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Diseases > Immune System
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Defense System

  • Skin is composed of dead cells containing the indigestible protein keratin
  • Sebum produced by the skin lowers the pH to inhibit growth of pathogens
  • Lysozymes in salvia, sweat and tears are anti-bacterial enzymes
  • Many ingested bacteria in the stomach are destroyed by acid (HCl)
  • A sticky substance, mucus, traps pathogens in the respiratory tract
  • Cilia moves away mucus towards the throat to protect gas exchange surfaces
  • The immune system targets foreign materials and pathogens

Inflammatory response

  • Histamine is released into the wound by white cells
  • This increases vasodilation and increases vascular permeability
  • Vasodilation increases the local blood flow → area becomes red, warm
  • Increased permeability allows escape of tissue fluid into the tissues
  • Tissue fluid contains plasma proteins (antibodies) and may cause swelling


  • White cells (phagocytes) contain digestive enzymes within lysosomes
    • Neutrophils primarily engulf bacteria
    • Macrophages engulf larger particles; including old and infected red blood cells
  • Found in blood, lymph systems and tissues
  • Squeeze through gaps in the walls of venules to enter tissues
  • This allows them to move faster to tissues infected with pathogens
  • Phagocytes are attracted by chemotaxis
  • Opsonisation by antibodies (bacteria becomes coated with antibody)
  • As a result, binding between bacteria and phagocytes is improved
  • Phagocytes form pseudopodia around the particle
  • This positions the particle into a phagocytic vacuole (also called phagosome)
  • Lysosome fuses with the phagosome
  • Intracellular killing by digestive enzymes from the lysosome
  • Pus if formed at the site of infection if no extensive vasculature is present


  • Molecule that stimulates an immune response
  • Usually proteins (polysaccharides, nucleic acid, lipids can also act as antigens) and other inorganic molecules important forself-recognition
  • Self-antigen
    • Only found on the host's own cells and does not trigger an immune response
    • As these are proteins, their structure depends on the amino acid sequence
    • The gene for this sequence is highly polymorphic, having several alleles at each loci
    • There is great genetic variability between individuals
    • Thus, Antigen is different in other people → would cause an immune response
    • There is only 1:4 change that siblings will possess an identical antigen
  • Non-self-antigen
    • Found on cells entering the body (e.g. bacteria, viruses, another person's cell)
    • Will cause an immune response

Antibody (immunoglobin protein)

  • Secreted by B-lymphocytes and produced in response to a specific (foreign) non-self antigen
  • B-lymphocyte's receptor site matches the non-self-antigen
  • Each antibody is produced by one type of B-lymphocyte for only one type of antigen
  • An antibody is Y-shaped
    • The two ends of the Y are called the Fab fragments
    • The other end is called the Fc fragment
    • Fab fragment is responsible for the antigen-binding properties
    • Fc fragment is the effector component and triggers the immune response
  • B cells divide and form memory cells and antibody-secreting plasma cells:
    • Agglutination makes pathogens clump together
    • Antitoxins neutralise toxins produced by bacteria
    • Lysis digests bacterial membrane, killing the bacterium
    • Opsonisation coats pathogen in protein that identifies them as foreign cells

Types of Immune Response

  • Lymphocytes undergo maturating before birth, producing different types of lymphocytes
  • Humoral response - B lymphocytes
    • Produce and release antibodies into blood plasma
    • Produce antibodies from B plasma cells
    • Recognize foreign antigen directly
  • Cellular response - T lymphocytes
    • Bind to antigen carrying cells and destroy them and/or activate the humoral response
    • Recognize foreign antigens displayed on the surface of normal body cells
  • Primary response produces memory cells which remain in the circulation
  • Secondary response new invasion by same antigen at a lower state. Immediate recognition and distraction by memory cells -faster and larger response usually prevents harm

B-Lymphocytes: The Humoral Response

  • Response for pathogens not entering our cells (e.g. bacterium)
  • Each B-lymphocyte recognizes only one specific antigen / need T-helper cell to be activated
  • Maturation / B-cells develop to give many different variants / specific immune system responds to
  • any type of pathogen entering the body
  • Primary response:
    • Pathogen is ingested by macrophages / macrophage displays the pathogens surface non-self antigen on its surface (antigen presentation)
    • It then joins with specific T-helper cells and B lymphocytes that have membrane receptors and are complementary in shape to the non-self antigen
    • T-helper cells will release cytokines to activate selected B-cell/lymphocyte
      • Secretes antibodies of the same type into the blood
      • Divided by mitosis to produce a clone
      • Cells grow to form plasma cells producing masses of free antibodies
    • Some of the cells remain in the blood as memory cells.
  • Secondary response = new invasion by same antigen at lower state. Immediate recognition and distraction - faster, larger response usually prevents harm. Antibodies are produced more rapidly and in larger amounts

T-Lymphocytes: Cell-Mediated Response

  • Virus enter cell and more difficult to remove
  • No antibodies involved / work directly on the infected cell by destroying it
  • Special proteins called Major Histocompability Complex (MHC) are present on all human cells
  • Non-self antigen interacts with MHC as human cell becomes infected by a pathogen


  • Specific T-lymphocyte recognises specific non-self antigen only with a chemical marker next to it (MHC)
  • Activated T-lymphocytes multiply by mitosis and enter circulation
  • Cells differentiate into different types of cell
    • Cytotoxic T-Cells destroy pathogens and infected cells by enzyme action, and secrete chemicals which attract and stimulate phagocytes
    • Helper T-Cells stimulate the activity of the cytotoxic T-Cells and B-lymphocytes by releasing chemicals (cytokines andinterleukins). Destroyed by HIV
    • Suppressor T-Cells switch off the T and B cell responses when infection clears
    • Memory T-Cells Some activated T-Cells remain in the circulation and can respond quickly when same pathogen enters body again

IMG 3-12-3
Table 3-12-3: Different types of immunity

Active (Antibodies made by the human immune system, long term acting due to memory cells)

Passive (Given-Antibodies, short term acting)



- Response to disease
- Rejecting transplant

- Acquired antibodies
(via placenta, breast milk)


- Vaccination
(Injection of the antigen in a weakened form)

- Injection of antibodies from an artificial source, e.g. anti venom against snake biter


- Antibody in response to antigen
- Production of memory cells
- Long lasting

- Antibodies provided
- No memory cells
- Short lasting

How vaccines produce responses by the immune system

Artificial active immunity

  • Vaccine containing dead pathogens. Antigen is still recognised and an immune response made
    • Salk polio vaccine (Polio vaccine is injected)
    • Influenza
    • Whooping cough
  • Vaccine containing a toxin
    • Diphteria
    • Tetanus
  • Vaccine containing an attenuated (modified or weakened) organism which is alive but has been modified so that it is not harmful
    • Sabin polio vaccine (Taken orally, often sugar pumps)
  • Purified antigen - genetically engineered vaccine
    • Hepatitis B (A gene coding for a surface protein of the hepatitis B virus has been inserted into yeast cells which produce the protein when grown in fermenters)