AGQ^UIRED HAEMOLYTIG ANAEMIAS 



seems to speak strongly in favour of the view that there is 

 some mechanism by which a deviated immune response can 

 be prevented from developing a vicious circle and giving 

 rise to gross lesions and symptoms, and perhaps suggests that 

 the change induced is dependent on the continued presence 

 of virus in the cells of the clones concerned. 



{a) Sedormid purpura 



Sedormid purpura is of considerable interest in any dis- 

 cussion of auto-immune reactions since it is probably the 

 most closely studied example of an auto-immune reaction due 

 to a foreign chemical substance. Clinical findings point to an 

 active development of the condition some time after the 

 drug (allyl zVopropyl acetyl urea) was first administered to 

 the patient. Detailed studies such as those reviewed by 

 Ackroyd (1955) have been made after the condition was 

 recognized and the drug withdrawn. In a sensitive subject 

 the clinical signs are those of a thrombocytopenic purpura — 

 symptoms disappear on withdrawal of the drug. The signifi- 

 cant immunological findings are: (a) a patch test of sedormid 

 on the skin provokes a purpuric rash limited to the area of 

 contact; {b) patient's fresh serum + normal platelets + sedor- 

 mid causes agglutination and lysis of platelets; [c) patient's 

 platelets in normal serum are not affected by sedormid; 

 {d) patient's blood mixed with saturated sedormid in saline 

 shows no clot retraction. 



Ackroyd's interpretation is that platelet (or vascular endo- 

 thelial cell) makes a loose union with sedormid and in this 

 way the sedormid becomes an antigenic determinant. Circu- 

 lating antibody is produced and when we have the complex, 

 endothelial cell or platelet + sedormid + antibody + comple- 

 ment, a damaging reaction results. The antibody has no 

 action on normal platelets and is clearly directed against the 

 simple chemical determinant. In this case therefore there is 

 no suggestion of an auto-immune response. 



Where thrombocytopenic purpura is not associated with 



9 129 BC 



