BIOL 1612 – Anatomy & Physiology II

CHAPTER 21 – The Immune System

 Immune system

• Nonspecific defenses
  • First line of defense – surface defenses
    • Skin
      • Intact skin epidermis
      • Acid mantle
      • Keratin
    • Intact mucous membranes
      • Mucus – traps microorganism in the digestive and respiratory passages
      • Nasal hairs – filters and traps microorganism
      • Cilia – propels debris away from the respiratory tract
      • Gastric juice – hydrochloric acid and proteases
      • Acid mantle of vagina – inhibits growth of microorganisms and fungi in the female reproductive tract
      • Tears and saliva – enzymes destroy bacteria
      • Urine – low pH cleanses the urinary tract
  • Second line of defense – internal defenses
    • Phagocytes – macrophages, neutrophils, mast cells, eosinophils
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    • Natural killer (NK) cells – lyse and kill cancer cells and virus-infected body cells; NK cells are not phagocytic – they perforate microbal cell membranes
    • Inflammatory response
      • Benefits of the inflammatory response
        • Prevents the spread of damaging agents to nearby tissues
        • Clears the area of cellular debris and pathogens
        • Encourages repair
      • Four cardinal signs of inflammation
        • Redness
        • Heat
        • Swelling
        • Pain
        • Impairment of function (fifth cardinal sign)
    • Antimicrobial proteins
      • Interferon – secretes PKR which helps protect cells that have not yet been infected
        • Types of interferon
          • Alpha
          • Beta
          • Gamma
      • Complement – enhances the immune response
    • Fever – abnormally high body temperature; activated by pyrogens secreted by macrophages and leukocytes
• Specific defenses
  • Important aspects of the adaptive immune response
    • It is specific
    • It is systemic
    • It has memory
  • Specific defenses respond to the presence of antigens
    • Antigen – substances that invoke the immune response
      • Classes of antigens
        • Complete antigens
          • Functional properties
            • Immunogenicity – able to stimulate proliferation of specific lymphocytes and antibodies
            • Reactivity – able to react with the activated lymphocytes and the antibodies released by the immune response
          • Examples of complete antigens – foreign proteins, nucleic acids, some lipids, large polysaccharides
          • Small molecules do not generate an immune response
        • Incomplete antigens (haptens)
      • Antigen structure
        • Antigen determinants – the parts of the antigen that are immunogenic
      • Self-antigens
        • Major-histocompatibility complex (MHC) protein markers
          • Two groups
            • Class I MHC markers – displayed on all cells except RBCs
            • Class II MHC markers – displayed on mature B-cells, some T-cells, and antigen-presenting cells
  • Specific defense system
    • Cell-mediated immunity
      • Cell populations associated with cell-mediated immunity
        • T lymphocytes – provide cellular/cell-mediated immunity
          • Helper T cells (aka, CD4 cells) – initiates humoral-mediated response
        • Cytotoxic T cells (aka, CD8 cells) – directly attack and kill other cells
        • Memory T cells – immunological memory
        • Suppressor T cells – end the immune response
        • Antigen-presenting cells (APCs) – engulf antigens and present the antigens on their own cell surface so that they can be recognized by T-cells.
    • Humoral Immunity (antibody-mediated)
      • Cell populations associated with humoral immunity
        • B lymphocytes (B cells)
          • Plasma cells
            • Produce antibodies (immunoglobulins)
              • Mechanisms of antibody action
                • Neutralization – covering up the toxic portions of bacteria viruses, or toxins
                • Agglutination – clumping of cells
                • Precipitation – forming insoluble substances (small molecules)
                • Complement fixation – causes lysis (rupture of cell membranes)
              • Antibody structure
                • Heavy chains (long)
                • Light chains (short)
                • Variable regions – altered for each specific antigen – contains the antigen binding site
                • Constant regions – remain the same
              • Classes of antibodies
                • IgD – found on the surfaces of B-cells
                • IgM – responds to antigens in food or bacteria – the first class of antibodies produced during the primary response
                • IgG – attacks bacteria, viruses, and toxins – the only class that crosses the placental barrier
                • IgA – found in exocrine secretions
                • IgE – found in exocrine secretions – integral in inflammatory and allergic responses
              • Monoclonal antibodies
          • Memory B cells – immunological memory
      • Types of humoral immunity
        • Active (lasts longer than passive)
          • Naturally acquired – contact with pathogen
          • Artificially acquired - vaccines
        • Passive
          • Naturally acquired – mother-to-fetus
          • Artificially acquired – gamma globulin

Immune Response – happens in two stages

• Primary immune response – occurs when the B-cell and T-cells first encounter a specific antigen
  • Triggers proliferation of the specific B-cell and T-cell populations
  • Produce antibodies for several weeks
  • Individual shows symptoms
• Secondary immune response – occurs when memory B-cells or memory T-cells are activated
  • Secondary response is faster and shorter in duration than the primary immune response

Organ Transplants and Prevention of Rejection

• Four major types of grafts
  • Autografts – tissue grafts transplanted from one body site to another in the same person
  • Isografts – grafts donated to a patient by a genetically identical individual
  • Allografts – grafts transplanted from individuals that are not genetically identical but belong to the same species
  • Xenografts – grafts taken from another animal species

Clinical Disorders or Diseases of the Immune System

• Immunodeficiencies – insufficient or abnormal immune cells, phagocytes, or complement that are produced. (Ex. AIDS)
• Autoimmune diseases – when the immune system attacks its own body parts
  • Triggers:
    • Ineffective or insufficient lymphocyte programming
    • Appearance of self-proteins that have not been previously exposed to the immune system
    • Cross-reaction of antibodies with self-antigens
  • Examples of autoimmune diseases
    • Multiple sclerosis
    • Myasthenia gravis
    • Grave’s disease
    • Type I diabetes mellitus
    • System lupus erythematosus
    • Glomerulonephritis
    • Rheumatoid arthritis
• Hypersensitivities
  • Immediate hypersensitivities – Ex. Anaphylaxis and atopy
  • Subacute hypersensitivities
  • Delayed hypersensitivities - Ex. Contact dermatitis