Ans. is 'c' i.e., Capsular polypeptide protein is responsible for virulence Pathogenicity of H. influenzae H. influenzae does not secrete any exotoxins, the polysaccharide capsule (not polypeptide) is considered to be the major factor in virulence. There are "6" types of capsule designated a,b,c,d,e, and f Of these, type 'b' is commonly associated with invasive H. influenzae disease in children. The polysaccharide capsule of type '13' strain is composed of polyribosyl ribitol phosphate (PRP) capsule and is the proven determinance of virulance. The PRP capsule exes its effect in the following ways : - i) It is antiphagocytic ii) It does not induce the alternative complement pathwary. This allows the bacterium to invade the blood or CSF without attracting phagocytes or provoking an inflammatory response and complement mediated bacteriolysis. For this reason anticapsular antibody which promotes both phagocytosis and lysis of bacteria is the main factor in immune defence against H. influenzae infection. Capsulated strains cause invasive diseases, amongst which meningitis is most common. Host defences against H. influenzae. H. influenzae is susceptible to lysis by antibody and complement. Therefore, host defenses against H. influenzae include : - i) Activation of the alternative and classical complement pathway. ii) Antibodies to PRP capsule. Antibodies to the Hib capsule play the primary role in conferring immunity against H. infleunzae. These bactericidal antibody directed against PRP capsule of H. influenzae type b is entirely responsible for host resistance. Newborns and infants are protected from infection upto the age of 6 months due to transplacental transfer of antibodies against the 'b' capsule from mother to fetus. When these transplacental antibodies to the capsule wane, infants are at higher risk for developing invasive H. influenzae disease.