The Immune System


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  • Created by: alex
  • Created on: 25-05-11 11:35

The Immune System


  • Acts as a physical barrier blocking pathogens from entering the body
  • Acts as a chemical barrier by protecting chemicals that are antimicrobial

Mucus Membranes

  •  Pretect body openings that are exposed to the environment
  • Membranes secrete mucus to trap pathogens and contain antimicrobial enzymes.


An immune response is the body’s reaction to a foreign antigen.

Antigens – molecules found on the surface of cells

When a pathogen enters a body, the antigens on its surface are identified as unknown.

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The Immune Response - 1

Phagocyte (e.g. a macrophage) - a type of white blood cell that carries out phagocytosis (engulfment of pathogens).


1) A phagocyte recognises the antigens on a pathogen as foreign.


2) The cytoplasm moves around the pathogen, engulfing it.


3) Lysosomes fuse with the phagocytic vacuole breaking down the pathogen.


4) The phagocyte presents the antigens on its surface.

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The Immune Response - 2

T Lymphocyte - another type of white blood cell covered in receptors, each one with different receptors.


1) Their receptors bind to complementary antigens presented by the phagocytes.


2) This activates the T Lymphocyte to divide and differentiate to carry out different functions.

  • some release substances to activate B lymphocytes
  • some attach to antigens on a pathogen and kill the cell
  • some become memory cells
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The Immune Response - 3

B Lymphocytes - another type of white blood cell, covered in proteins called antibodies.

1) the antibodies bind to antigens to form antigen-antibody complexes. 

2 ) this activates the B lymphocyte to divide by mitosis into:

  • plasma cells
  • memory cells
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Plasma Cells




Plasma Cells

-clones of B lymphocytes

-secrete lots of the antibodies specific to the antigen into the blood

-anitbodies bind to form antigen-antibody complexes:

  • variable region form the antigen binding sites - the shape differs to complement the antigen.
  • hinge region allows flexibility when the antigen binds.
  • constant region allows binding by antigens to immune system cells.
  • Disulfide Bridges hold the polypeptide chains together
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Antibodies help to clear an infection by:

Agglutinating Pathogens: each antibody has two binding sites so the pathogen can phagocytose a lot of pathogens all at once.

Neutralising Toxins: antibodies can bind to the toxins produced by pathogens preventing them form harming human cells by neutralising them. The toxin-antibody complexes are phagocytosed.

Preventing the pathogen binding to human cells: when antibodies bind to antigen they can block the cell surface receptors to prevent them binding with host cells.

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Memory Cells

Primary Response

- slow as there aren't many B Lymphocytes they can make the antibody.

- the B lymphocytes will produce the right antibosy but the person will show symptoms.

-after being exposed to an antigen the T and B Lymphocytes will produce memory cells which will stay in the body

- the cells will record the specific antibodies needed tobind to the antigen

...the person is now immune as their immune system has the ability to respond quickly to a second infection.

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Active Immunity

This is when your immune system makes its own antibodies after being stimulated by an antigen.

  • Natural: when you become immune after catching a disease
  • Artificial: when you become immune after having a vaccination of a dose of antigens.

Passive Immunity

This is when you become immune after being given antibodies made by a different organism and your immune system doesn't produce any antibodies of its own.

  • Natural: when a baby becomes immune due to the antibodies it receives from its mother via the placenta and breast feeding.
  • Artificial: when you become immune after being injected antibodies from someone else.
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Vaccinations - help your body produce memory cells as it contains antigens.








Herd Vaccination - when a lot of people in a community have been vaccinated against an infection, the people who haven't been vaccinated are unlikely to get the infection as there are less people to catch it off.

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-proteins on the surface of the influenza virus act as antigens triggering the immune system

-these antigens can change regularly forming new strains of the virus

-memory cells will not recognise new strains

-hence a new vaccine is made

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Sources of Medicines Need Protecting

-many medicinal drugs are manufactured using natural compounds found in plants, animal and microorganisms.

-only a small number of organisms have been investigated s far so it's possible that plants or microorganisms exist that could contain compounds that could be used to treat currently incurable diseases.

- possible sources of drugs need to be protected by maintaining biodiversity. Else a specie could die out before we get chance to study them.

- organisms that have been already studied could still prove to be useful sources of medicines as new techniques are developed for identifying, purifying and testing compounds.

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