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Antibodies, also known as immunoglobulins, are proteins produced by the immune system in response to foreign substances called antigens. These antigens can be viruses, bacteria, or other harmful substances that enter the body. Antibodies help the immune system to identify and neutralize these foreign substances effectively.

Antibodies are Y-shaped proteins composed of four polypeptide chains: two identical heavy (H) chains and two identical light (L) chains. These chains are connected by disulfide bonds and non-covalent interactions. Each chain contains a variable (V) region and a constant (C) region. The variable regions of both the heavy and light chains come together to form the antigen-binding site, which can specifically recognize and bind to a unique part of an antigen called an epitope.

The primary function of antibodies is to recognize and bind to antigens, thereby neutralizing or marking them for destruction by the immune system. There are several ways in which antibodies can achieve this, including: - Neutralization: By binding to antigens, antibodies can prevent them from interacting with host cells and inhibit their infection or toxicity. - Opsonization: Antibodies can coat antigens and enhance their uptake by immune cells called phagocytes, which will then engulf and destroy the antigens. - Complement activation: Antibodies can initiate a series of protein interactions known as the complement system, which can ultimately lead to the destruction of pathogens or the formation of immune complexes that can be cleared by phagocytes. - Antibody-dependent cell-mediated cytotoxicity (ADCC): Antibodies can recruit immune cells, such as natural killer cells, to destroy infected cells or tumor cells.

There are five main classes of antibodies, each with a specific role in the immune system. These classes are IgA, IgD, IgE, IgG, and IgM. Each class has a specific function, such as: - IgA: Found in secretions like saliva, tears, and mucus; provides protection against pathogens at mucosal surfaces. - IgD: Plays a role in the activation and development of B cells, a type of immune cell that produces antibodies. - IgE: Involved in allergic reactions and protection against parasite infections. - IgG: The most common class of antibodies in the blood; provides long-term immunity and can cross the placenta to protect a developing fetus. - IgM: The first class of antibodies produced during an immune response; helps with the activation of the complement system.