It seems desirable at this point, while we are trying to fix in mind the great value of recognizing syphilis in a person in order to treat it and thus prevent dangerous complications, to say something about the blood test for syphilis, the second great advance in our means of recognizing doubtful or hidden forms of the disease. The first, it will be recalled, is the identification of the germ in the secretions from the early sores. +Antibodies in the Blood in Disease.+--It is part of the new understanding we have of many diseases that we are able to recognize them by finding in the blood of the sick person substances which the body makes to neutralize or destroy the poisons made by the invading germs, even when we cannot find the germs themselves. These substances are called antibodies, and the search for antibodies in different diseases has been an enthusiastic one. If we can by any scheme teach the body to make antibodies for a germ, we can teach it to cure for itself the disease caused by that germ. So, for example, by injecting dead germs as a vaccine in typhoid fever and certain other diseases, we are able to teach the body to form protective substances which will kill any of the living germs of that particular kind which gain entrance to the body. Conversely, if the body is invaded by a particular kind of germ, and we are in doubt as to just which one it is, we can identify it by finding in the blood of the sick person the antibody which we know by certain tests will kill or injure a certain germ. This sort of medical detective work was first applied to syphilis successfully by Wassermann, Neisser, and Bruck in 1904, and for that reason the test for these antibodies in the blood in syphilis is called the Wassermann reaction. To be sure, it is now known that in syphilis it is not a true antibody for the poisons of the Spirochaeta pallida for which we are testing, but rather a physical-chemical change in the serum of patients with syphilis, which can be produced by other things besides this one disease. But this fact has not impaired the practical value of the test, since the other conditions which give it are not likely to be confused with syphilis in this part of the world. The fact that no true antibody is formed simply makes it unlikely that we shall ever have a vaccine for syphilis. +Difficulties of the Test.+--The Wassermann blood test for syphilis is one of the most complex tests in medicine. The theory of it is beyond the average man's comprehension. A large number of factors enter into the production of a correct result, and the attaining of that result involves a high degree of technical skill and a large experience. It is no affair for the amateur. The test should be made by a specialist of recognized standing, and this term does not include many of the commercial laboratories which spring up like mushrooms in these days of laboratory methods.