In addition to these various salts our body fluids contain in solution moderate amounts of very complex chemical substances belonging to the class of proteins. A fact about proteins which has not yet been emphasized is that they make liquids in which they are dissolved sticky or gelatinous. An excellent example of this is ordinary raw white of egg, which is a solution of protein in water, and which shows the gelatinous character very strikingly. Because of the protein that is in solution in the body fluids, they have also this gelatinous character, although to a much less extent than in the white of egg, because the amount of protein in solution is so much less. As we shall show later, this sticky quality of the fluid is of a good deal of importance in its actual use in the body. It may be that the protein in the body fluids serves other purposes as well. One interesting fact that needs to be emphasized is that it is not used for fuel or building material for the cells. The protein that comes to them as part of their food supply is entirely distinct from that about which we are now talking, which is part of the permanent structure of the body fluids. We shall speak of the protein that serves to nourish the cells in a moment, when we are talking about the relations of the fluids to the transportation of food material. In addition to the salts and proteins we have also dissolved a great many very complex materials which may be looked upon as permanent or relatively permanent constituents of the fluid, but about which we know practically nothing chemically. We are sure that they are present, because of certain effects which they produce, but the substances themselves have never been made out by chemical analysis. These are materials which are concerned with the resistance of the body to infectious disease, and it will be necessary to say just a word about infection to make clear the part played by them.