Learning Objective

Describe the differences between thymus-independent and thymus-dependent antigens with respect to structure, MHC presentation, immunogenicity, and ability to generate class switching and immunologic memory.

Thymus-Independent Antigens

Definition: Antigens lacking a peptide component, such as lipopolysaccharides (LPS) from Gram-negative bacteria and capsular polysaccharides.

MHC Presentation: Cannot be presented on MHC molecules → no T-cell help.

Immunogenicity:

• Produce weak immune responses.
• Primarily stimulates IgM production (minimal class switching).
• Generate poor immunologic memory.

Vaccine Implications: Often require boosters and adjuvants to enhance response.

• Example: PPSV23 (Streptococcus pneumoniae polysaccharide vaccine).

Thymus-Dependent Antigens

Definition: Antigens containing a protein component, such as diphtheria toxoid.

MHC Presentation: Processed and presented on MHC II to CD4⁺ T helper cells.

Immunogenicity:

• Enable class switching (e.g., to IgG, IgA, IgE).
• Support somatic hypermutation and affinity maturation.
• Generate strong immunologic memory.

Mechanism: Requires direct interaction between B cells and T helper cells via:

• MHC II–peptide presentation
• CD40–CD40L signaling
• Cytokine release

Activity