Final answer:
Binding sites formed by Ig constant regions on antibodies trigger effector immune responses such as complement activation, enhanced phagocytosis, and inflammation. IgG and IgM antibodies can particularly initiate the classical complement pathway to combat pathogens.
Step-by-step explanation:
Ag-binding sites formed by Ig variable regions bind pathogens; binding sites formed by Ig constant regions trigger effector immune responses. The constant regions on antibodies, which dictate their class, anchor to different immune cells or complement proteins and facilitate various functions such as opsonization, cell lysis, and the initiation of the complement cascade. For example, IgG and IgM can initiate the classical complement pathway by binding to the pathogen surface and then interacting with complement proteins, which results in enhanced phagocytosis, inflammation, or direct killing of pathogens through membrane attack complex (MAC) formation.
Each antibody class, like IgG, IgA, IgM, IgE, and IgD, triggers different immune mechanisms due to variations in their constant regions. For instance, IgM is effective at activating the complement system and is often the first antibody produced in a primary immune response, while IgG molecules are adept at neutralization and opsonization, as well as crossing the placental barrier to provide passive immunity to the fetus.