Table 2.

Major functions of human Ig antibodies [5]

Ig antibody	Function
IgM	• First immunoglobulin (Ig) expressed during B cell development (primary response; early antibody) • Opsonizing (coating) antigen for destruction • Complement fixation
IgG	• Main Ig during secondary immune response • Only antibody capable of crossing the placental barrier • Neutralization of toxins and viruses • Opsonizing (coating) antigen for destruction • Complement fixation
IgD	• Function unclear; appears to be involved in homeostasis
IgA	• Mucosal response; protects mucosal surfaces from toxins, viruses and bacteria through either direct neutralization or prevention of binding to mucosal surface
IgE	• Associated with hypersensitivity and allergic reactions • Plays a role in immune response to parasites