1. Abundance: IgG is the most abundant immunoglobulin class in the body, accounting for approximately 75% of total immunoglobulins. This high concentration allows it to play a significant role in immune defense.
2. Structure: IgG molecules are composed of two heavy chains and two light chains, forming a "Y" shape. Each IgG molecule has two antigen-binding sites, enabling it to bind to a wide range of antigens.
3. Versatile Effector Functions: IgG antibodies can activate various effector mechanisms to neutralize pathogens and toxins. These mechanisms include:
- Complement activation: IgG molecules can bind to and activate the complement system, a complex network of proteins that leads to the destruction of pathogens.
- Opsonization: IgG antibodies can coat the surface of pathogens, marking them for recognition and engulfment by phagocytic cells such as neutrophils and macrophages.
- Neutralization: IgG antibodies can directly block the infectivity of viruses or neutralize bacterial toxins by binding to their specific binding sites, preventing them from interacting with host cells.
4. Transplacental Transfer: IgG antibodies can cross the placenta during pregnancy, allowing mothers to transfer immunity to their developing fetus. This passive immunity protects newborns against infections until their own immune system matures.
5. Long-Term Memory Response: IgG antibodies are involved in the formation of immunological memory. Exposure to a pathogen induces the production of IgG memory cells, which can rapidly produce large amounts of IgG antibodies upon subsequent encounters with the same pathogen, providing long-lasting protection.
Therefore, immunoglobulin G (IgG) plays a crucial role in resistance against viruses, bacteria, and bacterial toxins through its abundance, versatile effector functions, transplacental transfer, and involvement in immunological memory. It is a vital component of the body's immune defense system.