Immunity

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Innate Immunity

• Based on genetic make-up
• Relies on already formed components
• Rapid response: within minutes of infection
• Not specific
  • Same molecules/cells respond to a range of pathogens
• Has no memory
  • Same response after repeated exposure
• Does not lead to clonal expansion
  • Clonal expansion: explosive increase in number of lymphocytes

Mechanisms

• Mechanical barriers/surface secretion
  • Skin
  • Acidic pH in stomach
  • Cilia
• Humoral mechanisms (mediated by macromolecules, e.g. cells)
  • Lysosomes, basic proteins, complement, interferons 
• Cellular defence mechanisms
  • Natural killer cells
  • Neutrophils
  • Macrophages
  • Mast cells
  • Basophils 
  • Eosinophils

Adaptive Immunity

• Based on resistance acquired during life
• Relies on genetic events and cellular growth
• Responds more slowly, over a few days
• Is more specific
  • Each cell responds to a single epitope on an antigen
• Has anamnestic memory
  • Repeated exposure leads to faster, stronger, response
• Leads to clonal expansion

Mechanisms

• Cell-mediated immune response (CMIR)
  • T-lymphocytes
  • Eliminate intracellular microbes that survive within phagocytes or other infected cells
  • Doesn’t use antibodies
• Humoural Immune Reponse (HIR)
  • B-lymphocytes
  • • Mediated by antibodies
  • Eliminate extra-cellular microbes and their toxins

Cell-mediated immune response

1. T-Cell
   • Recognises peptide antigen on macrophage in association with major histo-compatibility complex (MHS) class
   • Identifies molecules on cell surface
   • Helps body distinguish self from non-self
2. T-cell goes into effectors cell stage that is able to kill infected cells
   • Activation of phagocytes, t-lymphocytes and cytokines in response to an antigen
   • Doesn’t involve antibodies

Humoral Immune Response

1. B lymphocytes recognise specific antigens
   • Proliferate and differentiate into antibody-secreting plasma cells
2. Antigens bind to specific antigens on microbes: destroy microbes via specific mechanisms
3. Some B lymphocytes evolve into the resting state: memory cells
   • Mediated by macromolecules, e.g. antibodies, located in extracellular fluids
   • Uses antibodies

Once foreign antigen reaches the lymphoid tissue:

	Antibody Mediated (Humoral)	Cell Mediated
Main Cells involved	B-Cells	T-Cells
Where do cells develop	Produced and mature in the bone marrow	Produced in the bone marrow and mature in the thymus gland
Antibodies produced?	YES	NO
How are pathogens identified?	Antigens in the blood	Antigens on the surface of infected cells
How are pathogens killed?	Antibodies combine with antigen (antibody-antigen complex) and destroy or inactivate it	Killer T-Cells destroy the antigen, helper T-cells promote phagocytosis, suppressor T-cells inhibit T- and B-cell activity when infection has been dealt with

Immunisation 

• Naturally acquired immunity is acquisition of adaptive immunity through natural events
• Immunisation mimics these events by inducing artificially acquired immunity
• Natural or artificial immunity can be divided into
  • Active immunity
  • Passive immunity

Active immunity

• Immunity that is attained from exposure to an antigen/pathogen
• Results from immune response upon exposure to an antigen
• Active immunity can develop naturally
  • Following illness
• Or artificially
  • After immunisation
• Better than passive immunity
  • Body has already gone through the motion of defeating the pathogen
  • Don’t need to go through the infection itself, as vaccine has deactivated version of pathogen/virus

Passive Immunity

• Immunity that is acquired from antibodies being introduced into the body from an external source
• Occurs naturally during pregnancy
  • IgG from mother crosses placenta
    • Infers protection to the baby
• Occurs naturally as a result of breast-feeding
  • IgA antibodies in breast milk given to child
• Artificial passive immunity involves transfer of antibodies produced by another person or animal
  • Can be used to prevent disease before or after likely exposure

	Active Immunity	Passive Immunity
Natural	clinical, sub-clinical infection	via breast milk, placenta
Artificial	Vaccination: Live, killed, purified antigen vaccine	immune serum, immune cells