Q: Is it possible for blood cells to have CD4 and CD8 antigens in the same cell? If yes, what is the percentage of such cells in a normal healthy person? As far I know, such cells having both CD4 and CD8 are never possible. Is it true?

A:Mature T lymphocytes expressing the alpha/beta T cell receptor are generally classified as either CD4+ or CD8+, based on the mutually exclusive expression of these two lymphocyte co-receptors. The expression of CD4 or CD8 on T cells is pivotal in defining them as T helper or T cytotoxic cells, respectively. Aside from an intermediate stage during T cell maturation in the thymus (where they may constitute as much as 80% of all cells of the thymic cortex), CD4 and CD8 molecules were initially thought to be mutually exclusive on mature T cells. However, there is accumulating evidence that these two molecules can be co-expressed on mature T cells as well as some neoplastic cells (ALL, lymphoma, thymoma etc.) and in some disease states (arthritis, kidney transplant rejection, some skin disorders etc.). Populations of CD8+ T cells re-expressing CD4 have been designated CD4 (dim) CD8 (bright) or CD4 (low) CD8 (high) T cells, and likewise, populations of CD4+ T cells re-expressing CD8 have been designated CD4 (bright) CD8 (dim) or CD4 (high) CD8 (low) T cells.

Circulating CD4/CD8 double-positive cells are mature memory cells that mount rapid Th1/Tc1 recall responses to vaccine and viral antigens from self-limited, past, or highly replicative, persistent infections. They can be detected both in the peripheral blood and at the sites of inflammation and viral replication. The percentage of CD4/CD8 double-positive cells representing a population of mature lymphocytes with a normal phenotype seems to vary in different studies, but on an average is <5% in the peripheral blood. Their numbers increase during viral infections and can go up to 20% or so. Interestingly, there is a sub-population of NK cells too that co-expresses both CD4 and CD8 while in contrast, there are some T-cells that lack both CD4 and CD8 expression (called gamma/delta T-cells).