The C-reactive protein present in the albumin fraction of the serum of patients during certain acute bacterial infections is highly antigenic upon injection into rabbits. The antiserum thus prepared reacts specifically with this protein and does not react with the proteins of normal human serum. Immunological specificity has been demonstrated by both precipitin and complement-fixation tests.

Antiserum prepared in rabbits to the C-reactive protein from human sources also reacts specifically with the similar protein in the serum of monkeys acutely ill with experimental pneumococcus infection.

By means of immunological reactions it is possible to detect amounts of reactive protein which are too small to yield a visible precipitate in tests with the C polysaccharide.

Certain of the properties are discussed which distinguish the C-reactive protein from the proteins of normal human serum.

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