Ans. a. Positive selection during development (Ref: Lippincott's Immunology (Illustrated Reviews) 2007/1 13; The Immune Response; Basic and clinical principles by Sounders (2005)/346; 'Elements of Immunology' by Fahim Halim Khan (Pearson Education) 2009/206)During development, T cells undergo both positive selection and negative selection, while B cells undergo only negative selection.Class I MHC is expressed in on both T cells and B cellsClass I MHC molecules are expressed on nearly all nucleated cells.Since both T cells and B cells are nucleated, both of them express class I MHC molecules.Antigen receptors are found in both T cells and B cellsThe interaction of lymphocytes with antigens takes place through binding to specialized antigen specific receptors working as recognition units.Surface markersT cellsB cellsCD-3 receptor+Q-Surface lg-+QThymus specific+Q-Ag+Q-Ag receptor-+QFc receptor-+QComplement receptorSRBC or E rosetteEAC rosetteQRosettes(CD-2; measles receptor)(C3 receptor; CR-2; EBV receptor)During development, T cells undergo both positive selection and negative selection, while B cells undergo only negative selectionDuring development. T cells undergo both positive selection and negative selection, while B cells undergo only negative selection - Elements of ImmunologyDevelopment of LymphocytesB-cell Development (Negative selection)T-cell Development (Positive selection followed by negative selection)* Immature B cells expressing membrane IgM do not proliferate and differentiate in response to antigens.* Their encounter with antigens (which are usually 'self as only' self antigens are present at that time) in the bone marrow leads to death (apoptosis) or functional irresponsiveness This property is called negative selectionQ.* B cell negative selection occurs in the bone marrow.* T cells are selected for survival much more rigorously since they should be able to recognize 'self' MHC molecules, but should not recognize 'self' peptides.* Initially immature T cells that recognize MHC molecules are selected and saved from programmed cell death. This is called positive selection and ensures that T cells only recognize antigens in association with MHC (This is important since T cells need to identify and react with infected host cells).* Next amongst positively selected immature T cells, those T cells that react strongly with 'self-peptides' on MHC molecules undergo negative selectionQ and apoptotic death leading to elimination of immature T cells specific for self antigens. This second step is called negative selection.* T cells positive and negative selection occurs in thymusQ. Central T cell tolerance is established by positive and negative selection in the thymus while central B cells tolerance is established by negative selection in the bone marrowQ.