(3) It has been suggested that the zeta potential, mentioned previously, is
the most important factor In explaining why most lgG antibodies do not directly
agglutinate RBCs, the span of the lgG molecules not being sufficient to bridge adjacent
RBCs under the conditions created by the electrostatic forces keeping the RBCs apart.
The same workers suggested that lgG antibodies agglutinated RBCs in the presence of
albumin because albumin raises the dielectric constant (charge dissipating power) of
the suspending medium, thus lowering the zeta potential, allowing RBCs to come close
enough together for agglutination to occur. They also suggested that proteolytic
enzymes (for example, papain, ficin, bromelin, and trypsin) produce the same final
effect by cleaving sialic acid from the RBC membrane, thus reducing the zeta potential.
It should be noted that lgG antibodies (for example, lgG anti-A and -B) do sometimes
directly agglutinate saline-suspended RBCs; this may be a result of the large number of
antigenic sites present, the orientation of these antigens above the surface of the RBC
membrane, and/or the clustering of these antigens during the antigen-antibody
interaction. Recently some workers have argued that zeta potential may not be the
most important factor involved in these reactions.
Some blood group antibodies can activate the complement cascade (see
paragraph 1-11), leading to Iysis of RBCs possessing the appropriate antigens.
Antibodies showing this characteristic are termed hemolysins and usually will
agglutinate or sensitize RBCs in the absence of complement. Examples of blood-group
antibodies that can sometimes act as hemolysins are anti-A, -B, -A, B, -I, -i, -Lea, -Leb, -
Lex, -Jka, -Jkb, -PP1Pk(TJa), -Vel. Some of these antibodies and others may sensitize
the RBCs with complement, but not hemolyze them. This complement sensitization can
be detected by the antiglobulin test.
a. The complex complement system is involved in the humoral portions of the
inflammatory response arid interacts broadly with portions of the clotting sequence, the
fibrinolytic system and the kinin-generating sequence. The function of complement
appears to be activation of secondary immunologic actions resulting in cytolysis,
phagocytosis, chemotaxis, and so forth The complement system consists of at least 20
globulins, many exerting their effect by enzymatic activity. The components are
numbered C1 to C9. They contribute about 5 percent of the total plasma proteins. Most
of them are beta globulins and are good immunogens. C3 is by far the most abundant
complement protein (1,500 :g/ml) and C4 the next at 450 :g/ml; most of the other
components being present only in small amounts. It is C3 and C4 that most interest us
in lmmunohematology, particularly in reference to the antiglobulin test.
b. Two major pathways of complement activation have been identified, the
classic pathway and the alternative pathway. The classic pathway is of most
importance in blood transfusion science.