Capsular polypeptide protein is responsible for virulence Hemophilus Infl uenzae Hemophilus influenzae is a grain negative rod. The name hemophilus influenzae shows some of its features. Hemophilus means "blood loving" This organism requires blood containing medium.for growth H. influenzae requires 2 erythrocyte factors for growth. Factor X hemin --> It is required, for synthesis of cytochrome Factor V nicotinanti- --> It is essential for oxidation de adenine dinucleotide reduction In the laboratory it is usually grown on chocolate blood agar which is prepared by adding blood to an agar base at 80degC. The heat releases X and V factors from the RBC and turn the medium a chocolate brown colour. Pathogenesis :? The pathogenesis of H. influenzae infection is not completely understood although the presence of type B polysaccharids capsule is known to be the major factor in virulence. H. influenzae does not secrete any exotoxins, the polysaccharide capsule 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 conunonly associated with invasive Hemophilus influaenzae disease in children. The polysaccharide capsule of type 'b' strain is composed of ribose ribitol and phosphate "(PRP)" capsule and is the proven determinance of virulence. The "PRP capsule" exes its effect in the following way :? It is antiphagocytic It does not induce the alternative complement pathway This allows the bacterium to invade the blood or cerebrospinal .fluid 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 defense against H. influenzae infection. Non typable or nonencapsulated strains of bacteria are less virulent. They lack the invasiveness of their capsulated strains and can only cause local reactions. They frequently cause otitis media in children as well as respiratory disease in adults weakned by preexisting lung disease such as chronic bronchitis from smoking or recent viral influenza infection. Hemophilus influenzae is associated with two types of infection -Invasive infections - Non invasive infections Host defenses Host defenses include the :- - Activation of the alternative and classical complement pathways and - Antibodies to the PRP capsule H. influenzae is susceptible to lysis by antibody and complement. Antibodies to the Hib capsule plays the primary role in conferring immunity against H. influenzae. These bactericidal antibody directed against PRP capsule of H. influenzae type h is entirely responsible for host resistance. Newborns have low risk of infection The mother possesses antibodies against the "b" capsule which she has acquired in her life time. She passes these antibodies to the fetus transplacentally and in her breast milk. These passively acquired antibodies last for about 6 months as a result infants are immune to H Influenzae infections. When these transplacental antibodies to the capsule wane infants are at high risk for developing invasive H. influenzae disease. It takes 3-5 years of hemophilus influenzae colonization and infection for children to develop their own antibodies. So there is a "window" during which children are prone to be infected with hemophilus influenzae.