Antibody-Mediated Effector Functions

Opsonization

• promotion of phagocytosis of  the antigen by macrophages, neutrophiles
• FcR present on the surfaces of macrophages, neutrophiles bind the constant region of Ig

                                                                            ↓
                                     secures pathogen to the phagocyte membrane
                                                                            ↓
                                           initiates signal transduction pathways
                                                                            ↓
                                     phagocytosis of Ag═Ab complex takes place

Complement Activation

• IgM, most subclasses of IgG activate collection of serum glycoproteins (complement activators) that perforate cell membranes
• Binding of the Ag═Ab complex to C3b receptors of red blood corpuscles (RBCs)

                                                                           ↓
                                                     RBC delivers to liver/spleen
                                                                           ↓
                       resident macrophages remove complex without destroying the RBC


Antibody-dependent cell-mediated cytotoxicity (ADCC)

                         Infected cell═Ab complex
                                                ║
                                             FcR of NK cells
                                                     ↓
                     activates cytotoxic activity of the effector cells

Antibodies can also cross epithelial layers by a process called transcytosis; for example, transfer of IgG from mother to fetus ("passive immunization")