The 2 major subgroups of blood group A are A₁ and A₂. Anti-A₁ Lectin is used to differentiate these subgroups. Group A red blood cells which are agglutinated with Anti-A₁ Lectin are said to be of the subgroup A₁. Those that do not agglutinate are majorly classified as A₂. Agglutination of red cells that carry the A₁ antigen occurs with the addition of the Anti-A₁ reagent, following centrifugation. In the absence of the A₁ antigen, no agglutination is generally observed.

Biorex Anti-A₁ blood grouping reagent is prepared from a Dolichos biflorus seed extract, diluted with a sodium chloride solution containing bovine albumin. This reagent is supplied ready for use and does not require any further dilution or addition.