120 Self and the Phenomenon of Life
b2726 Self and the Phenomenon of Life: A Biologist Examines Life from Molecules to Humanity “9x6”antigen-presenting cells (such as dendritic cells) and displayed on the
latter’s cell surface. In other words, cell-to-cell contact is needed for T
lymphocyte function.
MHC (acronym for “major histocompatibility complex”) is a clus-
ter of glyco-protein molecules located on the surface of all mammalian
cells. When a cell is invaded by a virus, viral proteins are digested by the
host cell into peptides (protein fragments) and displayed on the surface
as a complex with MHC, called peptide-MHC complex. Dendritic cells
present this peptide-MHC to cytotoxic T lymphocytes and turn them
into killer cells. When an activated cytotoxic T subsequently encoun-
ters a virus-infected cell (a non-immune cell) whose surface displays the
same peptide-MHC, the infected cell is destroyed (Fig. 6.4).
Three features of MHC are crucial to the understanding of molec-
ular self: (1) the same MHC is present on the surface of all cells of
an individual whether or not they are infected (i.e., it is a normal cell
Fig. 6.4. Activation and function of cytotoxic T lymphocytes. (A) A dendritic cell (star
shaped) with MHC protein on the surface. (B) A dendritic cell displaying a piece of viral
peptide (shown as a dot) on the cell surface as peptide-MHC complex. (C) A dendritic
cell presenting a viral peptide to the receptor of a cytotoxic T lymphocyte (round cell).
(D) An activated cytotoxic T cell (now a killer cell) circulating in the blood. (E) A killer T
cell encountering a virus-infected non-immune cell (square shaped) whose surface pep-
tide-MHC complex matches that of the dendritic cell. (F) Demise of the virus-infected
cell. Note that all cells (immune and non-immune) that are infected by virus display the
viral peptide as the peptide-MHC complex, but only the immune cells (shown here as a
dendritic cell) are capable of triggering T lymphocytes to kill.