ABSTRACT
Pathogens and their products can be found either within infected cells or outside cells in the intercellular fluid and blood. In an adaptive immune response, the body is cleared of extracellular pathogens and their toxins by means of antibodies, the secreted form of the B-cell receptor for antigen. Antibodies are produced by the effector B lymphocytes, or plasma cells, of the immune system in response to infection. They circulate as a major component of the plasma in blood and lymph and are always present at mucosal surfaces. Antibodies can recognize all types of biological macromolecule, but in practice, proteins and carbohydrates are the antigens most commonly encountered. Binding of antibody to a bacterium or virus particle can disable the pathogen and also render it susceptible to destruction by other components of the immune system. Antibodies are the best source of protective immunity, and the most successful vaccines protect through stimulating the production of high-quality antibodies.
