Thymus-independent antigens
- Antigens lacking a peptide component (eg, lipopolysaccharides from gram ⊝ bacteria); cannot be presented by MHC to T cells. Weakly immunogenic; vaccines often require boosters and adjuvants (eg, pneumococcal polysaccharide vaccine).
Thymus-dependent antigens
- Antigens containing a protein component (eg, diphtheria vaccine). Class switching and immunologic memory occur as a result of direct contact of B cells with Th cells.