Conjugate Vaccine Technology

A conjugate vaccine links a bacterial capsular polysaccharide covalently to an immunogenic carrier protein. This coupling recruits CD4 T-cell help for a polysaccharide-specific B cell, producing durable antibody responses even though T cells recognize carrier-derived peptides rather than the polysaccharide itself.

Why conjugation is needed

Unconjugated polysaccharides are mainly T-cell-independent antigens: their repetitive structures can cross-link B-cell receptors without conventional T-cell help. They generally induce limited class switching, affinity maturation and immune memory, and responses are especially weak in young children.

Mechanism of linked recognition

• A B cell with a polysaccharide-specific B-cell receptor binds the polysaccharide component and internalises the entire conjugate.
• The B cell degrades the carrier protein and presents a carrier-derived peptide on MHC class II molecules.
• A cognate helper T cell, particularly a T follicular helper cell, recognizes this peptide and provides CD40-CD40L interactions and cytokine signals.
• Because the B cell and T cell recognize different parts of the same linked antigen, this process is called linked recognition.

Formation of immune memory

T-cell help initiates a germinal-centre reaction, enabling immunoglobulin class switching, somatic hypermutation and affinity maturation. It generates polysaccharide-specific memory B cells and long-lived plasma cells, allowing faster, stronger responses to booster vaccination or later exposure.