By Dr Ananya Mandal, MD
Allergies are when the immune system over reacts to everyday proteins and relatively harmless substances like pollen, animal dander, dust, dust mites, foods and some commonly used drugs.
The exact cause of allergies is not known. It is unclear why some people are prone to certain allergies to an extent that the condition may be life threatening while others are seemingly unaffected by such allergens. (1-4)
Sometimes allergic tendencies run in families. This is called atopy. The exact genetic cause for atopy is still unclear, however, there seems to be a rise of atopic cases over the past four decades.
Causes of the rise in atopic cases
It is not known why this rising trend occurs. Some researchers believe the rise in pollution, and addition of newer chemicals and allergens into the environment, is adding to the rise of cases of atopy.
On the other hand, some believe that increased upbringing in a cleaner and more germ free environment could be responsible for a relatively “under exposed” immune system.
This may be leading to over reactivity of the immune system when exposed to common proteins and substances in the environment.
There is evidence that better armaments against infectious diseases, and better prevention of infections by increased cleanliness, could be a reason for increased risk of allergies.
Some believe global warming also has an impact and changing patterns of natural vegetation and pollen production could be the reason for rising trends of allergies.
How does the immune system develop?
When a baby is born the immune system develops based on the genetics and environment. It either becomes predominantly TH2 or TH1. The former makes the individual allergy prone; whereas the latter makes the individual non-allergy prone.
TH cells are T helper cells or lymphocytes that fight invading microbes and organisms. TH1 immunity works in defending against bacteria and viruses and protects against allergies. TH2 immunity on the other hand fights parasite infections and makes an individual prone to allergies.
Allergies and TH2 immunity
Children with a family of allergic individuals are more likely to switch on TH2 immunity. This leads to increased amounts of allergy-related immunoglobulin E or Ig E in the blood.
The IgE goes on to attach with an allergen or a foreign protein and triggers an allergic reaction.
For example, when a pollen particle is inhaled it attaches to the IgE that is present in the nasal mucosal membranes. This complex molecule of IgE/pollen further attaches to the mast cells and this releases naturally occurring defence chemicals that include:
histamine and serotonin
leukotriene C4 etc.
Histamine and other allergic chemicals lead to itching, sneezing, runny nose and eyes etc.
Who is at risk of developing allergies?
Those at risk of developing allergies include atopic individuals who have a family member (usually a parent or a sibling) with a known allergic tendency.
Children who have no allergic family members have a 12% risk of developing allergies and those with one parent and two parents with allergies have a 20% and 40% risk of allergies respectively.
If both parents have the same allergic condition like hay fever, asthma or eczema the child’s risk of the same allergic condition rises to about 70%.
Other risk factors for developing allergies include:
Sex - men are found to be more prone to allergies than women. The cause of this is unknown.
Smoking - children who belong to families where parents or family members smoke are at greater risk of allergies
Childbirth - babies born by caesarean section are found to have higher allergic tendencies. The cause is unknown.
Environmental conditions - children who belong to smaller families with just one or two children or who are overweight or obese are at a greater risk.
Frequency of antibiotics - children who have frequent courses of antibiotics are at a greater risk of developing allergies.
It is speculated that this could be a two-way association. Children with allergies like asthma often get frequent respiratory tract infections and may need frequent courses of antibiotics.
Exposure to allergens - children who are exposed to very low amounts of dust mites, pollens, certain food items and pets and animals often have an increased likelihood of allergies.
Reviewed by April Cashin-Garbutt, BA Hons (Cantab)
Last Updated: Sep 29, 2013