Re: allergies

Dear Jacinta, Rafael and Daniel -

Allergies are throught to be caused by a 'hyperreactivity' of the immune system to specific substances (or antigens as referred to by immunologists). You can think of it as having an edgy immune system that overreacts to certain stimuli such as specific kinds of foods, dust or pollen. The reason for this overreaction is not entirely known though it can have a genetic component - if your parents have allergies, you may be 'predisposed' to having them too.

The allergic response involves different parts of your immune system, the collection of white blood cells and lymphocytes that protect your body from infection with bacteria and viruses. These cells develop in your bone marrow, enter the blood stream and migrate through all tissues in your body, the skin, your brain, intestines, lungs, etc. B cells, one kind of lymphocyte, produce antibodies - small Y-shaped proteins that stick to the surface of bacteria and viruses, making it easier for other cells called macrophages to come in and remove the invaders. Antibodies are also referred to as immunoglobulins (immuno = immune system; globulin = a kind of protein), and one type of immunoglobin produced by B cells is IgE. Unlike other types of antibody, IgE specifically binds to the surface of another immune cell called the mast cell. These cells reside in the skin, mucus membranes of the mouth and nose and in the lungs. If something sticks to the IgE on the cell's surface, it wakes up and starts secreting chemicals called immune mediators. Some of these mediators include cytokines, and histamine - signals that other immune cells recognize as a 911 call, or come fast, we're under attack!

In the event of a potential microbial or viral 'attack' the release of histamine and other chemicals causes a number of things to happen in your body.

1. Runny nose - make lots of mucus to wash out any invading critters.
2. Coughing and sneezing - try to expell anything that has made it into the throat or lungs.
3. Reddness and swelling in the skin, or throat - this reaction is due in part to histamine which makes blood vessels leaky so other white blood cells have an easier time getting to the site of infection.

Sounds just like an allergy, right?

Thus people with allergies mount an immune response to particles and compounds that are not infectious - pollen, dust, etc. In some cases allergies come about due to an excess production of IgE, putting the mast cells on edge. In other cases some people have been found to have 'trigger happy' mast cells that fire with minimal stimulation (thus the 'genetic component' I mentioned earlier).

Allergic reactions usually occur in the skin or respiratory tract - where mast cells most commonly reside. Incidentally, asthma is definitely related to allergic reactions. It involves a hyppereactivity of the immune cells living in the respiratory tract to dust and other inhaled particles.

Some allergies can be induced by placing the offending substance on the skin in which case a rash deveops around the area of contact. In severe cases people may develop urticaria (large red rashes that deveop suddenly) and enter anaphylactic shock a serious condition caused by the massive release of histamine and other substances throughout the body. Blood pressure drops markedly, and without any treatment people can die from the shock. Treatment involves giving people epinephrine a hormone produced by the body that raises the blood pressure and helps overcome the effects of the immune mediators. For those of us suffering from hay fever, we often take histamine blockers such as dimetapp or benadryl that block the effects of histamine released by the mast cells.

P.R.