The second type of thymus-independent antigens, called TI-2 antigens, activate only mature B cells. TI-2 antigens are typically composed of repetitive carbohydrate or protein epitopes present at high density on the surface of a microorganism. Typical antigens of this kind are bacterial cell wall polysaccharides. It is thought that they cross-link the B cell receptors and co-receptors to such an extent that the need for additional signals is overridden.
Human B cells develop their full function only when a person is 5 years old, perhaps explaining why infants have a poor antibody response to polysaccharide antigens. TI-2 antigens induce both IgM and IgG but have no long-term immunological memory and thus provide no long-lasting immunity against re-infection.
(Text taken from “the immune system”, Parham, Garland Publishing 2000 page 163.)
Pneumococcal vaccines need to be given:
Extra info: Close The pneumococcal vaccine is given yearly based on two factors. 1) the vaccine does not confer long-term immunity and 2) the composition of the serotypes changed each year depending on risk.