Haemophilus influenzae is a gram-negative bacteria that is present in approximately three-quarters of children and adults. In infants and young children, Haemophilus influenzae, especially type B, may cause bacteraemia and meningitis. In children and older individuals Haemophilus influenzae may cause respiratory tract infections. One of the great advances in modern medicine has been the development of an effective vaccine against HIB. A patient's immunological response to HIB vaccine can be determined by measuring anti-HIB IgG antibody using this EIA technique.