ADAPTIVE IMMUNITY
The immune system is a collection of cells and tissues scattered throughout the body that protects us from invading pathogens. An antigen is a molecule (e.g. a protein) that can be specifically recognized as foreign by cells of the immune system. Specific immune responses (also referred to as adaptive immunity) are directed toward a specific antigen that has gained entrance into the body. Specific immune responses become more rapid and more effective upon subsequent encounters.

Specific immune responses are described as either antibody- or cell mediated. Antibody-mediated immunity involves the production of antibodies by immune cells known as B cells. Cell-mediated immunity involves a group of immune cells known as cytotoxic T cells, which are able to kill cells the immune system regards as foreign.