An antibody, also known as an immunoglobulin (Ig), is a protein molecule produced by the immune system in response to the presence of foreign substances called antigens. Antibodies play a critical role in the immune response by recognizing, binding to, and neutralizing antigens, such as pathogens (e.g., bacteria, viruses) or foreign substances (e.g., toxins).

Antibodies are produced by specialized white blood cells called B lymphocytes (B cells). Each B cell can produce a unique type of antibody that is specific to a particular antigen. Antibodies are Y-shaped molecules composed of two heavy chains and two light chains, linked together by disulfide bonds. The tips of the Y-shaped molecule, known as the antigen-binding sites, are highly specific and can recognize and bind to specific antigens.

Once an antibody binds to its target antigen, it can initiate various immune responses. These include neutralizing the pathogen or toxin by preventing its harmful effects, facilitating the removal of pathogens by activating other immune cells, promoting the engulfment and digestion of pathogens by phagocytes, and activating the complement system, which helps in the destruction of pathogens.

Antibodies are not only involved in the immediate immune response but also play a crucial role in long-term immunity. Memory B cells retain information about previously encountered antigens, enabling a faster and stronger immune response upon subsequent exposure to the same antigen.

In addition to their natural role in the immune system, antibodies have widespread applications in various fields, including biomedical research, diagnostics, therapeutics, and vaccine development. They can be used as tools to detect, quantify, and isolate specific molecules or cells, and they can be engineered or modified for targeted therapy or as diagnostic reagents.