Antigens are the key players in immune responses, with their structure and characteristics determining how effectively they trigger immunity. From size and complexity to foreignness and stability, various factors influence an antigen's ability to stimulate the immune system.

T-cell and B-cell epitopes differ in location, size, and recognition mechanisms. Understanding these differences is crucial for grasping how the immune system processes and presents antigens, leading to targeted responses against pathogens and other foreign substances.

Antigen Structure and Characteristics

Characteristics of immunogenic antigens

• Size impacts immunogenicity with optimal molecular weight range 10,000 to 100,000 daltons enhances immune recognition (proteins)
• Complexity refers to chemical composition and structural features influencing antigen processing (carbohydrates, lipids)
• Foreignness determines immune response strength based on difference from self-molecules (bacterial cell wall components)
• Stability affects antigen persistence in body resisting degradation prolonging immune system exposure (viral capsid proteins)
• Repetitive structural motifs enhance immune cell recognition increasing binding avidity (bacterial flagella)
• Accessibility of exposed regions facilitates immune cell interaction crucial for effective response (surface glycoproteins)

T-cell vs B-cell epitopes

• B-cell epitopes located on intact antigen surfaces recognized by B-cell receptors and antibodies (hemagglutinin on influenza virus)
• T-cell epitopes are linear peptide fragments presented on MHC molecules recognized by T-cell receptors (internal viral proteins)
• Epitope size varies B-cell epitopes larger (5-15 amino acids) T-cell epitopes smaller (8-11 for MHC I, 13-17 for MHC II)
• B-cell epitopes often conformational depending on 3D structure while T-cell epitopes are linear sequences
• Recognition mechanisms differ B-cells directly bind epitopes T-cells require antigen processing and MHC presentation

Antigen processing and presentation

• Antigen uptake occurs through various mechanisms:
  1. Phagocytosis engulfs large particles (bacteria)
  2. Receptor-mediated endocytosis targets specific antigens (viruses)
  3. Pinocytosis captures soluble antigens (toxins)
• Intracellular processing involves:
  • Endosomal pathway breaks down exogenous antigens (extracellular pathogens)
  • Proteasomal pathway degrades endogenous antigens (intracellular pathogens, tumor antigens)
• MHC loading couples processed peptides with appropriate MHC molecules:
  • MHC class II presents exogenous antigens to CD4+ T cells
  • MHC class I presents endogenous antigens to CD8+ T cells
• Peptide-MHC complex transport moves assembled complexes to cell surface for T cell interaction
• Presentation to T cells involves TCR recognition and co-receptor engagement initiating immune response

Cross-reactivity in immune responses

• Cross-reactivity occurs when antibodies or T-cells recognize similar epitopes on different antigens (seasonal flu strains)
• Molecular basis stems from structural similarity between epitopes and flexibility in antigen-binding sites
• Positive implications include broader pathogen protection and potential for cross-protective vaccines (HPV types)
• Negative implications involve autoimmune reactions and allergic cross-reactions (pollen and fruit allergies)
• Factors influencing cross-reactivity include epitope similarity threshold and affinity of antibody or T-cell receptor
• Cross-reactivity plays role in vaccine design aiming for broad-spectrum protection (universal flu vaccine research)