Which CD marker differentiates naïve T cells from activated or memory T cells?

The correct answer is CD45RA because it is a surface marker that distinguishes naïve T cells from activated or memory T cells. Naïve T cells express the CD45RA isoform, which is associated with a functional state that has not yet encountered an antigen.

When T cells become activated in response to an antigen, they start to express the CD45RO isoform instead, which reflects changes in their function and cell surface markers as they transition to memory T cells. This differentiation in the expression of CD45RA and CD45RO is crucial for understanding the immune response and the status of T cells within the immune system.

Other markers mentioned, such as CD28, play more of a role in the costimulatory signals required for T cell activation rather than distinguishing between naïve and memory T cells. CD19 is primarily a marker for B cells, and CD4 is a co-receptor found on T helper cells, but it does not differentiate naïve from activated or memory T cells. Therefore, the role of CD45RA is specific and significant in identifying the naïve T cell population accurately.