optimal detection of IgG anti-Fya or Jka. Generally speaking, though, the optimal
dilutions are similar for most lgG antibodies.
d. Anticomplement Standardization.
(1) The FDA requires that reagents marketed as polyspecific AHG contain
anti-C3d activity at a level that equals or exceeds the FDA's anti-C3d reference serum.
(2) When the C3 (B10) molecule is acted on by C3 convertase (C142) in an
immune reaction, the molecule is cleaved into two fragments. The smaller C3a
fragment does not attach to the RBC membrane, but the larger C3b fragment does.
(3) (Thus, if RBCs are sensitized with complement "in vitro" by a
complement binding blood group antibody, they will be sensitized with C3b. They will
give a positive antiglobulin test with antiglobulin serums containing antibodies to
determinants on the C3b molecule (for example, anti-C3b, anti-C3c [(1A or anti-C3d2D)]).
(4) When RBCs are sensitized with complement "in vivo" (for example,
autoimmune hemolytic anemia or transfusion reactions), or following prolonged
incubation "in vitro", the C3b is acted on by the C3 inactivator, which is present in all
normal plasma, and the molecule is cleaved into C3c and C3d. The C3c (B1A) fragment
is lost from the RBC, and the C3d (2D) fragment remains on the cell membrane. (see
figure 3-6). Thus, in order to detect C3 bound to the RBC "in vivo", the antiglobulin
serum must contain activity against C3d (see figure 1-6).
Figure 1-6. Reaction products of C3. (From Garratty and Petz).