IMMUNE BODIES 25 



The cell molecules may be destroyed as the result of this com- 

 bination or it may be stimulated by defensive action to the 

 production of receptors; continued excitation results in the 

 production of more receptors than are necessary for the func- 

 tions of the cell and it is assumed that these receptors are set 

 free in the fluids surroimding the cells, and that they possess 

 a greater affinity for the antigen than the same receptors of the 

 cell molecule. These free receptors constitute the antibodies. 

 Three varieties of antibodies are known. 



(1) Uniceptors, such as antitoxins, which are regarded 



as comparatively simple and which combine 

 directly with the antigen. 



(2) Uniceptors, which have an enzyme-producing group 



in addition to the haptophoric receptor (agglu- 

 tinins, precipitins). 

 (2) Amboceptors, which require the presence of a third 

 substance before combination with the antigen can 

 be effected; this third substance is known as com- 

 plement. 



Antigens, and uniceptors produced by them, are specific 

 in their action, and this applies equally to the amboceptor- 

 complement-antigen system of the third order of receptors. 

 For instance, tetanus antitoxin acts on tetanus toxins and on 

 no others, and typhoid serum agglutinates only B. typhosus. 

 This statement, however, is not absolutely true, as antigens 

 produced by allied groups of organisms possess receptors which 

 are common to all, but as the specificity becomes more definite 

 with increased dilution of the antibody, the afiinity between 

 the specific receptors must be considered to preponderate. 

 The aiSiboceptors of the third order of antibodies also show 

 relative rather than absolute specificity. 



The antibodies generally are distinguishable from comple- 

 ments by their resistance to heat. The uniceptors and ambo- 

 ceptors are thermostabile, i.e., are not destroyed by heating to 



