^{page 224 note 1} The term “side-chain” is unfortunate, as when applied to a chemical molecule whose constitution is known it has a definite meaning. Ehrlich has only used the term to express an analogy. It cannot but be wrong to speak, as is sometimes done, of the “side-chains” of a cell, though such side-chains may occur in molecules within the cell. The word “receptor” is much more fitting to express the group within the cells which may carry an affinity capable of saturation by a molecule outside the cell.

^{page 238 note 1} For a knowledge of the bearings of physical chemistry on this subject I am indebted to the kindness of my friend, Mr D. Nagel, Fellow of Trinity College, Oxford.

^{page 243 note 1} Considerable confusion arises through the variety of terms applied to the “complement” and “ immune body.” Complement is often called by Ehrlich “addiment” and by the French school, constantly, “alexine.” By the latter the immune body is called “la substance sensibilatrice.” All through this paper we have used the terms complement and immune body. As we shall see substances analogous to the latter sometimes occur in ordinary sera. These Ehrlich calls “ Zwischenkörper,” which we have translated “go-betweens.”.