Immune Response Mechanisms

Immune System

• The immune system helps to guard against disease and tissue damage
• Immune cells are distributed throughout the body, but are highly concentrated in blood and lymphatic tissue

Functions of the Immune System

• Defense: against invading microorganisms
• Clean up: removes damaged tissue
• Surveillance: identifies and destroys abnormal body cells

Lymphoid Tissue

• Bone Marrow: site of white blood cell production
• Lymphatic Vessels: carry lymph from tissues to lymph nodes and lymph organs before returning to blood
• Lymph Nodes, Tonsils, MALT, appendix: collections of lymphocytes for immune response and macrophages to remove debris
• Spleen: replaces worn out blood cells, collections of lymphocytes for immune response and macrophages to remove debris
• Thymus: site of T-lymphocyte (T-cell) maturation

White Blood Cells

• White Blood Cells (leukocytes) make up less than 1% of total blood volume
• General Functions: major cells of the immune system, internal defense against foreign cells, remove debris, destroy cancer cells
• 5 Types: Neutrophils, Lymphocytes, Monocytes, Eosinophils, Basophils

Leukocyte Type and Appearance

• Granular leukocytes take up certain dyes to look grainy under the microscope
• Agranular leukocytes appear smooth
• The shape of the nucleus and the size of the cells are also characteristic

Functions of WBCs

• Neutrophils: phagocytic, engulf and destroy bacteria, clean up debris
• Lymphocytes:
  • B-cells: produce antibodies against foreign cells or viruses
  • T-cells: kill infected cells, cancerous cells
• Monocytes: phagocytic, clean up debris, foreign material
• Eosinophils:
  • allergic reactions
  • attach to and kill parasites
• Basophils:
  • Histamine: allergic response
  • Heparin: fat removal, prevent clotting

Infected Body Cells

• Can be infected by:
  • Virus
  • Mutated cancer cells
  • Transplanted donor cells
• Killed by:
  • Lysis
  • Granzymes that induce apoptosis

Helper T Cells (CD4)

• Recognize APC MHC II antigen presenting cells
• Release cytokines, chemicals that are necessary to activate other immune cells
• Activate:
  • B cells
  • Cytotoxic cells
  • Attract neutrophils and macrophages

Regulatory T Cells (CD4-25)

• Inhibit innate and adaptive immune responses to keep the system in check
• Under research for autoimmune disease control

Hypersensitivity

• Abnormal, vigorous immune responses (overzealous immune system)
• Immediate hypersensitivity: occurs within seconds to 20 minutes
• Allergies and Asthma:
  • Plasma cells produce IgE antibodies which attach to Mast cells
  • Mast cells release histamine, which causes inflammation and itching
  • Also releases non-specific immunity

Adaptive Immune System

• Antigen-specific: requires the production of specific lymphocytes and antibodies against a specific antigen
• Systemic: not restricted to the initial infection site
• Has memory: second encounter causes a more rapid and vigorous response

B-cells and T-cells

• Adaptive immune responses are mediated by B cells and T cells
• B-lymphocytes (B-Cells):
  • Born and mature in the Bone Marrow
  • Housed in lymphoid tissues
  • Recognize free pathogens (bacteria, toxins, viruses)
  • Involved in Antibody-mediated Immunity
• T-lymphocytes (T-cells):
  • Born in Bone Marrow, mature in thymus
  • Housed in lymphoid tissues
  • Recognize infected or cancerous body cells
  • Involved in Cell-Mediated Immunity
  • Involved in activating total, combined immune responses

Immunocompetent B or T cells

• Immunocompetent: cells display a unique type of receptor that responds to a distinct antigen
• Become immunocompetent before they encounter antigens they may later attack
• Are exported to secondary lymphoid tissue where encounters with antigens occur
• Mature into fully functional antigen-activated cells upon binding with their recognized antigen

Antigens

• Any molecule or partial molecule that triggers an immune response
• Examples: cell proteins, carbohydrates, lipids that are part of bacterial cell structure, bacterial released toxins
• B-cells and T-cells have specific receptors for specific antigens
• 2 trillion lymphocytes in the body, each lymphocyte can only recognize ONE out of 100 million possible types of antigen

B-Cells

• Pathogen invades the body
• Specific B-Cell with pathogen receptor recognizes specific pathogen
• Specific B-Cell differentiates into:
  • Plasma Cells: produce specific antibodies for that pathogen
  • Memory Cells: store memory of antigens to produce antibodies upon later infection, lay dormant until the 2nd infection by the same pathogen

Antibodies

• Also called immunoglobulins
• Y-shaped proteins that recognize specific antigens
• 5 types:
  • IgM: stays on B cell to act as BCR for antigen
  • IgG: MOST COMMON, secreted to respond to most antigens
  • IgE: secreted to respond to parasitic worms, allergic responses
  • IgA: secreted mostly in mucous membranes
  • IgD: stays on B cell, function unknown