A 2-year-old child receives her first dose of Hib conjugate vaccine (PRP-T). Which immunological mechanism explains why conjugate vaccines produce T-cell-dependent immunity, unlike plain polysaccharide vaccines, in children <2 years?
- A Conjugation increases the molecular weight of polysaccharide above 100 kDa to allow B-cell receptor cross-linking
- B The adjuvant aluminium hydroxide added to conjugate vaccines activates dendritic cells via Toll-like receptor 4
- C The protein carrier (tetanus toxoid) enables T-helper cell recognition, providing germinal centre reactions, class switching, affinity maturation and immunological memory ✓
- D Conjugation converts T-independent antigen type 1 to T-independent antigen type 2
Explanation
Pure polysaccharide antigens stimulate B cells directly (T-independent type 2 antigens) through cross-linking of surface immunoglobulin without T-cell help, producing predominantly IgM without class switching or memory — this response is absent/poor in children <2 years due to immaturity of marginal zone B cells. Conjugation to carrier proteins (tetanus toxoid, diphtheria toxoid CRM197, OMP) converts the polysaccharide into a T-dependent antigen; carrier-specific T helper cells are recruited, germinal centre reactions occur, affinity maturation and isotype switching to IgG ensue, and long-lived plasma cells/memory B cells are generated. This is the entire basis of infant Hib, PCV and meningococcal conjugate vaccine immunogenicity.
Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.