Memory T cells are a small, heterogenous population of T cells that remain following pathogen clearance and provide long-lasting protection. They are generally characterized as either central memory (T CM) or effector memory (T EM) based on differences in their surface and intracellular markers, anatomic location, effector function, and cytokine production. It is well known that patients with chronic lymphocytic leukemia (CLL) display differences in both T cell function and population compared to healthy individuals. Previous studies have shown that CLL patients skew towards the CD4 +memory compartment and among these, a skewing toward T EMis especially associated with unmutated IGHV and more progressive disease. Moreover, T EMcells appear to protect CLL cells, possibly via IFNγ and IL-4 mediated pathways.