Which of the following features is not shared between T cells and B cells

Ans. (a) Positive selection during development B cell undergoes negative selection, whereas T cells undergoes both positive and negative selection. Mechanism of central tolerance: B cell: The recognition of antigens by the immature B-cells in the bone marrow is critical to the development of immunological tolerance to self. For proper immunological function it is essential B cell do not recognize self antigen and should recognize antigen derived from pathogens. In the bone marrow the immature B cells bearing surface IgM, if recognizes self molecule undergo negative selection. This self antigen induce loss of cells from the B cell repeoire is called as clonal deletion. Thus only those B cells that do not recognizes self molecules as antigen comes out of bone marrow. T cells tolerance: T cells are selected for survival more rigorously than B cells. They undergo both positive and negative selection to produce T cells that recognizes self MHC but do not recognizes self peptides. T cell tolerance is induced in thymus. (i) Positive selection occurs in thymic coex, if a maturing T cell is able to bind to a surface MHC molecule it is saved from program cells death; whereas cells fails to recognize MHC dies. Positive selection ensures that T cell recognize antigen in association with MHC molecule only. (ii) Negative selection occurs in coex, coico-medullary junction, and the medulla. Here MTEC (modullary thymic epithelial cells) signal self reactive T cells to die apoptosis. Other Options Option b: Class I MHC are expressed by all nucleated cells and platelets. Ref. Robbin's 8/e 190 Option c: Both B cells and T cell possess antigen receptors. On B cells surface Ig acts as antigenic receptor whereas on T cell, TCR along with CD3 acts as antigenic receptors. Ref. AA 8/e 128, 9/e, p 133