The immune system distinguishes two groups of foreign substances. One group consists of antigens that are freely circulating in the body. These include molecules, viruses, and foreign cells. A second group consists of self cells that display aberrant MHC proteins. Depending on the kind of foreign invasion, two different immune responses occur: The humoral response (or antibody¿mediated response) involves B cells that recognize antigens or pathogens that are circulating in the lymph or blood (¿humor¿ is a medieval term for body fluid). The cell¿mediated response involves mostly T cells and responds to any cell that displays aberrant MHC markers, including cells invaded by pathogens, tumor cells, or transplanted cells.