A study of neutralization reaction of trypanosomes with immune sera made against them is recorded, together with a description of the technique.
Neutralizing antibody appears in rabbits infected with T. brucei 5 days after infection and reaches its peak (1/320) within 28 days.
The same titre of neutralizing antibody can be produced in rabbits by a series of injections of dead formalized trypanosomes.
Antibody titres persist at high levels for a longer period in animals infected and subsequently treated than in animals immunized with dead trypanosomes.
Neutralizing antibody can be demonstrated in infected guinea-pigs but on a much lower level than in rabbits. Mice and rats infected with T. brucei and not treated failed to show demonstrable neutralizing antibody.
Mice infected with T. brucei can be protected from infection by inoculation at the same time, or even 1 hr. later, with 0·5 ml. of antiserum showing a high level of neutralizing antibody.
Mice inoculated with the same antiserum showed passive immunity lasting for 10 days.
The evidence that T. brucei ‘O‘ after passages through rabbits failed to be neutralized by homologous or heterologous sera which, on the other hand, were able to neutralize the parent strain, suggests that strains after passage through rabbits were not genetic variants, but developed a protective mechanism against immune bodies.