(2) In most situations where RBCs are sensitized with complement by
alloantibodies (for example, "in vitro" or transfused RBCs "in vivo"), the responsible
immunoglobulin (for example, IgG) is readily detected by the antiglobulin serum. In
certain situations, described below, the immunoglobulin is not readily detectable by
routine methods or is not present on the RBC any more.
(a) When lgM antibodies sensitize RBCs without causing direct
agglutination, they are very difficult to detect by the anti-globulin test with
anti-IgM. The reason for this is unknown. In addition, commercial antiglobulin serums
contain very little, if any, anti-lgM. lgM antibodies invariably bind complement and are
detected by the anticomplement properties of the antiglobulin serum. This is
theoretically a more sensitive method, as every one lgM molecule will cause hundreds
of C3 molecules to sensitize the RBC.
(b) About 10 to 20 percent of "warm" autoimmune hemolytic anemias
have positive direct antiglobulin tests, resulting from sensitization with complement
(coating due to C3 alone), no lgG, lgM or IgA being detected on the RBCs. Some of
these patients are thought to have lgG present on their red cells, but it is present in
amounts below the threshold of the antiglobulin test as performed routinely.
(c) In cold agglutinin syndrome, the patient's lgM "cold" autoantibody
usually reacts up to 30C to 33C. Thus, the patient's RBCs become sensitized with
antibody in the peripheral circulation when the skin temperature drops to this range.
The antibody usually binds complement to the RBCs, and, if conditions are optimal,
hemolysis of the cells occurs. If the cells escape hemolysis, they will recirculate to
37C. At 37C, the cold autoantibody elutes back from the cell into the plasma, leaving
complement components firmly bound to the RBC. Thus, when a positive direct
antiglobulin test is obtained on these patients, it is because the anticomplement in the
antlglobulin serum has reacted with the RBC-bound complement. No inmunoglobulins
are present on the RBC.
b. Activation of Complement by Immune Complexes.
(1) Red blood cells can become sensitized with complement because of
activation of the complement cascade by immune complexes. Sometimes these
immune complexes are attached to the RBC membrane; at other times, they are remote
from the cell. A good example of this is the formation of immune complexes involving
certain drugs, for example, phenacetin or quinidine. The drug-antidrug complex can
attach nonspecifically to RBCs, and cause activation of complement with subsequent
attachment of complement compounds to the RBC membrane.
(2) It is important that anticomplement (in particular anti-C3d) activity be
present in antiglobulin serums used for direct antiglobulin tests in the diagnosis of
autoimmune hemolytic anemia; however, as alloantlbodies detectable only by their
ability to bind complement are so rare, the Importance of anticomplement activity in
reagents used for compatibility tests is open to debate.