4. Direct vs. indirect agglutination
a) Direct agglutination
(1) Antibody binds to multiple RBCs and causes agglutination without
additional manipulation
(2) IgM is much better able to do this than IgG
(a) IgM maximum diameter is 30 nm (vs 14 nm for IgG), which is wide
enough to more easily overcome RBC zeta potential
(b) Occasional IgG antibodies can do this, if antigens “stick up” fairly far
from RBC surface (ABO-related & M and N antigens most commonly)
b) Indirect agglutination
(1) Antibody binds to, but does not form bridges with, RBCs
(a) Requires additional step to see agglutination (AHG phase described
above; enzyme treatment of RBCs may also make an IgG capable of
direct agglutination)
(2) Classically IgG rather than IgM, for reasons mentioned above
(3) Most significant antibodies cause this type of agglutination