Vel is a blood group with a high-prevalence antigen that may induce a famously sneaky antibody in Vel-negative patients. Anti-Vel is notorious for looking like a benign, boring, cold-reactive IgM antibody, but unlike most such antibodies, this IgM is very capable of fixing complement at body temperatures, and they can cause severe hemolytic transfusion reactions. Patients with anti-Vel need Vel-negative blood, but fortunately, enough of these donors have been identified over the years that frozen Vel-negative units are not that hard to find in most blood centers.