The liver tissues of man and certain animals (dogs, rabbits, guinea pigs, etc.) yield, upon alcoholic extraction, various substances which may be divided by their physical and chemical properties into several groups. While many substances are present in the alcoholic extract, the ones possessing antigenic properties are comparatively few. The latter are responsible for the antigenic properties exhibited by the whole alcoholic extract. The substances extracted with alcohol were fractionated into the following four groups.

(a) Substances Insoluble in Ether and Hot Alcohol.—These are chiefly proteins and salts. The proteins are probably the minute particles of larger molecules held in apparent suspension in alcohol until all other substances are removed. The water extracted from the tissues and admixed with alcohol is also an essential factor in extracting these particles in an alcoholic solution. The salts present are the usual physiological constituents of the liver, notably, sodium chloride. The quantity of these substances extracted with alcohol varies greatly with different specimens. Biologically considered, they are neither markedly hemolytic nor anticomplementary and possess no antigenic property for the Wassermann reaction. It is important, however, to note that the proteins bind complement when mixed with certain active human sera. For this reason a preparation of antigen containing this group of substances is unsuitable for use in combination with an active serum, and should, therefore, be rejected.

(b) Substances Insoluble in Ether and Soluble in Hot Alcohol.—This group contains soaps, cleavage products of proteins, and small amounts of the bile salts. Soaps and bile salts are very strongly hemolytic and are absolutely unfit for use as antigen. Moreover, their antigenic properties are very slight. It is best to eliminate this group of substances from the preparation of antigen. The quantity of the substances of this group extracted from different specimens of tissue is very variable.

(c) Substances Soluble in Ether, Alcohol, and Aceton.—In this group are found varying amounts of fatty acids, both saturated and unsaturated, some neutral fats, cholesterin and many unidentified lipoidal bodies. This group causes either hemolysis or inhibition, of hemolysis. In other words, it is anticomplementary as well as hemolytic in the majority of preparations. At the same time, in some preparations it is, to a certain extent, antigenic. This great variation in the amounts of these substances in given extracts renders their presence in the antigen preparation undesirable. It is not denied, however, that, when added in adequate quantities, some of these substances may improve the activity of the antigenic lipoids.

(d) Substances Insoluble in Aceton.—This group of substances consists of phosphatids. The best known among them is, of course, lecithin. Besides lecithin, however, there must be various other phosphatids present in this fraction. It will be noticed that the precipitate formed by mixing the ethereal solution with aceton contains a certain amount of lipoids insoluble in ether as well as in alcohol. Before the fractionation in aceton, all lipoids were soluble in ether or ethyl alcohol. Further analytical work on the nature of the phosphatids contained in this fraction is necessary. This fraction, in general, is more constant in amount in the various liver extracts. Biologically considered, it is the most important. It is usually non-hemolytic, frequently anticomplementary, but much more strongly antigenic than the other fractions. The antigenic strength varies with different preparations, being almost absent in the extracts derived from fatty livers. An aceton insoluble fraction may be strongly antigenic without any other auxiliary effects, or may be accompanied by an anticomplementary property. This fraction does not cause the so-called non-specific reaction with an active human serum. For these reasons it is recommended (as Noguchi has repeatedly done before) that the antigen should be selected from the aceton-insoluble fractions.

Iodine Value in Relation to Antigenic Value.—Speaking generally of the aceton-insoluble fraction, a high antigenic value is associated with a high iodine value of lipoids obtained from the liver and heart. This rule does not, however, apply to the corresponding fraction from the brain.

Standard Antigen.—The preparation of a standard antigen for Noguchi's method for the diagnosis of syphilis is given just before the summary. The directions will enable one to make a stock solution, from which, at any time, a suitable antigenic emulsion may be obtained.

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