Antibodies are special plasma globulins having the feature of entering into the combination with the corresponding antibodies and creating the "antigen-antibody" complexes.

All types of antibodies are jointly called immunoglobulins. There are five classes of immunoglobulins IgG, IgA, IgM, IgE, IgD.

Antibodies to red blood cell antigens are immunoglobulins of M, G and A classes.

Scheme of "ANTIGEN-ANTIBODY" complex creation

Antibody heterogeneity directed against one antigen X

Schematic structure of immunoglobulins

IgG molecule has two active centers to be connected with the antigen. IgM molecule has ten active centers to be connected with the antigen, that's why IgM immunoglobulins enter into the reaction with erythrocytes easier than with IgG.

Reaction of erythrocyte direct agglutination by IgM antibodies and indirect agglutination by IgG antibodies

Reaction with anti-IgG is marked with green color - the Coombs test

Availability and absence of erythrocyte agglutination when adding reagents

А2 and А2В erythrocytes are not agglutinated by several specimens of serum and monoclonal antibodies anti-A. That's why, if the research of the blood group is made using standard serums (or monoclonal antibodies) without applying the cross method with А, 0, В erythrocytes, the blood group of such erythrocytes can be identified incorrectly as 0 or B, accordingly

Varieties of D antigen

Availability of agglutination in the drop with anti-D reagent confirms positive reaction and denotes the Rh-positive affinity. Absence of agglutination with anti-D reagent confirms the Rh-negative affinity. The result is considered actual in case of absence of agglutination with the control specimen. Anti – D, IgM reagents do not agglutinate some blood specimens, therefore, when examining Rh affinities of donors' blood all Rh-negative specimens must be examined using anti – D, IgG reagents.