Final answer:
The fractions of T lymphocytes circulating in the bloodstream carrying CD8 and CD4 co-receptors depend on their functional types. CD4+ cells become helper T lymphocytes, while CD8+ cells become cytotoxic T lymphocytes.
Step-by-step explanation:
The fractions of T lymphocytes circulating in the bloodstream carrying CD8 and CD4 co-receptors depend on their functional types. T lymphocytes recognize antigens based on a two-chain protein receptor. The most common and important of these are the alpha-beta T cell receptors. Naïve T cells can express one of two different molecules, CD4 or CD8, on their surface. CD4+ cells become helper T lymphocytes, while CD8+ cells become cytotoxic T lymphocytes.