The patient's γ/δ T cells express KIR (killer inhibitory receptors). When KIR bind HLA class I antigens on the surface of a target cell, the ability of the effector cell to lyse its target is inhibited. Normal BFU-E express class I determinants, but this expression is down-regulated as CFU-E mature proerythroblasts. This patient had a clonal expansion (LGL leukemia) involving γ/δ T cells, resulting in the “autoimmune” lysis of proerythroblasts and PRCA.