That's because its surface protein haemagglutinin doesn't contain many short amino acid sequences — called epitopes — that trigger helper T - cells in the body to stimulate antibody - making cells. (newscientist.com)
Direct anti-inflammatory effect of a bacterial virulence factor: IL -10-dependent suppression of IL - 12 production by filamentous haemagglutinin from Bordetella pertussis. (cellr4.org)
They are all analysed using a venerable test for similarity called haemagglutinin inhibition. (newscientist.com)