Cross-reactivity of the O antigens among Haemophilus influenzae type b strains

The double-immunodiffusion technique was used to compare the O antigens of 18 meningeal Haemophilus influenzae type b strains with each other and with H. influenzae strains representing the capsular types a-f. The O antigens of the meningeal strains and the reference type b strain (strain Rab) cross-reacted to a large extent, while only a few cross-reactions were demonstrable with the other serotypes.