filamentous hemagglutinin

Filamentous hemagglutinin is a large, filamentous surface protein and secreted adhesin expressed by Bordetella pertussis (and related Bordetella species). It functions as one of the principal adhesins that allows the bacterium to bind to ciliated epithelial cells of the respiratory tract via interactions with carbohydrate structures (e.g., via its RGD motif binding integrins) and sulfated glycoconjugates on host cells, as well as mediating binding to macrophages. This attachment is a critical early step in pertussis pathogenesis, enabling colonization prior to the local and systemic effects of pertussis toxin and other virulence factors (e.g., adenylate cyclase toxin, tracheal cytotoxin). FHA also has immunomodulatory effects, promoting an anti-inflammatory response that may help the bacteria evade clearance by the host immune system, including effects on dendritic cells and T-cell responses. Expression of FHA, along with other virulence factors, is controlled by the BvgAS two-component signal transduction system, which senses environmental signals (e.g., temperature) and coordinately regulates virulence gene expression (phenotypic phase variation between virulent Bvg+ phase and avirulent Bvg- phase). Clinically, FHA is important because it is one of the antigens included in acellular pertussis vaccines (e.g., DTaP, Tdap) along with pertussis toxin, pertactin, and fimbriae, aimed at inducing protective immunity against colonization and disease. Understanding FHA is high-yield for USMLE because it ties together bacterial adhesion mechanisms, pertussis pathogenesis, and vaccine composition questions.