Cold isohaemagglutinins in Plasmodium berghei-infected rats reacting with parasitized reticulocytes

Significant levels of cold IgM and IgG isohaemagglutinins were detected in the serum of rats infected with Plasmodium berghei KSP 11. Peak titres occurred 15 days after initial infection at the time when the parasitaemia was dropping rapidly, or 7 days after a second challenge infection. Infected reticulocytes were much more sensitive to agglutination than uninfected cells, but absorption experiments demonstrated isoantigenicity in the determinants involved. This result indicated that the presence of the parasite resulted in exposure of membrane isoantigens normally masked. Agglutination could be inhibited with fractions with pIs 7.7-7.8 obtained from parasitized reticulocytes. Formaldehyde and glutaraldehyde-fixed infected cells each gave distinct agglutination reactions, different from unfixed cells.