Topic 24- Blood and Immunity

I. Components of Blood

A. Plasma

• General Facts:

  • Humans have about 5 liters of blood.

  • Blood is crucial for:

    • Transport of nutrients, oxygen, ions, hormones.

    • Fluid balance between blood and tissues.

    • Pathogen defense as part of immunity.

  • Blood is composed of cells and plasma.

• Plasma Composition:

  • About 90% water.

  • Inorganic salts (electrolytes):

    • Functions:

      • Buffer blood pH.

      • Regulate osmosis between blood and interstitial fluid.

    • Key ions:

      • Calcium (Ca²⁺)

      • Magnesium (Mg²⁺)

      • Sodium (Na⁺)

  • Plasma proteins:

    • Albumin – maintains osmotic balance.

    • Immunoglobulins (antibodies) – immune defense.

    • Apolipoproteins – transport fats in blood.

    • Fibrinogen – clotting factor.

    • Additional role: regulate fluid exchange and buffer pH.

B. Cellular Components

• All blood cells are produced in bone marrow from multipotent stem cells.

• Three Major Types:

  1. Erythrocytes (Red Blood Cells, RBCs):

     • Structure:

       • Biconcave disc shape for high surface area.

       • No nucleus (in mature RBCs of mammals).

       • No mitochondria (use glycolysis for energy).

       • Packed with hemoglobin to bind and carry oxygen.

     • Function: Efficient oxygen transport.

  2. Leukocytes (White Blood Cells, WBCs):

     • Part of the immune system.

     • Defense against infections and foreign substances.

  3. Platelets (cell fragments, not whole cells):

     • Formed from pieces of megakaryocytes in bone marrow.

     • No nuclei.

     • Essential for clotting.

II. Blood Clotting

A. Platelet Plug Formation (Immediate Response)

• When a wound occurs:

  1. Blood vessel constricts to reduce blood flow.

  2. Collagen fibers under the vessel lining are exposed.

  3. Platelets:

     • Stick to exposed collagen.

     • Release chemicals to attract more platelets.

     • Form a platelet plug quickly to limit bleeding.

B. Fibrin Clot Formation (Permanent Clot)

• Longer-term repair:

  1. Clotting factors, calcium ions (Ca²⁺), and platelet compounds trigger a cascade.

  2. Prothrombin (with vitamin K) → converted to thrombin.

  3. Thrombin catalyzes fibrinogen → fibrin.

     • Fibrin forms long insoluble threads that strengthen the clot.

• Blood Clotting Disorders:

  • Hemophilia: Missing one clotting factor → can't form proper clots.

  • Thrombus: Excess clotting → dangerous blockage of blood flow.

    • In the brain → stroke.

    • In the heart → heart attack.

III. Immune System Overview

A. General Role

• The immune system protects the body from foreign invaders like bacteria, viruses, fungi, and parasites.

• All animals have some form of immune defense.

B. Two Major Branches

1. Innate Immunity (General, Immediate, Non-Specific)

   • Present from birth.

   • Includes physical barriers and general cellular responses.

2. Adaptive Immunity (Specific, Learned, Acquired)

   • Develops after exposure to specific pathogens.

   • Includes B cells and T cells that have memory.

IV. Innate Immunity

A. Barrier Defenses (First Line of Defense)

• Non-specific and always active.

• Types:

  • Skin:

    • Physical barrier.

    • Surface microorganisms provide protective competition.

    • Oil and sweat glands create an acidic environment (pH 3–5).

  • Mucous membranes: Trap pathogens at body entrances.

  • Ciliated cells: Line the respiratory tract; sweep out debris.

  • Stomach acid and digestive enzymes: Kill ingested pathogens.

  • Lysozymes: Enzymes in tears, saliva, etc., that break down bacterial cell walls.

B. Cellular Innate Defense (Second Line of Defense)

• Activated if a pathogen crosses physical barriers.

• Major Components:

  1. Phagocytic Cells:

     • Use toll-like receptors (TLRs) to recognize pathogen molecules.

     • Examples:

       • TLR3: Recognizes double-stranded RNA (viruses).

       • TLR4: Recognizes lipopolysaccharides (bacterial surfaces).

     • After recognition, pathogen is engulfed and destroyed.

  2. Natural Killer (NK) Cells:

     • Detect abnormal surface proteins on infected or cancerous host cells.

     • Destroy compromised host cells, not pathogens directly.

  3. Antimicrobial Peptides and Proteins:

     • Interferons:

       • Produced by virus-infected cells.

       • Warn neighboring cells to resist viral replication.

     • Complement System:

       • ~30 proteins in plasma.

       • Activated upon pathogen detection.

       • Lyses invaders by forming pores in membranes.

  4. Inflammatory Response:

     • Local response to injury or infection.

     • Steps:

       1. Wound triggers release of mast cell contents (histamine, cytokines).

       2. Blood vessels dilate (expand).

       3. Increased blood flow → redness, warmth, swelling (edema).

       4. Attracts phagocytes to engulf pathogens and debris.

       5. Clotting mechanisms help seal the wound.

     • Systemic inflammation:

       • If widespread → fever induced to enhance immune function.

Review Key Points:

• Plasma carries nutrients, hormones, and proteins critical for homeostasis and defense.

• RBCs transport oxygen; WBCs fight infections; platelets clot blood.

• Blood clotting involves a platelet plug followed by a fibrin clot.

• Innate immunity provides immediate, general protection through barriers and cellular responses.

• Inflammation is a key innate immune response to infection or injury.