Papain, a proteolytic enzyme derived from papaya (Carica papaya), induces a marked decrease in the electronegative charge on the surface of red blood cells enabling their agglutination by normally 'non-agglutinating' antibodies in saline medium.
 

The method used is based on the principle of haemagglutination. Red blood cells bearing an antigen, pretreated with papain, agglutinate in the presence of the corresponding antibodies.