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Why do We have Allergies?

What Causes Allergies? The Role of Antibodies:

The immune system is similar to a sensory system in that it receives input from the environment and produces an adaptive response. Its purpose is to recognize foreign invaders, such as bacteria and parasites, and launch an attack to neutralize the threat of infection. It also recognizes and disposes of ill or defective internal cells, to prevent a disease from spreading. 

The way the immune system accomplishes this is by recognizing what is foreign or abnormal,  and separating it from what is normal. This is done through a complex process that produces millions of unique antibodies which serve as recognition agents that can trigger  an immune response. 

After an antibody becomes activated through recognition/interaction with a foreign protein it is then mass produced by immune cells and circulates the body to form an immunological memory. This is how vaccinations work, where exposure to weakened or partial components of disease-causing microbes allows the body to prepare itself for launching a large immune response when the microbe is encountered in the future. 

During production by the immune system, these antibodies are supposed to be edited to remove ones that attack the self. In cases where this does not occur, autoimmune disorders, such as multiple sclerosis, result. In cases where the antibodies react to non-threatening proteins, such as those found in grass pollen, then allergy results. In a sense, allergy can be thought of as one extreme on a spectrum that has autoimmune disorders on the other. Health is the balance between the two where the immune response is always appropriate and controlled.



How Do Allergies Evolve?
Allergic disease and autoimmune disorders occur because the immune system gets conditioned to respond to inappropriate targets. If it attacks the body, it is an autoimmune disorder, and if it attacks a harmless environmental protein, then it is an allergy. In the case of allergy, pollen seasons can produce an enormous immune response because of the significant amount of exposure. Interestingly, these problems appear to be much more common in wealthy, industrialized nations.  One theory about why this occurs is called the Hygiene Hypothesis.

In population studies, prevalence of allergy and asthma is low in developing countries, which lack the advanced sanitation found in wealthy countries. This lack of sanitation leads to an early exposure to parasites and other microbes that is likely similar to what humans have experienced over centuries of evolution.  Since our immune systems evolved over long periods of time under constant exposure to microbes, if those microbes are removed, the system’s potential is still active but no  longer has a suitable target. 

Studies have shown that early immune system exposure to parasites and other microbes allows the system to develop the regulatory mechanisms that keep it focused and under control. In the normal course of childhood development, the immune system is supposed to shift away from the anti-parasitic inflammatory pathway predominant at birth toward the anti-bacterial non-inflammatory pathway. In clean environments that lack parasite exposure, this transition fails to occur and the child predisposed to allergy begins to develop an inflammatory immune response against harmless environmental proteins. 

This is not to romanticize unsanitary conditions, of course. Though rates of allergy and asthma are low in developing countries, rates of preventable diseases are much higher. This suggests that biological systems require balance, where overcorrection from one problem has unforeseen negative consequences.