The nature of age immunity to Plasmodium berghei in the rat.

The intensity of Plasmodium berghei infections decreases as the age of the rat host increases. The nature of this 'age immunity' was investigated. No experimental support was found for innate resistance involving either serum non-antibody factors or changes in the erythrocytes that inhibit parasites in older rats. A cross reacting immune response active against P. berghei was not found. Evidence is presented which shows that rats less than 7 weeks old lack at least part of the functional immunological apparatus by which older rats produce a protective immune response. It is suggested that the defect might involve T lymphocytes.