

• Some therapeutic monoclonal antibodies neutralize a particular function without otherwise engaging the immune system (e.g. immune checkpoint inhibitors that block a specific receptor/ligand interaction without eliciting ADCP, ADCC, or CDC).
• Some therapeutic mAbs use CDC, ADCP, and ADCC to kill target cells (e.g. anti-CD20 rituximab, which depletes B cell leukemic cells).
• Some therapeutic mAbs are loaded with toxic payload that is activated upon internalization to enable directed cell killing (e.g. antibody-drug conjugates (ADCs) like rovalpituzumab tesirine).
• Some antibodies are engineered and reformatted as single chain Fv chimeric antigen receptors (CARs) and used to transduce T cells and/or NK cells for use in cancer immunotherapy.