Defense Against Infectious Diseases

Overview

• Topic: Defense Against Diseases (focus on infectious diseases).
• Covers: barriers (skin, mucous membranes), blood clotting, innate and adaptive immunity.
• Emphasis: how pathogens are recognized and eliminated, and how antibodies are produced.

Primary Defenses (Physical Barriers)

• Skin: outer layer consists of dead cells forming an impenetrable barrier.
• Mucous Membranes: line openings (eyes, ears, mouth, nose, reproductive and excretory openings).
• Mucus: sticky secretion that traps pathogens and prevents entry to bloodstream.
• Wounds: create entry points for pathogens; clotting needed to seal and prevent infection.

Clotting Cascade (Sealing Wounds)

• Platelets: attach to wound and release clotting factors.
• Prothrombin → Thrombin: clotting factors activate prothrombin into thrombin.
• Fibrinogen → Fibrin: thrombin converts soluble fibrinogen (from liver) into insoluble fibrin.
• Fibrin Mesh: forms a network trapping blood cells and platelets to form a clot.

Overview Of Immune System Branches

• Primary defense (skin, mucous membranes) is distinct from immunity.
• Two immunity types:
  • Innate Immunity: non-specific, constant throughout life.
  • Adaptive Immunity: specific, builds memory and improves with age.

Key Terms And Definitions

• Pathogen: organism or virus that causes infectious disease (bacteria, fungi, protozoa, viruses).
• Antigen: recognition protein (often glycoprotein) on cell or virus surface; identifies "self" vs "nonself".
• Antibody: Y-shaped protein produced by lymphocytes; binds specific antigens.
• Phagocyte (phagocyte/macrophage/fagocyte): innate immune cell that engulfs and digests nonself material.
• Lymphocyte: adaptive immune cell (T cells, B cells) located in lymph nodes and bloodstream.
• Clonal Selection: mitotic cloning of a specific B cell after activation.
• Plasma Cell: differentiated B cell that secretes large amounts of antibodies.
• Memory Cell: differentiated B cell that remains to provide faster response on reinfection.
• Antigen Presentation: display of pathogen fragments on phagocyte surface to activate T cells.

Innate Immunity (Phagocytes)

• Phagocytes are nonspecific defenders that recognize antigens indicating "nonself".
• Mechanism:
  • Recognition of nonself antigens on particle or cell.
  • Engulfment via endocytosis into a vesicle.
  • Fusion with lysosome containing digestive enzymes.
  • Destruction of pathogen inside phagocyte.
• Also perform antigen presentation by displaying pathogen fragments on their surface.

Adaptive Immunity (Lymphocytes And Antibodies)

• Main focus: production of specific antibodies to antigens.
• Lymphocytes reside in lymph nodes and circulate in blood.
• Antibodies:
  • Y-shaped proteins with antigen-specific binding sites.
  • Functions: tag pathogens for destruction or block antigens from enabling infection.
• Specificity requires many different lymphocyte types ready to respond to varied antigens.

Steps In Antibody-Mediated Adaptive Response

• 1. Phagocyte action: engulfs pathogen and presents antigen on its surface.
• 2. Helper T Cell Activation: recognizes antigen presentation and becomes activated.
• 3. B Cell Identification: activated helper T cell locates the specific B cell that can make the matching antibody.
• 4. Clonal Selection: correct B cell undergoes many mitotic divisions to increase numbers.
• 5. Differentiation:
  • Some B cells → plasma cells that produce and secrete antibodies.
  • Some B cells → memory cells for long-term immunity.
• Plasma cells develop extensive rough ER and Golgi for protein (antibody) synthesis and secretion.

Blood Groups And Immune Recognition (Example)

• Blood cell antigens are recognized as "self" by body (e.g., type B blood has B antigens).
• Individuals produce antibodies against antigens they lack (e.g., type B person produces anti-A antibodies).
• Transfusion with incompatible blood triggers immune response due to antigen mismatch.

Key Points Summary Table

Action Items / Next Steps (If Studying)

• Review antigen-antibody binding specificity and examples.
• Practice drawing the clotting cascade conversions (prothrombin→thrombin→fibrin).
• Memorize roles of phagocytes, helper T cells, B cells, plasma cells, and memory cells.
• Apply knowledge to blood transfusion compatibility problems.