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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.

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Alyaa Ahmed Farid, Associate Professor of Immunology, Faculty of Science, Cairo University. Born on 15 August 1978, Egypt. B.Sc in Chemistry- Zoology, M.SC and Ph.D in Immunology.

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