IMMUNITY 39 



nor will tetanus antitoxin neutralize diphtheria toxin or 

 snake venom. 



Lock and Key Theory. This specific action is explained by 

 supposing the molecule of toxin to have a shape peculiar to 

 itself. The molecule of diphtheria toxin is of such shape 

 that the haptophore end will fit only on certain receptors 

 of a cell; the molecule of tetanus toxin will fit on only certain 

 other receptors. 



Fig. 8. Graphic representation of receptors of the third order, and 

 of some substance uniting with one of them: c, Cell-receptor of the third 

 order, amboceptor; e, one of the haptophores of the amboceptor with 

 which some food substance or product of bacterial disintegration, /, may 

 unite; g, the other haptophore of the amboceptor with which complement 

 may unite; k, complement; h, the haptophore, and z, the zymotoxic 

 group of the complement (Ehrlich). 



An antitoxic serum is a suspension of receptors of the first 

 order in blood-serum. Antitoxins for diphtheria and tetanus 

 are the most common. 



Precipitins are bodies in serum which, when added to a 

 protein in solution, will cause a precipitate to form. The 

 precipitins are specific and act only with similar proteins. 



When a protein or food substance is injected into an 



