The core functions of antibodies within the immune system are highly diverse and important, serving as the body's primary defense against a variety of pathogens. These Y-shaped proteins, produced by B cells, are not only tasked with the direct neutralization of pathogens but also play a pivotal role in orchestrating a broader immune response. Through mechanisms such as neutralization, opsonization, complement activation, agglutination, and antibody-dependent cellular cytotoxicity (ADCC), antibodies allow the immune system to mount an effective defense against infections and maintain health.
Neutralization
Antibodies neutralize pathogens by binding directly to the surface proteins or other components critical for the pathogen's entry into host cells. This binding inhibits the pathogen's ability to infect host cells, effectively neutralizing its threat. For instance, in the context of viral infections, neutralizing antibodies can prevent viruses from attaching to and entering host cells, a mechanism well-documented in influenza and HIV research.1
Opsonization
Opsonization involves antibodies binding to antigens on the surface of pathogens, marking them for destruction by phagocytes such as macrophages and neutrophils. Macrophages can process and present the antigens to specific T cells during a primary immune response. This process enhances the efficiency of the phagocytic removal of pathogens from the bloodstream. The Fc region of the antibody interacts with Fc receptors on phagocytes, facilitating the engulfment and destruction of the opsonized pathogen.2
Complement Activation
Upon binding to antigens, certain classes of antibodies can activate the complement system—a group of proteins that aid in the clearance of microbes and damaged cells. The complement system consists of plasma proteins that are typically activated directly by pathogens or indirectly by pathogen-bound antibodies. This activation leads to a cascade of events culminating in the formation of the membrane attack complex, which can lyse pathogens directly. The complement system also helps with opsonization and inflammation.3
Agglutination
Antibodies can cause the agglutination of pathogens by cross-linking them, forming clumps that are more easily ingested by phagocytes. Agglutination effectively reduces the number of infectious units the immune system must deal with individually and is particularly important in the immune response to multicellular parasites and some bacteria. It can also be used in research to identify antigens or antibodies in bodily fluids, as well as blood typing and detecting bacterial strains in microbiological cultures.4
Antibody-Dependent Cellular Cytotoxicity (ADCC)
ADCC is a mechanism through which antibodies promote the destruction of infected or malignant cells. The antibodies bind to antigens on the surface of target cells with their Fab region and recruit effector cells (such as natural killer cells) through their Fc region. The effector cells then release cytotoxic substances that lead to the death of the target cell. ADCC, mediated by monoclonal antibodies against tumor antigens, plays a significant role in the therapeutic efficacy of these antibodies in cancer treatment.5
These functions underscore the versatility and critical importance of antibodies in the immune defense system, highlighting their role in both the direct neutralization of pathogens and the orchestration of broader immune responses.
References:
Lu, L. L., Suscovich, T. J., Fortune, S. M., & Alter, G. (2018). Beyond binding: Antibody effector functions in infectious diseases. Nature Reviews Immunology, 18(1), 46-61. https://doi.org/10.1038/nri.2017.106
Kutteh, W. H., Stovall, D. W., & Schust, D. J. (2014). Immunology and Reproduction. Yen & Jaffe's Reproductive Endocrinology (Seventh Edition), 287-307.e3. https://doi.org/10.1016/B978-1-4557-2758-2.00014-7
Janeway Jr, C.A., Travers, P., Walport, M. (2001). The complement system and innate immunity. Immunobiology: The Immune System in Health and Disease (5th edition). https://www.ncbi.nlm.nih.gov/books/NBK27100/
Ryan, K., Ahmad, N., Alspaugh, J. A., Drew, W. L., Lagunoff, M., Pottinger, P., Reller, L. B., Reller, M., Sterling, C., & Weissman, S. (2023). Ryan & Sherris Medical Microbiology (8th edition).
