Human red cells are classified as Rh positive (Rh +) or Rh negative (Rh -) depending upon the presence or absence of ‘D’ antigen on them. Major percentage of the population is Rh positive. Human red blood cells possessing D antigen will agglutinate in the presence of corresponding antibody. The red blood cells are first checked using Anti-D (IgM) monoclonal antibodies. Agglutination of red cells with Anti- D (IgM) monoclonal antibodies indicates the presence of D-antigen and hence Rh positive result. If no agglutination is obtained then the red blood cells are checked with Anti-D (IgG) monoclonal antibodies as it sensitize the Rh positive/ Du positive cells which will give agglutination on addition of coomb’s reagent (Indirect coomb’s technique).