Final answer:
Mature T lymphocytes leaving the thymus express either CD4+ or CD8+ receptors, which define their roles as either helper T cells or cytotoxic T cells.
Step-by-step explanation:
When mature T lymphocytes leave the thymus, their T cell receptors are either CD4+ or CD8+. Mature T cells express either the CD4 marker or the CD8 marker, but not both. These markers designate the type of T cell: CD4+ T cells become helper T cells (TH) that bind to class II MHC and assist other cells of the immune system, while CD8+ T cells become cytotoxic T cells (Tc) that bind to class I MHC and directly kill infected cells. The expression of these markers is a result of cell signals that cause the thymocytes to become self-tolerant and differentiate into the two distinct T cell types.