The Soluble Malarial Antigen in the Serum of Monkeys Infected with Plasmodium Knowlesi

When rhesus monkeys with acute Plasmodium knowlesi infection recover after administration of quinine, complement-fixing antibodies are present at maximum titer in the serum soon after the first acute infection has subsided (1), and protective (2) and agglutinating (3) antibodies are demonstrable somewhat later in the course of the chronic disease. Relapses cause an increase in the titer of complement-fixing antibodies. From previous studies it has been impossible to learn whether the antigens and antibodies concerned in the complement fixation reaction are identical with or different from the antigens and antibodies concerned in active immunity and passive protection. The results of the present work show that a soluble malarial antigen occurs in the serum of monkeys with acute Plasmodium knowlesi infection. This antigen, upon injection into normal monkeys, gives rise to complement-fixing antibodies which are similar to those produced by malarial infection, but no protective or agglutinating antibodies are formed. This paper will describe serological and chemical investigations of the soluble malarial antigen.