Specific

Specific Acquired Immunity

  • Presented by Dr. Rawah Faraj, DVM, MSc., Ph.D.

  • Course: VMED 809A Immunology, Spring 2026

  • Email: rfaraj@tuskegee.edu

Antigens

  • Definition: Substances that can induce an immune response and are disease-causing.

  • Types of Antigens:

    • Cell membrane components

    • Nucleic acid

    • Cell sap

    • Enzymes

    • Cell wall

    • O antigens

    • H antigens (Flagellar antigens)

    • F antigens (Pili)

    • K antigens (Capsule)

Definitions of Key Immunology Terms

  • Antigen (Ag): Any substance or foreign particle that stimulates the immune system to produce antibodies.

    • Sources include:

    • Bacteria, viruses, toxins, parasites, chemicals, pollen

    • Can be various biomolecules: proteins, peptides, lipids, polysaccharides, nucleic acids, or even self-cells in autoimmune disorders.

  • Immunogen: A substance that induces a specific immune response.

  • Epitope (Antigenic Determinant): The specific part of an antigen recognized by antibodies.

Types of Antigens Based on Functionality

  • Tolerogen: An antigen that induces immunologic tolerance.

  • Allergen: An antigen that causes an anaphylactic reaction, where mast cells release granules rapidly. Common allergens include:

    • Certain medicines

    • Flower pollen

    • Seafood

    • Toxins from insect bites

    • Animal hair

    • Dust and dust mites.

  • Tumor Antigens: Antigens presented by MHC class I molecules on tumor cells.

    • Distinction between tumor-specific antigens (TSAs), which arise from tumor-specific mutations, and those presented by normal cells.

  • Autoantigens: Normal proteins or DNA/RNA complexes recognized by the immune system in autoimmune diseases due to loss of immunological tolerance.

Immunogenicity vs. Antigenicity

  • Immunogenicity: The ability of an antigen to induce an immune response.

  • Antigenicity: The ability of a molecule to bind and react with immune response products (antibodies or T cell receptors).

  • Note: Not all antigens are immunogens; however, all immunogens are antigens.

Classification of Antigens (Ags)

  1. By Immunogenicity:

    • Complete Antigens: Immunogens that can stimulate an immune response independently. Typically proteins, large molecules (e.g., bacteria, viruses).

    • Incomplete Antigens (Hapten): Smaller molecules that are not immunogenic alone but become so when coupled with larger carrier proteins (e.g., penicillin).

  2. By Chemical Nature:

    • Proteins: Most potent immunogens (e.g., bacterial exotoxins).

    • Polysaccharides: Found on bacterial capsules (e.g., pneumococcal polysaccharides).

    • Lipids: Poorly immunogenic but antigenic when bound to proteins (e.g., lipopolysaccharides from Gram-negative bacteria).

    • Nucleic Acids: Weak antigens that can be immunogenic in autoimmune diseases (e.g., anti-DNA antibodies in lupus).

  3. Dependence on T Cells:

    • T-Dependent Antigens: Require help from CD4+ helper T cells for B cell activation. Generally protein-based.

      • Induce stronger and long-lasting immunity (like memory B cells).

    • T-Independent Antigens: Stimulate B cells without T cell help (usually polysaccharides/lipids).

      • Result in weaker immune responses with no memory cell formation.

  4. By Origin:

    • Exogenous Antigens: Enter the body from external sources (e.g., bacteria, viruses, toxins).

      • Processed by antigen-presenting cells (APCs) and presented via MHC-II to CD4+ T helper cells.

    • Endogenous Antigens: Generated internally (e.g., viral proteins from infected cells, tumor antigens).

      • Presented via MHC-I to CD8+ cytotoxic T cells.

    • Autoantigens: Self-antigens mistaken for foreign in autoimmune diseases (e.g., DNA in systemic lupus erythematosus).

    • Neoantigens: New antigens from mutations in tumor cells (e.g., p53 mutations in cancer) and targeted in cancer immunotherapy.

Antigen Processing & Presentation

  • Exogenous Ags

    • Engulfed by APCs, processed, and presented via MHC II.

  • Endogenous Ags

    • Produced within cells, presented via MHC I.

Superantigens

  • Superantigens (SAgs): A class of antigens that excessively activate the immune system leading to non-specific T cell activation, polyclonal T-cell proliferation, and massive cytokine release.

  • Produced by certain pathogens as a defense mechanism against the host immune system. Examples include:

    • Staphylococcal enterotoxins

    • Toxic shock syndrome toxin (TSST-1)

    • Streptococcal pyrogenic exotoxins.

Factors Influencing Immunogenicity

A. Properties of Immunogen

  1. Foreignness: Immune systems typically only react to foreign molecules, e.g., Bovine serum albumin is not immunogenic in cows but is in rabbits.

  2. Molecular Size: Generally, molecules less than 5,000-10,000 Da (such as insulin) are weak immunogens; larger molecules are typically more immunogenic.

  3. Chemical Composition: More complex substances (especially proteins) tend to be more immunogenic.

  4. Physical Form: Particulate antigens are generally more immunogenic than soluble ones; denatured forms are often more immunogenic than native ones.

  5. Susceptibility to Processing: Antigens that are easily phagocytosed tend to be more immunogenic.

B. Biological System of Host

  1. Genetic Factors: Species or individuals may lack or have variations in genes necessary for receptor recognition on B and T cells.

  2. Age: Immunogenic response capability can vary, particularly weakened in very young and very old individuals.

C. Method of Administration

  1. Dose: An optimal level of antigen administration is crucial; too low or high may not elicit a sufficient immune response.

  2. Route: Routes such as subcutaneous and intramuscular are often more effective than intravenous or intra-gastric for eliciting immune responses.

  3. Adjuvants: Substances that enhance the immune response to an immunogen are known as adjuvants.

Study Guide Questions

  • Describe the classifications and properties of antigens.

  • Compare antigenicity vs immunogenicity.

  • Identify factors that influence immunogenicity.

  • Distinguish between superantigens and conventional antigens.