I count myself lucky: my immune system doesn’t overreact to pollen, animal hair, peanuts, or anything else. But both my wife and daughter are sensitive to cats and dogs (as is about a tenth of the U.S. population). And, while not a true allergy, my daughter has celiac disease, which requires strict adherence to a gluten-free diet, because that protein—found in wheat and other grains—triggers a destructive autoimmune response. According to the American College of Allergy, Asthma & Immunology, some 50 million Americans suffer from exposure to one allergen or another. The college’s Advice from Your Allergist page, offers guidance on a host of such conditions.

For a video primer on the science behind true allergies go to Allergies: Hypersensitivity Type 1. There are, however, four distinct types of hypersensitivity in which the immune system may go awry. At this online learning site created by McGraw Hill, Animation: IgE Mediated (Type I) Hypersensitivity you can find another cartoon of how allergies are triggered, or you can click on the animations in the menu to the left to learn about the other three recognized types of hypersensitivity. (There is nothing simple about the immune system.) View the animation on the left from About.com, showing airborne allergens, such as pollen, entering via the nasal passages to trigger a response. See also related video animations on the About.com page.

The Washington, D.C.–based Asthma and Allergy Foundation of America has an overview of the impact of allergies and asthma (which is often triggered by allergens) on our population. At Allergy Facts and Figures, I learned that some 700 people die each year in America from anaphylaxis, the most extreme allergic reaction. The mortalities are triggered most often by penicillin, followed by foods, insect stings, and latex. Another group dispensing information on allergies is the American Academy of Allergy Asthma and Immunology. Click on “pollen counts” in the menu at the top of their homepage and you will be directed to the National Allergy Bureau, their network of pollen- and mold-spore-counting volunteers who monitor the seasonal changes. Pollen.com has a map of the United States that you can click on to get local pollen conditions. According to the site when I checked, my neighborhood has medium-high levels due mainly to ash, alder, and cedar trees

Pollen is undoubtedly the most widespread allergen, affecting the millions of people. The National Pollen and Aerobiology Research Unit, at the University of Worcester in the United Kingdom, tells you which kinds of plants are the most allergenic. Contrary to popular belief, flowers are rarely the source of the offending pollen. “The majority of flowering plants are insect-pollinated and so their pollen does not need to be dispersed on the wind and they therefore produce smaller quantities of it.” Grasses, many trees, and plants like the infamous ragweed (which can spew 8 billion pollen grains in five hours) are the problem. In the U.K., only fifteen plant species produce the majority the airborne allergens.

The “enemy” may come in a variety of forms. See the photo agency Mediscan’s homepage and type in “allergen” to see images, many of them taken with high magnification, of the most potent allergens, from colorized ragweed pollen grains to dander-covered cat hairs. Take a look at this scanning electron microscope image of pollen grains; depending on your susceptibility, you will see beautiful, geometric shapes, or a house of spiny, inflammation-producing horrors. The tough outer layers of the pollen make them ideal for preservation in ancient sediments. For palynologists, fossil pollen grains are the key to recreating environments thousands or even millions of years ago. Irritating as they are, they are an important window into past climate change. Owen K. Davis, a professor in the University of Arizona’s department of geosciences, has a brief account of pollen biology. Pollen evolved some 300 million years ago, freeing “the seed plants from dependence on standing water for fertilization, which is needed by the spore-reproducing plants like ferns.”

Unfortunately, global warming is likely to make allergies worse. According to this research report from Harvard University, “Pollen Production—and Allergies—May Rise Significantly over the Next 50 Years,” when scientists grew ragweed in an atmosphere with double current levels of carbon dioxide (a likely increase in this century) the plant emitted 61 percent more pollen.

At a Web site devoted to dust mites (one of the chief sources of indoor allergens), you can learn about these ubiquitous, microscopic pests. According to the site, their digestive proteins are to blame. “These secretions are very hard on the respiratory systems of us unwitting hosts. This secretion from the guts of dust mites is extremely potent. There is no cure for allergies to dust mites. The only way to avoid allergy symptoms when it comes to dust mites is to prevent exposure in the first place.” Cockroaches, too, are a major problem, contributing greatly to incidence of inner city asthma attacks. At the Asthma and Allergy Foundation of America I learned that cockroach allergens come from the insect’s feces, saliva, and bodies. And most disturbing of all: “Studies show that 78 to 98 percent of urban homes have cockroaches. Each home has from 900 to 330,000 of the insects.” That little fact brings back bad memories.

At the Food Allergy and Anaphylaxis Network I found a list of the eight common ingredients that are responsible for 90 percent of the food allergies. I clicked on wheat (my daughter’s poison), which can trigger a true allergic response (the immediate inflammation mediated by IgE) or the intestinal damage associated with celiac disease, an autoimmune disease that affects 1 in 133 people.

Genetics is another important facet of the problem. At several Web sites, I read that specific allergies are not inherited, but your tendency to develop them is, and more so from your mother’s genes. At Bryn Mawr College, student Melissa Teicher wrote and researched “Are Genetics Responsible for Allergies? A Study in Identical Twins.”

The “Hygene Hypotheses” from the PBS Evolution Library. Speaking at the end is renowned biologist E.O. Wilson. (Click arrow to begin video.) the bottom are links to the sources, which can followed for more information. At TheTech Museum of Inonvation, in San Jose, California, Dr. Barry Starr, a geneticist at Stanford University, answers the question: Are Peanut Allergies Genetic? (At TheTech's Online Exhibits page are a number of exhibits on other subjects.)

An emerging explanation for why allergies are generally on the rise is called the Hygiene Hypothesis. See the PBS Evolution Library video clip on the right, which focuses on the work of a German immunologist who finds that exposure to microbes at an early age, particularly the microbes in the livestock and stables in the countryside, is associated with lowered incidence of allergies. “In the late 1990s, Dr. Erika Von Mutius, a health researcher, compared the rates of allergies and asthma in East and West Germany. Her hypothesis was that children growing up in the poorer, dirtier, and generally less healthful cities of East Germany would suffer more from allergy and asthma than youngsters in West Germany, with its cleaner and more modern environment. When the two regions were reunified in 1999, von Mutius compared the disease rates. ‘What we found was exactly the opposite’ of her hypothesis, she recalls. Children in the polluted areas of East Germany had lower allergic reactions and fewer cases of asthma than children in the West. What was going on? As sometimes happens to scientists, von Mutius was forced to abandon her original hypothesis and rethink the question based on her new observations.”

Examining humanity’s coevolution with bacteria, viruses, and parasites is one of the keys to finding new ways of mitigating allergic reactions. Some research has shown that exposing patients to parasitic nemotodes can, in some cases, help. See this short video on helminth therapy. This Medical News Today article, “Clues to How Humans Evolved Allergies Offered by Ancient Antibody Molecule,” offers another angle on how evolution has shaped our immune system. And finally, at the New York Academy of Sciences Web site on a symposium at Hunter college about Darwinian Approaches to Medicine, I found an interesting article by Paul Sherman of Cornell University: “Allergies and Cancers: Are Complex Relationships Comprehensible?” Here are the highlights:

“Darwinian medicine suggests that troublesome bodily responses can persist if they sometimes protect the body from much more serious problems.”

“Previous studies have sometimes indicated an association between allergies and cancer, but not consistently.”

“Combining results from many studies shows that allergies correlate more often with lower cancer risk than with higher risk.”

“The reduced risk is strongest for cancers of tissues that are exposed to the environment, supporting the idea that allergic responses directly reduce exposure to carcinogens.”