Microbiology || Antigens, Antibodies, and Antigen-Antibody Reactions || For Nursing student ||

 

Antigens, Antibodies, and Antigen-Antibody Reactions


Antigens

  • Definition: Antigens are substances that can provoke an immune response in the body. They are typically foreign molecules such as proteins, polysaccharides, lipids, or nucleic acids.
  • Types:
    • Exogenous Antigens: Enter the body from the external environment (e.g., bacteria, viruses, toxins).
    • Endogenous Antigens: Produced within cells, such as proteins resulting from viral infection or cancer.
    • Autoantigens: Normal body components that the immune system mistakenly identifies as foreign, leading to autoimmunity.
  • Structure: Antigens often have specific regions called epitopes or antigenic determinants, which are recognized by antibodies.

Antibodies

  • Definition: Antibodies, or immunoglobulins (Ig), are Y-shaped glycoproteins produced by B-lymphocytes in response to antigens.
  • Structure:
    • Consist of two heavy chains and two light chains linked by disulfide bonds.
    • Have variable regions that bind to specific antigens and constant regions that determine their class.
  • Classes:
    • IgG: Most abundant, provides long-term immunity.
    • IgA: Found in mucosal areas, protects against infections at these sites.
    • IgM: First antibody produced during an immune response.
    • IgE: Involved in allergic reactions and parasitic infections.
    • IgD: Plays a role in initiating B-cell activation.

Antigen-Antibody Reactions

  • Definition: Specific binding between an antigen and its corresponding antibody.
  • Mechanisms:
    • Neutralization: Antibodies block the active sites of toxins or prevent pathogens from binding to host cells.
    • Opsonization: Antibodies coat pathogens, making them easier to be phagocytized by immune cells.
    • Agglutination: Antibodies cause antigens to clump together, aiding in their removal.
    • Complement Activation: The antigen-antibody complex activates the complement system, leading to cell lysis.
  • Applications:

Hypersensitivity

  • Definition: Hypersensitivity refers to an exaggerated or inappropriate immune response to an antigen, resulting in tissue damage.
  • Types:
    1. Type I (Immediate Hypersensitivity):
      • Mediated by IgE antibodies.
      • Examples: Allergies, anaphylaxis.
      • Mechanism: Antigen triggers mast cells to release histamine and other mediators.
    2. Type II (Cytotoxic Hypersensitivity):
      • Mediated by IgG or IgM antibodies.
      • Examples: Hemolytic anemia, Rh incompatibility.
      • Mechanism: Antibodies target cells, leading to complement activation and cell destruction.
    3. Type III (Immune Complex Hypersensitivity):
    4. Type IV (Delayed-Type Hypersensitivity):
      • Mediated by T-cells.
      • Examples: Tuberculin reaction, contact dermatitis.
      • Mechanism: Cytokine release causes tissue damage.

Autoimmunity

  • Definition: Autoimmunity occurs when the immune system attacks the body's own tissues, mistaking them for foreign antigens.
  • Mechanism:
    • Breakdown of self-tolerance mechanisms.
    • Involves both genetic predisposition and environmental triggers.
  • Examples:
    • Systemic: Systemic lupus erythematosus (SLE), rheumatoid arthritis.
    • Organ-Specific: Type 1 diabetes, Hashimoto's thyroiditis.
  • Management:
    • Immunosuppressive drugs.
    • Symptom management (e.g., insulin for diabetes, thyroid hormones for hypothyroidism).

Immunization (Vaccines and Schedules)

Immunization

  • Definition: The process of inducing immunity against specific diseases by administering vaccines.
  • Types:
    • Active Immunization: Administration of antigens to stimulate the body’s immune response (e.g., vaccines).
    • Passive Immunization: Administration of pre-formed antibodies (e.g., immunoglobulin therapy).

Vaccines

  • Types of Vaccines:
    1. Live Attenuated Vaccines: Contain weakened forms of pathogens. (e.g., measles, mumps, rubella (MMR)).
    2. Inactivated Vaccines: Contain killed pathogens. (e.g., polio vaccine).
    3. Subunit Vaccines: Contain specific antigens of the pathogen. (e.g., hepatitis B vaccine).
    4. Toxoid Vaccines: Contain inactivated toxins. (e.g., tetanus, diphtheria vaccines).
    5. mRNA Vaccines: Use messenger RNA to produce antigens in the body. (e.g., COVID-19 vaccines).

Vaccination Schedule

  • Infants and Children:
    • BCG at birth.
    • Hepatitis B at birth, 6 weeks, and 6 months.
    • DPT, IPV, Hib, Hep B, Rotavirus, PCV at 6, 10, and 14 weeks.
    • Measles and MMR at 9 months and 15 months.
  • Adults:
    • Tdap every 10 years.
    • Influenza vaccine annually.
    • HPV vaccine for young adults (9–26 years).
    • COVID-19 booster as recommended.
  • Elderly:
    • Pneumococcal vaccine at 65 years.
    • Shingles vaccine after 50 years.


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