Question

B cells and T cells both undergo positive and negative selection during development. Describe a positive selection process that is SIMILAR between the two cell types. Describe a negative selection process for each cell type that is NOT used by the other cell type.

Answer 1

CELL	POSITIVE REACTION	NEGATIVE REACTION
T Cell	Autoimmunity is the common positive reaction for both T and B cells Positive selection in case of T cells is the ability of the receptors on the thymocyte to interact with self peptide–MHC complexes expressed by the cortical epithelial cells of the thymus. If they fail to receive the signal, they are consequently deleted as not being useful to an immune system that requires T cells to recognize antigen that is bound to self-MHC molecules. Thymocytes surviving this hurdle are said to have been “positively selected" Thymic selection results in deletion by apoptosis of the vast majority of differentiating thymocytes by mechanisms that place stringent boundaries around the viability of a thymocyte with a newly expressed TCR specificity.	Negative selection is the means by which an immature T-cell clone encoding a self-reactive αβ TCR is eliminated from the host. The deletion of self-reactive T-cell clones mediated by negative selection is key to establishing central tolerance.During negative selection, self antigens are displayed by the MHC on the surface of thymic epithelial cells and in the cortex and the medulla, or thymic-resident, bone marrow–derived DCs and macrophages. The self antigens presented in the thymus during negative selection directly impacts the deletion of autoreactive αβ T-cell clones, thereby influencing autoimmunity.
B cell	lymphocytes are activated, proliferate, differentiate, and carry out effector functions. It is equally important, however, that this positive response be tightly regulated by mechanisms that operate to turn off the response and to eliminate cells no longer required. Under physiological circumstances, once an immune response fades, commonly as a consequence of antigen depletion, two pathways to terminal lymphocyte differentiation become available: apoptosis or differentiation into memory cells.But the majority of lymphocytes in an active response are not required for maintenance of immunological memory, and the necessity for homeostasis leads to apoptosis of cells no longer required. Apoptosis is a unique process of cellular death, widely conserved phylogenetically, and distinguished from death by necrosis by cellular shrinking, DNA fragmentation, and breakdown of cells into “apoptotic bodies” containing nuclear fragments and intact organelles that can be eliminated by phagocytosis without release into the extracellular space of the majority of intracellular, especially nuclear, components.	The ability of passively administered soluble antibody to inhibit humoral responses has long been appreciated and was initially thought to occur by soluble antibody effectively masking all available antigen epitopes. The molecular mechanism accounting for this suppression is known to be mediated by the binding of IgG to FcRγIIB ( single chain molecule)and the subsequent recruitment of cytosolic phosphotases.Thus the inhibitory effect of IgG on BCR-mediated B cell activation is explained by the interaction of the FcγRIIB ITIM, and specifically associated phosphatases, with the BCR