The Origins of Ocular Allergy

The Origins of Ocular Allergy

The Origins of Ocular Allergy

Allergies may have evolved as an immune response to help humans defend against parasites. Now this response inappropriately targets benign substances such as dust and pollen.

By Bob Lanier, MD

Allergies affect approximately half of the US population, and ocular symptoms—such as itching, redness, and tearing—occur in most cases. While individuals forced to deal with these symptoms may find it hard to imagine that allergies could have a useful function, a current theory holds that allergies originated as a mechanism to help humans fight parasitic infections. Because parasites are now much less of a health threat in developed countries, this powerful defense is frequently triggered by benign substances like pollen and mold, causing the maladaptive allergic response familiar to many individuals.

Roots of Allergy
   
In its broadest sense, “allergy” sometimes refers to conditions such as contact dermatitis that do not involve the production of an antibody response. But 95% of ocular allergies occur as a result of an IgE-mediated immune response, and most allergists limit their definition of “allergy” to include only those conditions resulting from this IgE mechanism.
   
To determine how humans evolved to have an immune mechanism that today seems worse than useless, researchers have considered its possible benefits, and many believe that allergies developed as a response to parasitic infection. Throughout most of human history, parasites have been an important cause of death in children—possibly the most important cause—and the IgE-mediated immune response seems to be an effective defense against this threat.
   
In the past several hundred years, however, the prevalence of parasite infection has declined dramatically in much of the world, leaving many people with a well-developed immune response that never encounters its target organism. As a result, this immune response is now triggered most often, not by parasites, but by benign substances that may have proteins similar to those the immune system associates with parasites. Instead of a parasitic infection triggering a beneficial immune response, now dust, pollen, and other allergens trigger the runny nose and itchy eye symptoms characteristic of allergy.

Nature vs. Environment
  
While we know that the immune system recognizes and responds to some proteins much more than to others, researchers still do not know why these substances trigger allergies in some individuals and not in others. Genetics almost certainly plays a key role, but environmental exposure must also have some influence. Physicians commonly see identical twins (who have precisely the same genetic code) in whom one will have significant allergies and the other does not. Genetic traits must interact with environmental factors in some way, but we don’t know whether genetic traits are uncovered by specific viral or bacterial agents or whether specific infections protect against allergy.
   
Whatever the circumstances under which people develop allergies, once the immune system is activated by aeroallergens, such as pollens and molds, an allergic response tends to be more or less permanent. Unlike with food allergies, to which people often gain some tolerance, people do not commonly gain a tolerance to aeroallergens.

The Increasing Prevalence of Allergy
   
Another big question in allergy research is whether the prevalence of allergy has increased and, if so, why. One factor that explains the increased prevalence in reported cases of allergy is simply the fact that patients and physicians are now paying more attention to allergy complaints and treating symptoms that might previously have been ignored. With overall improvements in medicine and the increasing attention given to quality of life issues, doctors can now treat conditions like allergy that make patients uncomfortable even if these conditions are not life-threatening. Thus, while people may previously have tolerated the discomfort allergies cause, with the availability of treatments more people are now seeking relief from allergy symptoms. Whether or not the actual incidence of allergy has increased, doctors are almost certainly diagnosing and treating it more frequently.
   
In addition to this increased attention to allergy, the actual prevalence of allergy may have increased. One theory holds that allergies have become more common as a side effect of improved hygiene. As public health measures have cleaned up the areas where most people live, and fewer people live and work in close proximity to animals, people’s exposure to parasites has decreased, thus lessening the need for an immune response tailored to fight them. Instead of lying dormant, however, the immune system has continued to seek out targets, sometimes overreacting to substances that are not inherently harmful.

Types of Allergies
   
Among patients who have significant ocular symptoms, the two main types of allergy are seasonal allergic conjunctivitis and perennial allergic conjunctivitis. The allergic mechanism is the same for both, but they differ in their clinical presentation because they represent responses to different allergic triggers. Perennial allergies are triggered primarily by indoor allergens, such as house dust and animal dander, which people encounter throughout the year. Seasonal allergies, on the other hand, typically result from an allergic response to outdoor allergens, such as pollen, which tends to occur only during certain times of the year.
   
While some patients have clear cases of seasonal allergic conjunctivitis, many prove difficult to diagnose. Often patients have a mixture of both seasonal and perennial allergic conjunctivitis, and in southern parts of the country, patients with seasonal allergies may have symptoms throughout much of the year because of overlapping pollination periods. In addition, eyecare providers may have difficulty differentiating perennial allergies from other, nonallergic ocular conditions since perennial allergies are always present and tend to cause chronic, low-grade symptoms.

Treating Allergy

While not everyone with allergies complains of ocular symptoms, and only 1% of people who see a health care professional for their allergies consult an eyecare provider, ocular symptoms probably occur alongside nasal sensitivity in most, if not all, cases (Figure 1). Since patients frequently don’t mention allergy symptoms unless asked about them specifically, eyecare providers need to include questions about allergies as part of their exams.
   
Also, the oral antihistamine medications used to treat nasal allergy can worsen ocular symptoms in some patients, so eyecare providers need to ask about medications that patients are taking, both eye drops and oral medications. Since these drugs can alter the tear film and may be drying to the eye, doctors need to check for ocular side effects in patients taking certain medications, including the popular over-the-counter oral antihistamines such as loratadine (Claritin®) and diphenhydramine (Benadryl®).
   
For most patients, treatment of ocular allergy involves antihistamine eye drops, which are generally effective in controlling symptoms. One such medication, olopatadine hydrochloride (Patanol®, Alcon), is now available in a once-a-day formulation, which may prove more convenient for patients, particularly contact lens wearers. Its reduced dosing schedule allows contact lens wearers to instill their eye drops once in the morning before putting in their contact lenses, and then leave their lenses in until they are ready to remove them in the evening. With more frequent dosing schedules, patients must remove their contact lenses midday, leaving them unable to perform visual tasks during a sometimes critical period.

A Practical Tip for Patients
   
In addition to using antihistamine eye drops, patients can also alleviate some of their symptoms by keeping their hair clean. While most people have never thought of their hair as a source of pollen or dust, hair can act as a reservoir for allergens, and removing those allergens can make patients significantly more comfortable. Because hair carries a partial electric charge, it attracts particles such as pollen and dust, and it can accumulate a considerable amount of allergen in the course of a day. Allergens on patient’s hair can then get into their eyes, particularly while they sleep, so patients should wash their hair before sleeping, or pull it away from their face, in order to reduce the severity of their ocular allergy symptoms.

BOTTOM LINE

Allergy may be a troublesome parasite defense mechanism gone awry, but eyecare providers can relieve ocular allergy symptoms in many patients by asking about ocular allergies and prescribing antihistamine drops as appropriate. Eyecare providers should also advise patients with allergies to wash their hair before sleeping, or at least pull it away from their eyes, to prevent allergens on the hair from getting into the eyes.

Bob Lanier, MD, is clinical professor of pediatrics and immunology at the University of North Texas Health Science Center, in Fort Worth, TX. Assistance in writing this article was provided by Refractive Eyecare senior editor Kay Downer.

Figure 1: Of patients who consult a healthcare professional about their allergies, only
1% see an eyecare provider, despite the fact that many patients experience ocular symptoms. (Note: Sum of all sections is greater than 100% since some patients consult more than one type of health care professional.)