Sometimes triggering the immune system causes problems. Two examples are:

• allergic reactions
• tissue rejection in transplant operations.

Many foreign particles can trigger an immune reaction even though they are not pathogens and do not cause an infection. People become sensitive to them and asthma is an example of this type of allergic reaction.

Antigens on pollen, pet hair, dust or some other trigger are recognised by cells of the immune system found in the lining of the airways. Called Mast cells, they release histamine which is a powerful signalling molecule usually released when there has been tissue damage. It causes the walls of the trachea and the two bronchi to contract. This narrows the airways and makes breathing very difficult. In severe cases, an asthma attack can be fatal.

Inhalers, such as salbutamol, taken during an asthma attack cause the airways to relax and return to their normal diameter. Steroid inhalers may also be taken on a regular basis to reduce the sensitivity of the mast cells to the trigger.

People suffering from hay fever have a milder reaction to the pollen and may take anti-histamine medicines to lessen the effects of the histamine release.

See Breathing and asthma for more information on asthma attacks and how they happen.

Allergic reactions to peanuts works in a similar way.

Play the animation to see the sequence of events during an asthma attack.

All your body cells have markers on their surface that signal to your lymphocytes and make sure that they are not attacked by your own immune system. They are called the tissue-type markers.

Before a patient receives a donated organ, such as a kidney, great care is taken to match the donor's and recipient's tissues-type markers so that they are as similar as possible. They are usually quite close in family members and only ever a perfect match in genetically identical twins. Even a slight mis-match could mean that the organ was attacked by the recipient's immune system and rejected.

To try and prevent transplant rejection, recipients will also take medicines to reduce the efficiency of their immune system. These medicines are called immunosuppressants. They reduce the chances of the donated organ being attacked by the recipient's immune system but they also increase the likelihood of them catching infectious diseases. A balance needs to be maintained where there immune system does not reject the transplant while the recipient is not harmed by dangerous infections.