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Why the First Born Often Suffer from Allergies – While Their Siblings Don’t

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If you’re the eldest child and suffer from allergies or asthma while your younger siblings don’t – they may have you to thank.
Despite our shared DNA when it comes to allergies between siblings there are significant differences. Numerous studies into these differences indicate that younger children often have stronger more dynamic immune systems than their older siblings – making them less susceptible to many increasingly common ailments such as asthma, hay fever and common food allergies.

What still remains a mystery is why or how this phenomenon occurs.

Researchers aren’t sure why younger siblings appear to have more protection against allergies compared to their older siblings. Allergic diseases such as asthma, hay fever, eczema and food allergies have dramatically increased over recent generations, together with the threats of infectious disease pandemics. The scientific community who have been trying to understand what the underlying causes have been working on a number of theories.

The Hygiene Hypothesis

The hygene hypothesisa theory says that allergies happen when the immune system does not develop properly. It was first put forward in the British Medical Journal by David P. Strachan, a professor of epidemiology, Strachan suggested that as younger children are exposed to germs by older siblings helps their immune systems develop and strengthen, reducing their susceptibility to disease – resulting in fewer instances of hay fever .

Since this theory was first put forward in the late 80’s multiple studies have since demonstrated how early-life exposure to pets or siblings affords protection against allergic disease. It has also been found that children in developing areas of the world are less likely to develop allergies and asthma compared with children in the developed world.

The Microflora Hypothesis

More recently, the composition and diversity of the babys gut microbiota has also been linked to allergic disease and asthma.
The microflora hypothesis suggests that early life exposure that disrupts the composition of the bacteria in the gut affects the immune system in the form of hypersensitivity disorders such as asthma and allergies.
As a dramatic increase in allergic disease has been seen in western populations over a few decades, researchers believe changes in our lifestyle and modern behaviours that are to blame, such as antibiotics and diet which are known to adversely affect our microbiota.

They have found that the microbiota or microflora of allergy sufferers differs from that of individuals without allergies and that these differences can be identified even before the onset of the allergy, consistent with a possible causative role.

In-Utero Hypothesis

Another theory is that this increased immunity is a result of changes in the womb: Multiple pregnancies could be causing changes to the mother’s immune system, which may in turn affect the fetus, according to Takashi Kusunoki, of the Shiga Medical Center for Children in Shiga, Japan.
Susanna Brix Pedersen, a biologist at the Technical University of Denmark suggests that after the first pregnancy, the mother’s immune system recognizes the presence of a new fetus as similar to its previous occupant and aids in the development of its (the fetus) immune system differently. To support this theory, Danish researchers studied the mucus from 1 month old infants found that babies with elder siblings had higher quantities of signal proteins; responsible for recognising and triggering immune responses. The researchers think this helps younger siblings respond to foreign objects like pollen through means other than allergy.
A Further study showed that as more time passes between pregnancies, the lower is the additional boost given by the immune system. In this case, the fetus would be getting almost the same treatment its older sibling had received.

Linking this theory with the hygene hypothesis combines further benefits after birth which could play a role in this increase in immune strength. Studies show that babies, when exposed to microbes soon after birth, tend to build a stronger resistance to it. With the presence of an older sibling, who could be messy and quite ignorant towards hygiene, the changes of being affected by said microbe increases. This forces the immune system of younger sibling to be stronger. However, if there was a substantial gap between pregnancies, the chances are that the older sibling is quite hygienic which would mean that there was no reason for the immune system to develop more that it requires.

While nobody can be sure as yet the exact reason for this difference between people that share DNA, there is little doubt that there is considerable reason to believe that the younger a sibling, the more their ability to ward of diseases.

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