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Teacher 2011

Infectious diseases - immunity

Age range 14-16 Age range 16-19

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White blood cells

Natural active immunity (natural acquired immunity)

If a pathogen gets through all the barriers to infection a second line of defence is activated. This is the white blood cells of the immune system. The immune system responds to a particular pathogen to give you active or acquired immunity.

But how does your body recognise when a foreign pathogen has entered?

Friend or enemy?

The surface of every cell is covered with molecules that give it a unique set of characteristics. These molecules are called antigens. Antigens are generally fragments of protein or carbohydrate molecules. There are millions of different antigens and each one has a unique shape that can be recognised by the white blood cells of your immune system. The white blood cells then produce antibodies to match the shape of the antigens.

The antigens on the surface of pathogenic cells are different from those on the surface of your own cells. This enables your immune system to distinguish pathogens from cells that are part of your body. Antigens are also found on the surface of foreign materials like pollen, pet hairs and house dust where they can be responsible for triggering hay-fever or asthma attacks.

Primary response to infection

If a pathogen enters your body, white blood cells of your immune system quickly recognise its foreign antigens. This stimulates specific lymphocytes to grow, multiply and finally produce antibodies that will stick to the antigens on the invading pathogens and destroy them.

This initial response takes a few days before it is large enough to fight off the infection. During this time, damage to body tissues will happen and you will feel the symptoms of the infection. This can cause serious damage and some infections can be fatal. However, with most infections, your immune system is able to produce enough antibodies to kill the pathogens that are causing the infection and the symptoms disappear. Once this happens, your immune system switches off its response to this infection but remains active, waiting for other pathogens.

White blood cells are found all over your body, but especially in your lymph glands. These glands often become swollen when your body is mounting an immune response against an infection, which is what we describe as having swollen glands.

Different types of white blood cells respond in one of three different ways to the presence of a pathogen in the body:

  • They can produce antitoxins which stop the toxins produced by some bacteria from damaging the cells
  • Lymphocytes produce antibodies which attach to the antigens of a particular type of bacteria or virus. This may destroy the pathogen or make it easier for a phagocyte to ingest. Different organisms have different antigens so a different antibody is needed to recognise each different type of antigen.

Lymphocyte response to infection

  • Phagocytes ingest pathogens and digest them. They also ingest pathogens which have been destroyed by antibodies

Phagocyte in action

Protein that is produced by lymphocytes (white blood cells) and that attaches to a specific antigen.
Molecule on the surface of a pathogen that identifies it as a foreign invader to the immune system.
Single-celled organism. Has a cell wall, cell membrane, cytoplasm. Its DNA is loosely-coiled in the cytoplasm and there is no distinct nucleus.
Immune system
The body's natural defence mechanism against infectious diseases.
A type of white blood cell that make antibodies to fight off infections.
A micro-organism that causes disease.
Phagocytes are the white blood cells that protect the body by ingesting harmful foreign particles, bacteria, and dead or dying cells.
A polymer made up of amino acids joined by peptide bonds. The amino acids present and the order in which they occur vary from one protein to another.
A poisonous or toxic substance - produced by pathogens.
The smallest of living organisms. Viruses are made up of a ball of protein that contains a small amount of the virus DNA. They can only reproduce after they have infected a host cell.