Fig 1. Antibody structure and isotypes
Human antibodies can be classified into five main isotypes - IgG, IgA, IgM, IgE and IgD, with IgG and IgA being further divided into the subclasses IgG1, IgG2, IgG3, IgG4, and IgA1 and IgA2, respectively. Overall structural organisation of an antibody molecule is similar for all isotypes. It consists of two heavy and two light chains joined by disulphide bonds. Both the heavy and light chain have a highly diverse variable domain (VH, VL respectively) and one or more constant domains (CH1 CH2, CH3, and CL, respectively). The constant domains of heavy chain are identical for all antibodies of the same isotype/subclass. Antibodies can also be divided into two functional subunits: (1) Fab arm, responsible for the specific binding to the antigen, (2) Fc tail, responsible for the activation of antibody effector functions (CDC, ADCP, ADCC, antigen cross-presentation) through interaction with the complement system and binding to Fc receptors present on immune and other cells. A graphical overview of different isotypes and subclasses is shown.
Table 1. Ig isotypes mediate their Fc effect via different receptors and activate different immune cells