An antibody is a protein produced by the immune system in response to a foreign substance, known as an antigen. Antigens can be parts of pathogens like bacteria and viruses, or other foreign molecules. When an antibody encounters its specific antigen, it binds to it, marking it for destruction or neutralization. This binding process is crucial for the immune response. The interaction between antibodies and antigens helps the body identify and eliminate harmful invaders. This mechanism is fundamental to how vaccines work, as they stimulate the production of antibodies against specific antigens, providing immunity against future infections.