Adjuvants are mostly pharmacological agents of drug or biological origin used to modify the antigenicity of immunization components, i.e., to stimulate, potentiate, or depress the immune response or to inhibit or enhance specific subclasses of immunocytes. Adjuvants augment, stimulate, activate, potentiate, or modulate the immune response at either the cellular or humoral level. Classical agents (Freund's adjuvant, BCG, Corynebacterium parvum) contain bacterial antigens. Some adjuvants are endogenous (e.g., histamine, interferon, transfer factor, tuftsin, interleukin-1). Their mode of action is either non-specific, resulting in increased immune responsiveness to a variety of antigens, or antigen-specific, affecting a restricted type of immune response to a narrow group of antigens. Since adjuvants enhance the body's immune response, they can be considered a type of immune modulator.