By Jonathan Piel
September 1993 exact factor that includes the immune approach. includes 164 pages of textual content and illustrations
Read or Download [Magazine] Scientific American. Vol. 269. No 3 PDF
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Extra resources for [Magazine] Scientific American. Vol. 269. No 3
Finally, they leave the thymus. Not all potential T cells, of course, complete this line of development. Some undergo negative selection, in which signals from other cells (those carrying selfantigen attached to self-MHC ) cause apoptosis. Cells in the thymus can supposedly trigger positive or negative selection depending on the layer of primitive fetal tissue from which they derived: endoderm, mesoderm or ectoderm. The thymus is unusual among lymphoid organs in containing cells from all three sources.
The added phosphate groups change the activity of these proteins so that they ultimately signal the cell to grow and diÝerentiate. The CD4 and CD8 proteins on T cells, as well as a protein on B cells known as CD19, are examples of membrane proteins coupled to kinases inside cells. Another kind of molecule that goes into action is CD45, an enzyme that helps to mediate lymphocyte activation by removing phosphates from certain proteins, thereby deactivating them. But kinase-mediated signals cannot by themselves activate lymphocytes.
CellsÑhematopoietic stem cellsÑthat could both reproduce themselves and also generate all blood cell types. The establishment of the crucial part played by bone marrow cells was followed by discovery of a similarly essential role for the thymus. Removal of the thymus from newborn mice compromised the development of lymphocytes elsewhere in the body. ) The mice from which the thymus had been removed experienced severe lifelong immunodeÞciency. In another important group of experiments, researchers removed a lymphoid organ called the bursa of Fabricius from chicks (the bursa plays the role in chickens that bone marrow does in humans).