immunoglobbins

Activation of the Immune Response

  • The immune response activation begins upon pathogen entry into the body.

    • Pathogen: Any organism, such as bacteria or viruses, that can cause disease.

  • Role of Macrophages:

    • Macrophages encounter pathogens and ingest them.

    • They process and display antigen fragments from the pathogens on their cell surfaces.

    • Antigen Presenting Cells (APCs): Macrophages displaying these antigen fragments are termed antigen presenting cells.

  • Interaction with T Helper Cells:

    • An antigen presenting macrophage interacts with a T helper cell that recognizes the same antigen.

    • Chemical Alarm Signal:

    • The macrophage releases interleukin-1 (IL-1).

      • Function of IL-1: Stimulates T helper cells to secrete interleukin-2 (IL-2).

    • Effect of IL-2: Causes the proliferation of cytotoxic T cells and B cells.

Immune Response Pathways

  • The immune response diverges into two main pathways:

    1. Cytotoxic T Cells (Cell-Mediated Immunity):

    • Infected normal body cells digest pathogens and display antigen fragments on their surfaces.

    • Millions of specific types of cytotoxic T cells are produced, each recognizing a specific antigen.

    • These T cells bind to infected cells and produce chemicals that kill them, thus destroying pathogens.

    1. B Cells (Humoral Immunity):

    • B cells also come in millions of varied types, each recognizing a particular antigen.

    • Upon activation by T helper cells:

      • B cells differentiate into plasma cells which produce antibodies.

        • Function of Antibodies: Bind to antigens on pathogens, marking them for destruction by macrophages.

      • Some B cells become memory B cells, remaining for years.

        • Importance of Memory B Cells: Enable a faster and stronger secondary immune response to subsequent infections by the same pathogen.

        • This response provides immunity post-infection or vaccination.

Concepts of Antigens

  • Definition of Antigens:

    • Antigens can be proteins, viruses, bacteria, energy sources (like worms), fungi, parasites, and even hormones.

    • Environmental substances such as chemicals can also be antigens.

    • Antigens include various structures:

    • Proteins

    • Peptides

    • Lipopolysaccharides

    • Lipids

    • Nucleic acids

  • Autoimmunity:

    • Occurs when normal immune cells mistakenly attack the body's own cells.

  • Immunogen:

    • A substance specifically inducing an immune response.

  • Epitopes:

    • Specific parts of an antigen recognized by the immune system (antibodies and T cell receptors).

Definitions: Immunogenicity vs. Antigenicity

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

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

  • Important Clarification: Not all antigens are immunogenic; all immunogens are antigens but not vice versa.

Classification of Antigens

  • Chemical Nature:

    • Most immunogens are protein in nature.

  • Hapten:

    • A small molecule that is not immunogenic alone but becomes immunogenic when attached to a larger carrier protein.

  • T-dependent vs. T-independent Antigens:

    • T-dependent antigens: Require T helper cells for activation.

    • T-independent antigens: Activate B cells directly without T cell help, typically polysaccharides or lipids and induce a weaker immune response (no memory cells formed).

  • Origin Based Classification:

    • Exogenous Antigen: Enter from outside sources (e.g., bacteria, viruses).

    • Endogenous Antigen: Originates from within cells (e.g., tumor antigens).

    • Autoantigen: Derived from the body's own proteins.

  • Super Antigen:

    • A large antigen eliciting a more intense immune response, activating a greater percentage of T cells (20-30% vs. 0.1% for normal antigens).

    • Example: Methicillin-resistant Staphylococcus aureus (MRSA) can act as a super antigen.

Factors that Influence Immunogenicity

  • Foreignness:

    • An antigen may be immunogenic in one species and not in another (e.g., bovine serum is non-immunogenic to cows but immunogenic to mice).

  • Chemical Composition:

    • Complex proteins are generally more immunogenic than simple ones.

  • Physical Form:

    • Particulate antigens tend to be more immunogenic than soluble ones.

  • Antigen Processing & Presentation:

    • Antigens that are readily phagocytosed are generally more immunogenic.

  • Genetic Factors:

    • Individual genetic traits can affect antigen recognition and responsiveness.

  • Dose:

    • The administration dose can also influence immunogenicity; optimal doses vary by age and weight.

  • Route of Administration:

    • Different routes impact the immune response:

    • Subcutaneous and intramuscular routes tend to be more effective than intravenous or intragastric routes.

  • Adjuvants:

    • Substances that enhance the immune response to an immunogen and are commonly mixed with vaccines.

Overview of Immunoglobulins (Antibodies)

  • Types of Immunoglobulins (5 main classes): - IgM

    • IgG

    • IgA

    • IgE

    • IgD

Structure of Antibodies

  • Shape: Y-shaped.

  • Isotypes based on structure:

    • IgM: Pentamer (five units).

    • IgA: Dimer (two units).

    • IgD, IgE, IgG: Monomers (one unit).

  • Regions of Antibodies:

    • Fab (Fragment antigen-binding): The region that binds antigens.

    • Fc (Fragment crystallizable): The tail region that interacts with cell receptors and proteins of the immune system.

  • Hypervariable Regions:

    • Allow the antibody to bind diverse antigens due to variability in antigen shapes.

Functions of Antibodies

  • Neutralization: Inactivity of toxins and viruses.

  • Opsonization: Mark pathogens for phagocytosis.

  • Activation of Complement System: Key in the classical pathway.

  • Involvement in Allergic Reactions: Particularly with IgE antibodies.

  • IgM: The first antibody produced in response to pathogens, effective in blood-borne infections but does not cross the placenta.

  • IgG: Main antibody in secondary immune response, provides passive immunity to the fetus.

  • IgA: Predominant in mucosal immunity; present in secretions like saliva and tears.

  • IgD: Acts as a B cell receptor and is found in low concentrations.

  • IgE: Associated with allergic responses and is bound to receptors on mast cells and basophils.

Additional Notes

  • Immunoglobulin Diversity: Generated in various organs such as bone marrow in mice or appendix in sheep; unique immunoglobulin (IgY) exists in birds.

  • Applications in Medicine: Knowledge of the immune response, antibody functions, and classifications is essential for developing vaccines and treating immune-related diseases.