Scheme showing the mechanism of action of IgG antibodies. (a) IgG antibodies coated on cell surface antigens can bind to Fc receptors and initiate signalling through immunoreceptor tyrosine-based activation (ITAMs) or inhibitory motifs (ITIMs). (b) IgG antibodies can bind neonatal Fc receptors (FcRn) on endothelial cells to maintain serum IgG levels, or (c) bind to tumour cells and recruit the protein C1q to initiate the complement cascade, resulting in tumour cell lysis by the membrane attack complex (MAC). Adapted from ref. 17 with permission from Springer Nature, Copyright 2010.