Seed-derived substances that are very useful in the blood bank due to their reaction with certain red cell antigens. These extracts, which may come from reagent manufacturers or may be “homemade” by blood banks, will react in a characteristic pattern against red cells carrying particular antigens. Lectins are used for distinguishing subtypes of various ABO blood groups. For example, reactivity with Dolichos biflorus lectin separates subgroup A₁ red cells from negatively reacting A₂ red cells. Also, Ulex europaeus lectin is used to demonstrate the presence and strength of expression of the H antigen. Lectins may also be used in the workup of polyagglutination syndromes (T activation, for example).