What differentiates memory T cells from naïve T cells?

Memory T cells are differentiated from naïve T cells by the expression of specific surface markers, particularly the presence of CD45RO. This isoform of the CD45 protein is associated with cells that have previously encountered their antigen, indicating that they have undergone activation and differentiation during an immune response.

In contrast, naïve T cells, which have not yet been exposed to their specific antigen, predominantly express CD45RA. The switch from CD45RA to CD45RO as T cells progress from the naïve to the memory state is a key marker of their maturation and functional history.

The other options do not appropriately characterize the difference between memory T cells and naïve T cells. For instance, the absence of CD19 is not relevant because CD19 is a B cell marker, not a T cell marker, and does not distinguish between T cell types. Similarly, the absence of CD4 does not pertain to the distinction between memory and naïve T cells, as both cell types can express either CD4 or CD8 depending on their lineage. Therefore, the presence of CD45RO is a definitive marker that delineates memory T cells from their naïve counterparts.