1-18. ROLE OF COMPLEMENT IN THE ANTIGLOBULIN REACTION
Red blood cells can become sensitized with complement components both
"in vivo" or "in vitro." One of two main mechanisms is usually involved: (1) complement
components may sensitize the RBCs through the action of complement-binding blood
group antibody(ies), or (two) immune complexes may activate the complement cascade,
causing complement components to attach to RBCs nonspecifically. Whichever of the
mechanisms is operative, the end product is the same, in that the RBC becomes
sensitized with complement components and may or may not continue to hemolysis. If
the RBC does not hemolyse, complement components may still be detected on the
RBC membrane by the antiglobulin test. C3 and C4 are the most readily detected, and
C3 is the most clinically significant. C3 and C4 are beta globulins (B1O and B1E,
respectively); thus, if a RBC is sensitized with C3 or C4, the cell is, in chemical terms,
sensitized with beta globulin. When antiglobulin serum is made by injecting a rabbit with
human globulin (either fractionated or in human serum), the rabbit will make antibodies
to gamma globulin, and also to the "nongamma" globulins, (for example, beta globulin).
This anti-non-gamma fraction of the antiglobulin serum containing antibeta globulin will
react with the RBC-bound complement (C3 or C4), causing agglutination, a positive
antiglobulin test. It is also possible to prepare purified C3 and C4 as immuunogens, so
that specific anti-C3 and anti-C4 can be produced.
1-19. MECHANISMS FOR SENSITIZATION OF RED BLOOD CELLS BY
a. Activation of Complement by Blood-Group Antibodies.
(1) Some blood group antibodies bind complement to the RBC membrane.
Some of them do this very efficiently and commonly cause lysis of normal red cells (for
example, anti-A, -B, -Vel, -PP1Pk(Tja); others may sensitize the RBC with complement
components, not causing lysis of normal RBCs, unless they are enzyme-treated (for
example, anti-Lewis and -Kidd); still, other examples of anti-Lewis, -Kidd, -KeIl, -Duffy, -
S, -s, -I, -i, -H, and -P1, may sensitize RBCs with complement components without
causing lysis of untreated cells or enzyme-treated cells. It is not understood why some
antibodies sensitize RBCs with enough C3 to give strongly positive antiglobulin tests,
yet do not proceed to lysis. Some of these antibodies are lgM and cause agglutination
as well as complement sensitization of the RBCs (for example, anti-A, -B, -I, -i, and
some anti-Lewis). Other, rarer, examples may be lgM and do not cause agglutination
under normal conditions (for example, some anti-Lewis, -KeII, -Duffy, and -Kidd); but
may sensitize RBCs with complement, detectable by the antiglobulin test. Still others
are lgG (for example, some anti-Kell, -Duffy, anti-Kidd); in most cases, the lgG
sensitization is readily detected by anti- globulin serum in addition to the complement
sensitization, but in some cases (for example, some anti-Kidd), the lgG sensitization is
very weak, and the complement sensitization strong.