• Created by: Han2812
  • Created on: 08-01-14 18:34

Defence Mechanisms

Defence mechanisms are used to protect us from harful pathogens. There are 2 main types:


  • Do not distinguish between one type of pathogen and another
  • Respond to all of them in the same way - ACTING IMMEDIATELY. Two forms:
    • A barrier to the entry of the pathogen
    • Phagocytosis


  • Do distinguish between different pathogens - SPECIFIC 
  • LYMPHOCYTES - take 2 forms:
    • Cell-mediated response --> T LYMPHOCYTES
    • Humoral responses --> B LYMPHOCYTES

There are about 10 million different types of lyphocytes in the body with receptors specific to different pathogens. So when one with that receptor comes along a pathogen with that specific antigen, they have to divide which is why there is a time gap

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T Cells

T cells are matured in the thymus gland are associated with cell-mediated immunity involving body cells 

They respond to an organisms own cells that have been invaded by non-self materia eg. virus or cancer


  • Pathogens invade body cells or are taken up by phagocytes 
  • Phagocyte puts antigens from pathogen on its cell-membrane surface
  • Receptors on some T helper cells fit exactly these antigens
  • This activates other T cells to divide rapidly by mitosis and form a clone
  • These cloned T cells develop into:
    • Memory cells - enable rapid response to future infections by the same pathogen
    • Stimulates B cells to divide
    • Stimulates phagocytosis in phagocytes
    • Killer cells - kills infected cells
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How T Cells kill Infected Cells

T cells kill infected cells such as CANCER. They are good at destroying viruses as they live inside cells, so they can reproduce

They do this by:

  • They produce a protein that makes holes in the cell-surface membrane 
  • This makes the cell freely permeable to all substances and it dies

This shows the importance of the cell-surface membranes that cells have to survive

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B Cells

B Cells are matured in the bone marrow - involved in HUMORAL IMMUNITY - antibodies in body fluids. They are used to help fight off infections with T cells


1. Surface antigens of the invading pathogen are taken up by B cells

2. B cells process the antigens and present them on their cell-surface membrane

3. T helper cells activate them by attaching to the processed antigens

4. The B cells then divide by mitosis to give a clone of a PLASMA cell

5. The cloned plasma cell PRODUCE ANTIBODIES --> fit EXACTLY to the antigens on the pathogens cell-surface membrane

6. The antibodies attach to the antigens on the pathogen, destroying them. PRIMARY RESPONSE SYSTEM

7. Some B cells develop into MEMORY CELLS --> respond quickly in future infections of same pathogen --> SECONDARY RESPONSE SYSTEM

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Antibodies are proteins synthesised by B cells

Polyclonal Antibodies: Antibodies that are produced in our body

Monoclonal Antibodies: Antibodies that are produced outside our bodies


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There are two types of immunity

1. PASSIVE - Introduction of antibodies into individuals from an OUTSIDE SOURCE. These are not being produced by the individual themselves, not replaced when being broken down --> Immunity is SHORT

2. ACTIVE - Stimulating the production of antibodies by the individual's own immune system. Generally LONG LASTING 

Vaccinations are the introduction of a substance into your body with the intention of stimulating active ummunity against a particular disease for individuals and whole populations so ACTIVE IMMUNITY

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Vaccinations - Successful and Not

To have a sucessful vaccination program depends on a number of factors:

  • Must be economically avaliable in enough quantities to immunise all the vulnerable population
  • Only a few side effects. Unpleasant side effects may make populations not get immunised
  • Must be ways of producing, storing and transporting the vaccine - hyginic conditions ect
  • Must be ways of advertising the vaccine properly at the right time - staff training ect
  • HERD IMMUNITY - all vunerable people if possible vacinnated at one time --> so at a certain time, no-one in a population has the disease and transmittion of pathogen is interrupted 

Sometimes, vaccines don't always eliminate a disease. This is because:

  • They fail to immunise some people - eg. have weak/defective immune system. Some people just dont want to have them done - scared, religious, ethical or medical reasons
  • Individuals may develop the disease after getting the vaccine - immunity levels aren't high enough to prevent it
  • Vaccine is ineffective as pathogen has mutated as antigens aren't recognised by the immune system - ANTIGENTIC VARIABILITY - happens alot with influenza virus
  • May be many varieties of one type of pathogen - near immpossible to vaccinate against all of them  + some hide from bodies immune system 
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