Which of the following is NOT an example of a checkpoint in lymphocyte maturation that contributes to the selective maturation of cells with useful antigen receptors?

Question

A. B cell precursors that cannot express Ig ? heavy chains die by apoptosis.
B. T cell precursors that cannot express TCR ? chains die by apoptosis.
C. Immature B cells that bind to self antigen with high affinity die by apoptosis.
D. Developing T cells that fail to express TCR ? chains die by apoptosis.
E. Developing T cells that recognize self major histocompatibility complex (MHC) molecules with high affinity die by apoptosis.

Answer 1

ANS: A. B cell precursors must express Ig μ heavy chains (not Ig γ heavy chains) to survive. The μ chains associate with surrogate light chains in pre-B cells, which allows for the generation of survival signals. Otherwise, the cells die by a default apoptotic pathway. A similar checkpoint exists in T cell precursors, which must express TCR β chains that associate with pre-Tα chains to avoid apoptotic death. Developing T cells must progress to express TCR α chains and form αβ TCRs to be positively selected and continue to survive. Both immature B cells and developing T cells will undergo apoptosis if they express high-affinity receptors that bind to self antigens. In the case of developing T cells, high-affinity binding to self MHC will lead to apoptotic death.