Which marker is characteristic of memory B cells?

Memory B cells are characterized by the expression of specific surface markers that distinguish them from naive B cells and other types of B cells. One of the key markers associated with memory B cells is CD27. This marker indicates that the B cell has encountered an antigen previously and has undergone differentiation into a memory cell, capable of responding quickly upon re-exposure to the same antigen.

CD27 is a member of the tumor necrosis factor receptor superfamily and plays a role in B cell activation and survival. Its presence is often used as a reliable marker in distinguishing memory B cells from naive B cells, which do not express CD27.

In contrast, other markers such as CD45RA are associated with naive B cells rather than memory B cells, as naive B cells typically express CD45RA while memory B cells usually express CD45RO. CD19 is a pan-B cell marker that is present on both naive and memory B cells and does not provide information about memory status. CD40L, on the other hand, is primarily involved in T cell activation of B cells and is not used to identify memory B cells specifically.

Thus, CD27 is the correct answer because it specifically denotes the presence of memory B cells, making it a critical marker for