Antigens: The Structure, Function, and Immune Response

Classify antigens by origin, focusing on exogenous antigens that enter via inhalation, ingestion, or injection, and highlight antigen presenting cells (APCs) that present these foreign antigens to trigger immune responses.

Endogenous antigens are generated inside the cell through normal metabolism, including anabolism and catabolism, and may arise from viral or intracellular bacterial infection, reflecting intracellular antigen production.

Exogenous antigens processed by APCs and presented on MHC II to CD4 T cells; endogenous antigens formed in infected cells and presented on MHC I to CD8 T cells.

Antigen presenting cells capture exogenous antigens, process them, and present peptide fragments to T-lymphocytes and B-lymphocytes, driving adaptive immunity via MHC class I and II pathways.

Explore the major histocompatibility complex (MHC), or HLA, on human chromosome 6 and mouse chromosome 17, with class I, II, and III molecules governing antigen presentation to T cells.

Discusses T-cell classes: CD4 helper cells, MHC II restricted, signaling via cytokines and surface proteins to boost antibodies; CD8 cytotoxic cells, MHC I restricted, killing infected or altered self cells.

Self antigens, or auto antigens, are normal body components tolerated by the immune system; when tolerance fails, antibodies target self, causing autoimmune diseases, and even cancer cells can become immunogenic.

Define neoantigens as novel protein fragments from tumor-specific mutations unique to cancer cells, triggering strong immune responses and enabling targeted, personalized cancer immunotherapy with less damage to healthy tissues.

Classify antigens by mechanism into t cell dependent and t cell independent types. T dependent antigens require antigen-presenting cells and helper t cells to activate b cells, while t independent antigens stimulate b cells directly with no memory formation or class switching.

T-cell dependent antigens are complex proteins requiring helper T cells, enabling memory, antigen processing, and class switching; T-cell independent antigens are simple, triggering polyclonal B cell activation with no memory.

Identify super antigens as bypassing antigen-presenting cells to activate T cells directly. They interact with the VB region of the TCR, triggering non-specific cytokines and polyclonal B cell activation.

Explain immunogenicity versus antigenicity, noting that all immunogens are antigens but not all antigens immunogenic; define epitopes as the smallest antigenic determinants binding paratopes and sensitizing T and B cells.

Compare sequential (linear) and conformational (non-sequential) epitopes and how B cells and T cells recognize them, including accessible and inaccessible determinants, neo antigens, and effects of folding and denaturation.

Antigenic variation alters surface antigens to evade host memory cells and enable reinfection. It uses phase variation, multi-copy genes, and epigenetic controls to express a single variant at a time.

Explore cross reactivity, where a single paratope or a population of antibodies reacts with multiple epitopes due to shared oligosaccharide residues and ABO blood group antigens.

Antigen–antibody interactions are highly specific, governed by epitope–paratope binding and affinity, leading to irreversible, non-covalent associations that drive in vivo and in vitro responses.

Non-covalent bonds drive antigen-antibody binding, with electrostatic interactions, hydrogen bonding, hydrophobic interactions, and van der Waals forces operating at close proximity and yielding reversible binding.

Define affinity as the strength of antigen–antibody interaction, measured by Ka, involving epitope and paratope. Describe avidity as strength from affinity, valency, and structure, noting multivalency and IgG interactions.

explains the specificity of antibody-antigen reactions, detailing agglutination and precipitation, including prozone phenomena and post zone phenomena, zone of equivalence, and applications in blood grouping, serotyping, and serological tests.

Explain how vaccines use protein antigens with adjuvants, preservatives, stabilizers, and inactivation processes to safely train the immune system, develop memory cells, and ensure long shelf life.

Preservatives prevent microbial growth and contamination in vaccines, especially for multi-dose vials; manufacturing advances reduce their use, while stabilizers like sugar or gelatin help maintain integrity.

Explore types of vaccines—live attenuated, killed, toxoid, DNA, and recombinant—and distinguish heterologous (generic) from homologous vaccines with examples like cowpox and BCG, plus routine vaccines and boosters.

Assess vaccine considerations, including safety, immunogenicity, protection duration, booster need, routes of administration, antigenic variation, target populations, storage, and herd immunity.

This lecture reviews tumor antigens, detailing tumor-specific transplantation antigen and tumor-associated transplantation antigen, and outlines cancer immunotherapy strategies and hypersensitivity types one to four.