Allergy: Allergic reactions - text only -

• The wrong antibodies

• Histamine

• Mild and extreme reactions

• An antidote

The immune system offers protection against the outside world. But occasionally this system fails, and the mechanisms designed to protect the body from outside threats can themselves place the body in mortal danger.

• The wrong antibodies
When the body comes into contact with an intruder, B-cells — an integral part of the body's immune system — treat it as hostile and act accordingly, by manufacturing antibodies. This is known as the sensitisation phase. But sometimes the immune system manufactures the wrong type of antibodies. The reasons for this are not fully understood, but such factors as genetic make-up, the environment and exposure to dirt and disease in childhood may be important. See allergy theories.

Whatever the cause, the B-cells mistakenly create E-antibodies, also known as IgE-antibodies or immunoglobin E. E-antibodies are designed to tackle large intruders, such as tapeworms or ticks. Like other antibodies, E-antibodies are made as a response to a specific threat. However, unlike other antibodies, E-antibodies cling to mast cells, which produce and store histamine. When E-antibodies encounter the invader that first led to their production, they are activated and bind together, and a large amount of histamine is released into the system as a result. Put simply, the body overreacts to the threat.

• Histamine
Histamine has various effects on the body. When it is released, it causes the surrounding blood vessels to dilate and increase in permeability. This allows fluid and cells to leak from the bloodstream and migrate to the site of the invasion.

In the case of an allergy to cats or dogs, for example, the histamine causes the blood vessels in the nose to leak. A wave of surplus mucus is produced, which would sweep a parasite away — were there one present. Histamine also has an effect on the respiratory system: it causes the muscle in the bronchial tubes of the lungs to constrict, causing sneezing. If a parasite were present in the nose, sneezing would help to expel it.

• Mild and extreme reactions
In most cases allergic reactions are mild. Symptoms of a mild allergic reaction are a runny nose, itching eyes and sneezing. Some allergens may also cause the skin to itch and swell. Most allergies to pollen, animals and dust are of the mild variety.

However, in some cases, allergic reactions can be much more serious. They may even be life-threatening.

In the case of an extreme allergic reaction, the E-antibodies clustered around mast cells cause the cells to react violently. The venom in a wasp sting, for example, would cause the mast cells in the vicinity of the sting to erupt, releasing inordinate amounts of histamine. As traces of venom enter the blood stream, they trigger a similar reaction in mast cells all around the body. Fluid pours out of the blood vessels, just as though a wasp had stung the body in hundreds of places. Hives develop and the skin itches. Again, this is a typical reaction to a parasite: were one present, it would encourage the victim to scratch the parasite away.

As more and more fluid leaves the bloodstream, it causes the blood pressure to fall suddenly. This leads to dizziness and may result in loss of consciousness. The sudden fall in blood pressure may also cause the heart to beat irregularly and even provoke a heart attack. And the large amount of histamine has another nasty effect: it causes the muscles in the lower part of the airway to constrict, making breathing difficult. It may also cause swelling in the lips, tongue and throat, further reducing the amount of air reaching the lungs. As the supply of oxygen to the brain dwindles, anaphylactic shock can also result in seizures.

Anaphylactic shock is the most extreme form of allergic reaction. Why it occurs in some people is not fully understood. The most common allergens involved are nuts, shellfish, wasp and bee stings, dairy products, eggs, natural latex and drugs such as penicillin. A mere trace of an allergen can be enough to bring on anaphylactic shock in a susceptible person.

• An antidote
When the body goes into anaphylactic shock, it tries to fight the effects of histamine by releasing an antidote, adrenaline. Adrenaline makes the blood vessels contract, stopping them from leaking, which raises the blood pressure. It also opens up the airways. However, the body's own supply of adrenaline is not sufficient to counter the effects of histamine in anaphylactic shock: as the allergen moves around the body, it continues to trigger mast cells, meaning that more and more histamine is released into the blood.

However, if extra adrenaline is pumped into the system by a doctor, the body will recover quickly. A rapid reaction is essential: without treatment, a person suffering from anaphylactic shock will die within minutes.

People who are at serious risk of suffering a life-threatening allergic reaction are usually advised to carry an emergency kit containing an adrenaline injection. These are only available on prescription, although in an emergency they may be available from a pharmacy.

The immune system | Common allergies

Allergic reactions | Allergic people

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