Immunogens and Antigens Quiz

Immunogens and Antigens

• Immunogens are substances that stimulate a specific immune response (humoral or cell-mediated immunity).
• Antigens are substances that can bind specifically with antibodies or sensitized T lymphocytes.
• Epitopes are small, specific parts of an antigen that bind to antibody or T-cell receptors; they are also called antigenic determinants.
• Paratope refers to the antigen-binding site on an antibody.
• Haptens are small molecules incapable of inducing an immune response on their own but become immunogenic when coupled to a larger carrier protein.

Factors Affecting Immunogenicity

• Foreignness: The immune system generally recognizes and responds to foreign substances, not self-components.
• Molecular size: Molecules with a molecular weight of less than 5000-10,000 generally have weak or no immunogenicity.
• Chemical nature: Proteins are the most potent immunogens, followed by complex polysaccharides.
• Degradability: Immunogens are susceptible to enzymatic degradation during antigen processing by antigen-presenting cells.
• Methods of antigen administration:
  • Dosage: Very high or very low doses can induce tolerance. Repeated administration of booster doses stimulates a strong immune response.
  • Route: Parenteral routes (e.g., injection) are preferred for inducing strong immune responses compared to oral routes.
  • Adjuvants: Substances that enhance the immune response to an antigen.
• Host genetic factors: Individuals exhibit different responses to antigens: strong responders, weak responders, non-responders.

Forms of Antigen-Antibody Binding

• Lock and Key: The most efficient immunological responses involve high-affinity antibodies that perfectly match the antigen.
• Low Affinity: Antibodies can sometimes bind to antigens with a "poor fit," leading to weaker reactions.

Heterophile Antigens and Antibodies

• Cross Reactivity: Antibodies can react with similar epitopes on different antigens, sometimes leading to unintended immune responses.
• Heterophile Antibodies: Antibodies produced against one antigen that can also bind to a different, unrelated antigen.

Laboratory Tests Based on Heterophile Antigens

• Paul-Bunnell Test: Used to diagnose infectious mononucleosis (Epstein-Barr Virus) by detecting heterophile antibodies that agglutinate sheep red blood cells.
• Cold Agglutinins: Used to detect antibodies in patients with mycoplasma pneumonia that agglutinate human group O red blood cells at low temperatures.
• VDRL and RPR Tests: Used to detect antibodies against cardiolipin antigen (reagin) in sera of patients with syphilis.

Types of Antigens According to Source

• Bacterial Antigens: Include capsular, flagellar, soluble (e.g., exotoxins, enzymes), and cell-wall antigens.
• Viral Antigens: Include viral proteins and genetic material. Some viral antigens can activate multiple clones of T lymphocytes leading to increased cytokine secretion.
• Human Tissue Antigens: Include blood group antigens (A, B, RH) and histocompatibility antigens (MHC/HLA) present on tissue cell membranes.
  • MHC I: Present on all nucleated cells.
  • MHC II: Primarily expressed on antigen-presenting cells.

Test your understanding of immunogens, antigens, and the factors affecting immunogenicity. This quiz covers key concepts such as epitopes, paratopes, and the characteristics that influence the immune response. Challenge yourself to see how well you grasp the fundamentals of immunology.