The Hib (Haemophilus influenzae type b) conjugate vaccine generates a T-cell dependent immune response. In contrast, the unconjugated Hib polysaccharide vaccine (HbPS) fails to protect infants <2 years. What is the immunological principle that explains why conjugation to a carrier protein (e.g., tetanus toxoid) overcomes this limitation?
- A Conjugation adds TLR ligands (danger signals) that activate innate immunity
- B Pure polysaccharide antigens are T-independent antigens (TI-2); conjugation converts them to T-dependent antigens, enabling germinal centre reactions, class switching, affinity maturation, and immunological memory ✓
- C Carrier protein provides additional epitopes that directly bind and activate B cell receptors specific for the polysaccharide
- D Conjugation increases antigen size beyond the threshold required for complement activation
Explanation
Bacterial polysaccharides are thymus-independent type 2 (TI-2) antigens that can directly cross-link B-cell receptors and stimulate IgM production without T-cell help, but do not generate germinal centre reactions, class switching to IgG, affinity maturation, or memory B cells. Infants <2 years lack mature marginal zone B cells responsible for TI responses. Conjugation links the polysaccharide to a carrier protein (tetanus toxoid, diphtheria CRM197, meningococcal OMP): the carrier peptides are processed and presented to CD4+ T cells, providing T-cell help via CD40L-CD40 interaction, enabling IgG class switching, affinity maturation, and long-lived immunological memory.
Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.