Immunisation and the maturation of infant immune responses.

Maturation of the adaptive immune system occurs early in development and by 14 weeks gestation the developing foetus has circulating differentiated T cells and B cells capable of responding to antigen. While the immune system is capable of responding appropriately at birth to protein antigens, its capacity to respond to carbohydrates is limited. A consequence of this is the success of bacteria with carbohydrate capsules in causing invasive paediatric infections. Vaccines containing purified carbohydrates alone are thus limited in their immunogenicity and fail to provide protection for those most at risk. Conjugate vaccine technology, where a carbohydrate antigen is chemically coupled to a protein carrier, has overcome the limitation of carbohydrates as vaccine antigens. The first such vaccine to enjoy widespread use, the Haemophilus influenzae type b conjugate vaccines, have met with enormous success and have almost eradicated invasive Hib disease in those countries where their use in infancy is routine. Conjugate technology is thus being applied to a number of other vaccines in development, including Neisseria meningitidis groups A and C and Streptococcus pneumoniae vaccines.