Animation: allergic reaction

View our animation to see what happens inside your body when you have an allergic reaction to grass pollen. Use the ‘Next’ navigation button to move through the scenes, and the ‘Play again’ navigation button to review scenes. You can also use the pause and play buttons to control how quickly you move through the animation.

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Your immune system defends you from foreign invaders, such as bacteria and viruses, which can cause disease. In people who suffer from allergies, the immune system over-reacts to substances that are ordinarily harmless, such as pollen and animal dander. These so-called allergens trigger an unnecessary immune response, which can range from a mild allergic reaction (such as sneezing) to life-threatening allergic shock (doctors call this anaphylaxis or anaphylactic shock). Fortunately, severe, life-threatening allergic reactions are rare, but other allergic reactions, such as hay fever, are common.

  • Substances that cause allergies are known as allergens. Common allergens include pollen, mould, animal dander, certain foods (e.g. peanuts) and insect stings.
  • Breathing in allergens from the air (e.g. pollen) is a common way for a person to be exposed.
  • The grass pollen stimulates the immune system's B cells – a type of white blood cell.
  • The B cells produce a lot of antibodies to the grass pollen. These antibodies are known as IgE antibodies and are specific for the grass pollen.
  • The antibodies travel through your body until they come into contact with mast cells. Mast cells are immune cells found in your digestive system, respiratory system and skin.
  • The antibodies then attach themselves to the mast cell. They remain attached, waiting for you to come into contact with the grass pollen a second time (which may not be until years later).
  • You don’t experience any symptoms after this first exposure – your immune system has simply set itself up to create an allergic reaction if you have any subsequent exposures to the allergen.
  • The next time you are exposed to the grass pollen, the antibodies that are attached to the mast cells recognise the grass pollen immediately, and bind to it.
  • When the grass pollen attaches to the antibodies, the mast cells release several powerful chemical substances, including histamine.
  • Histamine is the chemical that causes many allergy symptoms, such as sneezing, a runny nose and watery, itchy eyes.
  • How severe your allergic reaction is will depend on how much histamine is released.


1. Australasian Society of Clinical Immunology and Allergy (ASCIA) [website]. What is allergy? (updated 2010, Jan). Available at: (accessed 2010, Jun 8)
2. Merck Manual Online Medical Library, Home Edition for Patients and Caregivers [website]. Allergic reactions: introduction (updated 2008, Sep). Available at: (accessed 2010, Jun 8)
3. Asthma and Allergy Foundation of America [website]. IgE's role in allergic asthma (updated 2005). Available at:⊂=16&cont=54 (accessed 2010, Jun 17)