T H E OCCURRENCE IN T H E RABBIT OF AN ACUTE PHASE PROTEIN ANALOGOUS TO HUMAN C-REACTIVE PROTEIN BY H. C. ANDERSON,* M.D., AND MACLYN McCARTY, M.D

C-reactive protein is a substance which appears in human blood during the acute phase of a variety of infectious diseases as well as during the course of certain non-infectious diseases. The presence of C-reactive protein in serum was first recognized as the result of its property of reacting to form a precipitate with the somatic C polysaccharide of pneumococeus (1). This reaction is responsible for the name that has been applied to the substance and provides a specific technique for its detection and isolation. As the result of a series of investigations, many of the properties of the Creactive protein have been defined (2-7). On salt fractionation of serum it is found in the albumin fraction (3), although electrophoretic studies indicate that it is probably an a globulin (6). Calcium ion is required for the reaction between the polysaccharide and the protein, and the precipitation can be prevented by the use of calcium-binding agents such as oxalate or citrate (3). A further relationship to calcium ion is indicated by the fact that the C-protein, as it occurs in the albumin fraction of serum, will precipitate upon dialysis against tap water or dilute calcium chloride solution (4). The precipitability of the protein under these conditions is determined by another component, presumably lipid in character, combined with the protein, and is eliminated by defatting the serum with alcohol and ether. However, defatting does not alter the reactivity of the protein with the C polysaccharide. The C-reactive protein is immunologically distinct from the proteins of normal serum, and antisera prepared against purified preparations are specific and do not react with normal human sera (5). The protein has been crystallized from human serous fluids, and the serological specificity has been confirmed with this material (7). Interest has been maintained for several years in the possible significance of the C-reactive protein, and numerous experiments have been undertaken in this laboratory in an effort to determine its function and site of origin. Recently, the clinical value of the determination of C-reactive protein in the serum as a