Defense and Immunity Comprehensive Notes

Defense and Immunity Overview

Non-Specific Defense Mechanisms

• Epithelial Barriers

  • Healthy intact skin and mucous membranes create a physical barrier.
  • Internal membranes of epithelial tissue protect organs/body systems and prevent the spread of pathogens from one system to another.
  • Epithelial cells produce secretions with antibacterial properties:
  • Acidic secretions can inhibit microbial growth.
  • Sticky secretions trap microorganisms.
  • Presence of antibodies and enzymes enhances antibacterial action.
  • Hair and cilia trap and sweep away potential pathogens.

• Phagocytes and Phagocytosis

  • Key phagocytic cells: Neutrophils and Macrophages.
  • They are attracted to sites of inflammation.
  • Engulf potential threats such as microorganisms, antigens, and debris:
  • Break down these threats using enzymes inside their cells.
  • Threats are not identified but are phagocytized.
  • Fragments of the threat are displayed on the surface for identification by T cells, stimulating adaptive immunity.
  • Also play a role in the inflammatory response.

• Antimicrobial Chemicals

  • Hydrochloric Acid: Found in gastric juice, kills microorganisms due to high acidity.
  • Lysozymes: Antibacterial enzymes present in tears, attack bacterial cell walls but are ineffective against viruses, fungi, or parasites.
  • Saliva: Helps remove food/debris that might promote microbial growth; contains enzymes.
  • Interferons: Released by T-cells and macrophages during viral infections to inhibit replication and spread.
  • Complement System: Approximately 20 proteins in blood activated by immune complexes and sugars on bacterial cell walls:
  • Binds to and destroys bacterial cell walls.
  • Stimulates phagocytosis and recruits phagocytic cells to infection sites.

• Inflammatory Response

  • Signs include: Redness, Pain, Heat, Swelling.
  • Caused by any form of tissue damage (infection, trauma).
  • Acute Inflammation: Lasts for days to weeks, generally beneficial for fighting infection.
  • Chronic Inflammation: Lasts months to years, can lead to tissue damage.
  • Increased Blood Flow:
  • Arterioles dilate due to histamine and serotonin release, contributing to heat, redness, and swelling.
  • Brings extra oxygen, nutrients, and circulating proteins.
  • Increased Tissue Fluid Formation (Edema):
  • Caused by increased capillary permeability; plasma proteins pull water into tissues via osmotic pressure.
  • Migration of Leukocytes: Neutrophils first to migrate, followed by macrophages; attraction due to chemicals at the site.
  • Increased Temperature: Inhibits pathogen growth and enhances phagocyte activity.
  • Pain: Caused by bradykinins and prostaglandins increasing nerve sensitivity; swelling compresses nerve endings.
  • Suppuration: Pus formation occurs in specific infections.

• Immunological Surveillance

  • Specialized lymphocytes called Natural Killer Cells circulate searching for abnormal host cells (infected or mutated).
  • They immediately destroy these abnormal cells.

Adaptive Immunity (Specific)

• Differentiated from Non-Specific Immunity by:
  • Specificity: T-cells and B-cells respond to only one antigen.
  • Memory: Exposure to an antigen allows for a stronger and faster response upon subsequent encounters.
  • Adaptation: Immune cells recognize healthy body cells and attack only non-self cells.
• Lymphocyte Development: Produced in red bone marrow and mature in lymphatic tissue; make up 20-30% of WBCs.

B-cells

• Produce antibodies against specific antigens. Antibodies (immunoglobulins) target pathogens including bacteria and viruses.

T-cells

• Programmed to recognize specific antigens and destroy infected cells:
  • Cytotoxic T-cells: Directly inactivate invader cells.
  • Helper T-cells: Facilitate both cell-mediated and antibody-mediated immunity.
  • Regulatory T-cells: Prevent autoimmunity.
  • Memory T-cells: Ensure a faster response during future encounters.

Antibody Mediated (Humoral) Immunity

• Mediated by B-cells located primarily in lymph tissue. B-cells respond directly to antigens, no APCs required:
  • Plasma Cells: Secrete large amounts of antibodies.
  • Memory B-cells: Recognize antigens upon re-exposure.
• Types of Antibodies:
  • Comprise 20% of plasma proteins, enter body fluids, neutralize toxins, attract T-cells, and activate complement.

Immune Responses

• Primary Immune Response: Initial exposure, slower response (mainly IgM).
• Secondary Immune Response: Faster and stronger (mainly IgG) due to Memory Cells.

Abnormal Immune Responses

• Hypersensitivity Reactions: Inappropriate immune response to usually harmless antigens:

  • Type 1: Anaphylaxis (IgE-mediated).
  • Type 2: Autoimmune responses (antibody-mediated).
  • Type 3: Immune complex-mediated reactions affecting organs.
  • Type 4: Delayed reactions (T-cell mediated).

• Acquired Immunodeficiency Syndrome (AIDS):

  • Caused by Human Immunodeficiency Virus (HIV).
  • Spreads through sexual contact or blood.
  • Affects Helper T-cells primarily, leading to severe immune deficiencies.
  • Initial flu-like symptoms followed by an asymptomatic period until severe immune dysfunction occurs, marked by an AIDS-defining illness.