Most memory T cells express which marker?

Memory T cells predominantly express the marker CD45RO. This marker signifies a more differentiated state of T cells and is associated with their ability to respond rapidly to previously encountered antigens. Unlike naive T cells, which typically express CD45RA, memory T cells have undergone activation and differentiation, leading them to express CD45RO instead.

The presence of CD45RO is crucial because it indicates that these T cells have historical exposure to antigens, allowing them to mount a quicker and more effective immune response upon re-exposure. This characteristic is essential for the functioning of the adaptive immune system, ensuring a robust defense against pathogens the body has already encountered.

In contrast, other markers like CD19 are primarily found on B cells, and CD3 is a general marker present on all T cells, regardless of their activation state. CD45RA, while expressed by naive T cells, is not indicative of the memory status. Thus, the expression of CD45RO is a defining feature that distinguishes memory T cells from naive T cells and other T cell subsets.