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Antibodies are proteins produced by the immune system to identify and neutralize foreign objects like bacteria and viruses. They are a crucial component of the adaptive immune response, which is responsible for recognizing and eliminating pathogens that the innate immune system cannot handle. Antibodies are typically Y-shaped molecules composed of four polypeptide chains: two heavy chains and two light chains. The heavy chains determine the class of the antibody, while the light chains contribute to the antigen-binding site.

There are five main classes of antibodies, each with distinct functions: IgA, IgD, IgE, IgG, and IgM. IgG is the most abundant and versatile, capable of crossing the placenta and providing long-term immunity. IgA is primarily found in mucosal secretions, protecting against pathogens entering through the respiratory and gastrointestinal tracts. IgE is involved in allergic reactions and parasitic infections. IgD and IgM are less common and have roles in the early stages of immune responses.

Antibodies recognize and bind to specific antigens, which are usually proteins or polysaccharides on the surface of pathogens. This binding triggers various immune responses, including phagocytosis, complement activation, and antibody-dependent cellular cytotoxicity. The production and affinity of antibodies are regulated by the immune system to ensure effective pathogen clearance and minimize damage to the host's tissues.

Antibodies play a vital role in vaccine development and immunotherapy. Vaccines stimulate the production of specific antibodies, providing immunity against particular diseases. In immunotherapy, antibodies are used as therapeutic agents to target and neutralize cancer cells or other pathological agents. The study of antibodies has significantly advanced our understanding of the immune system and has led to the development of numerous diagnostic and therapeutic tools.