Which of the following is NOT a required step in B and T lymphocyte maturation?
A. Proliferative expansion of precursor cells
B. Cell surface expression of complete antigen receptor complexes
C. Positive selection
D. Activation of precursors by foreign antigens
E. Pre-antigen receptor expression
ANS: D. Foreign antigens are not likely to be present in the generative lymphoid organs where lymphocytes mature and thus are not involved in the process. Both B cell and T cell maturation include sequential steps, including cytokine-induced proliferation and expansion of immature precursors, somatic recombination of Ig heavy chain or T cell receptor (TCR) β chain genes, expression of μ heavy chain or TCR β chain with a surrogate nonvariable partner to form a pre-Ig or pre-TCR, somatic recombination of Ig light chain or TCR α chain genes, expression of complete antigen receptors and associated signaling molecules, and positive selection by recognition of self antigens.
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