Jul 09

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What is an Allergy ? What causes allergic reactions ?

What is an Allergy ?

Each of you reading this article will have your own idea of what an allergy is. In very simple terms, an allergy is an adverse reaction. You will often hear people say things such as “I’m allergic to cats”, or “I’m allergic to this or that type of food”, or even “I’m allergic to Monday mornings”. In all these instances, the implication is that contact with whatever they are expressing concern about causes them some sort of unpleasant or adverse feeling or reaction.

When the term “allergy” was first coined in the early years of the twentieth century by the Austrian physician Clemens Freiherr von Pirquet, the word was used to mean a change – either good or bad – in the way that the body reacted. A good reaction provided immunity, while a bad one produced hypersensitivity. Over the years, the term “allergy” has come to be associated solely with immune hypersensitivity reactions, whereas the term “immunity” is linked to the body’s beneficial immune responses.

What causes allergic reactions ?

An adverse immune response is the body’s attempt to kill or expel a foreign protein that has invaded it. From the body’s perspective, therefore, an allergic reaction is a protective mechanism. While immune reactions can occur in response to viruses or bacteria – and, on occasions, inappropriately to the body’s own proteins allergic reactions usually occur in response to external, non-infectious substances.

When these protective reactions are directed against what are basically harmless proteins, it is the allergic reaction itself – rather than the external agent it is directed against -that becomes the focus of the problem. Most of the allergies that trouble people today fall into this category.

Which parts of the body are affected by allergies?

Allergic reactions tend to occur in the parts of the body where the immune system is activated. Thus, airborne proteins that are inhaled cause reactions in the nose and airways of the lung. If these airborne proteins also settle on the surface of the eye, then symptoms may also arise there. Proteins that are eaten will either cause a gastrointestinal upset or, if they are absorbed into the blood, they have the potential to cause a variety of symptoms involving the skin, joints, cardiovascular, and central nervous systems, as well as the nose and lungs.

Similar responses can occur with foreign proteins that are injected into the body, such as stings from bees or wasps, for example. Although the skin is an obvious site for contact, the barrier function of the skin protects the immune system from exposure unless it is damaged by the protein or the protein contains enzymes that enable it to penetrate the skin. Although it is easy to assume that an allergic reaction involving the skin is some sort of contact allergy, it could, in fact, be a response to a foreign protein that has entered the blood’s circulation system after being ingested, or possibly following its absorption from the mucous membranes of the eye, nose, or lungs.

The immune system reacts to the presence of a foreign protein by producing an area of inflammation. This happens when special immune cells within the affected tissue become activated. These immune cells release chemicals, and it is these that give rise to the localized symptoms you experience. This inflammation is often signified by the suffix “itis” that is added to the name of the organ involved, such as “dermatitis”, “sinusitis”, or “conjunctivitis”.

What is inflammation?

Your body’s immune responses are designed to protect you by killing or expelling the carrier of any foreign protein that is perceived as “attacking” the body. With an infection, this protective response kills the bacteria and so the response terminates itself. The local response involved in this process causes swelling, redness, pain, and an increase in the blood flow – this is why the swollen part of your body feels hot. This process, which involves the recruitment of white blood cells (known as neutrophil white cells) from the circulatory system, is called “inflammation”.

With an allergic reaction, a similar; -but lesser, inflammation occurs. However, since the proteins, or allergens, to which you are sensitized are inert (rather than alive, as bacteria are) and are associated with the environment you are in, this type of inflammation is persistent and results in the symptoms you experience.

In contrast to your body’s immune responses to fight bacteria, in which the neutrophil white cell is recruited to combat the infection, in an allergic response another type of white cell, an eosinophil white cell, is recruited. A different type of white cell is involved because the immune response to allergens is different to that against infection.

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