Polymorphism implies that each different MHC proteins bind a different peptide motif.

• Positive selection of developing T cells requires productive interaction with MHC proteins (“self”), which stimulate cell division; nonproductive interactions result in cessation of development and cell death (apoptosis).

• Negative selection occurs next after positively selected T cells are exposed to MHC proteins that are binding self-antigens. These T cells remain in the thymus and eventually die, whereas the positively selected T cells that did not interact migrate to the lymph nodes and spleen.

• The term clonal deletion is used to describe the overall process involving positive and negative selection during T cell maturation. B cells also undergo clonal deletion in the bone marrow, but also clonal anergy (i.e., “unresponsiveness”).

Positive selection of developing T cells requires productive interaction with MHC proteins (“self”), which stimulate cell division; nonproductive interactions result in cessation of development and cell death (apoptosis). Negative selection occurs next, after positively selected T cells are exposed to MHC proteins that are binding self-antigens. These T cells remain in the thymus where they eventually die, while the positively selected T cells that did not interact with self-antigens migrate to the lymph nodes and spleen. The term clonal deletion is used to describe the overall process involving positive and negative selection during T cell maturation. B cells also undergo clonal deletion in the bone marrow, but in addition undergo selection via clonal anergy (unresponsiveness).