Effector mechanisms by which drugs mediate a positive direct antiglobulin test. Relationships of drug, antibody-combining site, and red blood cell membrane protein are shown. Panels A, B, and C show only a single immunoglobulin Fab region (bearing one combining site). A. Drug adsorption/hapten mechanism. The drug (▾) binds avidly to an unknown red blood cell membrane protein in vivo. Antidrug antibody (usually immunoglobulin [Ig] G) binds to the protein-bound drug. The membrane protein is not known to be part of the epitope recognized by the antidrug antibody. The direct antiglobulin test (with anti-IgG) detects IgG antidrug antibody on the patient’s circulating (drug-coated) red blood cells. The indirect antiglobulin test detects antibody in the patient’s serum only when the test red blood cells have been previously coated with the drug by incubation in vitro. B. Ternary complex mechanism. Drug binds loosely or in undetectable amounts to red blood cell membrane. However, in the presence of appropriate antidrug antibody, a stable trimolecular (ternary) complex is formed by drug, red blood cell membrane protein, and antibody. In general, the antibody-combining site (Fab) recognizes both drug and membrane protein components but binds only weakly to either drug or protein unless both are present in the reaction mixture. In this mechanism, the direct antiglobulin test typically detects only red blood cell–bound complement components (e.g., C3 fragments) that are bound covalently and in large number to the patient’s red blood cells in vivo. The antibody itself escapes detection, possibly because of its low concentration but also because washing of the red cells (in the antiglobulin test procedure) apparently dissociates antibody and drug from the cells, leaving only the covalently bound C3 fragments. The indirect antiglobulin test also detects complement proteins on the test red blood cells when both antibody (patient serum) and a complement source (fresh patient serum or fresh normal serum) are present in the reaction mixture together with the drug. C. Autoantibody induction. Some drug-induced antibodies can bind avidly to red blood cell membrane proteins (usually Rh proteins) in the absence of the inducing drug and are indistinguishable from the autoantibodies of patients with autoimmune hemolytic anemia. The direct antiglobulin test detects the IgG antibody on the patient’s red blood cells. The indirect antiglobulin test usually detects antibody in the serum of patients with active hemolysis. D. Drug-induced nonimmunologic protein adsorption. Certain drugs cause plasma proteins to attach nonspecifically to the red blood cell membrane. The direct antiglobulin test detects nonspecifically bound IgG and complement components. If special antiglobulin reagents are used, other plasma proteins, such as transferrin (TF), albumin (Alb), and fibrinogen (Fibr), also may be detected. In contrast to the other mechanisms of drug-induced red blood cell injury, this mechanism does not shorten red blood cell survival in vivo. Source: Hemolytic Anemia Resulting from Immune Injury, Williams Hematology, 9e Citation: Kaushansky K, Lichtman MA, Prchal JT, Levi MM, Press OW, Burns LJ, Caligiuri M. Williams Hematology, 9e; 2015 Available at: http://accessmedicine.mhmedical.com/DownloadImage.aspx?image=/data/books/1581/kau_ch54_f001.png&sec=94305682&BookID=1581&ChapterSecID=94305662&imagename= Accessed: October 20, 2017 Copyright © 2017 McGraw-Hill Education. All rights reserved