Are peanut allergies genetic?
-A middle school student from California
June 13, 2007
Peanut allergies are definitely genetic. But genes aren't the whole story. What happens to us plays a role in whether we'll be allergic to peanuts too.
A peanut allergy happens when our immune system gets confused. As you probably know, our immune system protects us from invaders like bacteria and viruses.
These invaders make proteins that we don't. Our bodies recognize that these proteins are foreign and sometimes attack
But sometimes an immune system has trouble telling whether something is dangerous or not. It will then overreact to the foreign protein. Like maybe a protein in a peanut.
Some immune systems see a peanut protein and leave it alone. Others go berserk and launch such a powerful attack that it ends up killing the person. Why is this?
Well, part of it has to do with the different versions of certain genes that each person carries. Some of us have genes that tell our bodies to mount an attack against certain proteins. But this is only the first step.
The immune system also needs to be primed somehow to recognize a peanut protein as a bad thing. One way this may happen is through being exposed to peanuts while in the womb or early on as a baby. And there are probably lots of other ways that we don't know about.
But the first step is having the right set of genes. We know this for a couple of reasons.
First, peanut allergies tend to run in families. If you have a close relative with a peanut allergy, your risk of being allergic to peanuts is 7%. If you don't, then your risk is only 0.5%.
So you are 14 times more likely to have a peanut allergy if you have a relative with one. But these kinds of results alone are never that convincing. Why not?
Because close relatives tend to live close together. They eat the same foods, breathe the same air, etc. In other words, close relatives usually live in similar environments.
So if something is caused by the environment, it will tend to affect people living together more. Think about people living near a toxic waste dump.
Most members of the family will develop whatever illness the dump causes. But this doesn't mean the disease runs in the family. Just that they all live next to some awful disease causing thing.
One of the ways scientists figure out what is really going on is with twin studies. There are two kinds of twins -- fraternal and identical.
Identical twins share all the same DNA. Which means they have the same versions of all 25,000 of their genes. Fraternal twins only share about 50% of their DNA -- as much as any brother or sister.
If something happens more often to both twins in an identical pair, then genes are important. Because these kinds of twins have the same DNA.
When scientists have looked at peanut allergies, they have found that twins in an identical twin pair are both more likely to be allergic to peanuts. For example, in one study, both twins in a fraternal twin pair had peanut allergies 7% of the time. For the twins in identical pairs, both had the allergies 64% of the time. So genes are definitely important.
Which specific genes are important isn't yet known. Clearly they will have to do with the immune system but we just don't know which are the important ones yet.
But we do know that genes are not the whole story. If they were, then if one identical twin was allergic to peanuts, the other one would always be allergic too. Because they have the exact same gene versions.
The fact that in 36% of the cases this wasn't true suggests that the environment plays a role too. Scientists don't know everything in the environment that can trigger a peanut allergy, but they suspect that what mom eats can affect a child's chances of developing the allergy.
A recent study showed that if mom eats peanuts while pregnant, the risks for the child of being allergic to peanuts goes up 4-fold. For example, if the risk was 0.5%, it would now be 2%.
And if mom eats peanuts while breastfeeding, then the child's risk goes up by 2-fold. If these studies hold up, then clearly being exposed to peanuts at an early age can increase a child's risk of being allergic later.
And the baby or mother may not even need to eat the peanuts. For awhile in Great Britain scientists thought a diaper rash cream might be part of the reason that peanut allergies have been increasing there.
Apparently, this cream was made with a small amount of peanut oil. The thought was that spreading it on the baby exposed the child to peanuts. And may have triggered the peanut allergy.
The jury is still out on this particular cause. But it shows that triggers can be anywhere. Who knows whether something out there might just look like a peanut protein and so trigger the allergy even though mom and baby were careful to avoid peanuts. We just don't know enough yet.
What we do know is that certain genes make someone's immune system more likely to overreact to a peanut. And that coming into contact with peanuts early on can increase someone's risk.
Interesting facts about peanut allergies:
- Between 0.5% and1% of the population is allergic to peanuts. And the percentage is apparently rising.
- Only 20% of people outgrow their peanut allergy as adults. This is a much smaller percentage than other food allergies.
- People in France and the United States tend to be allergic to peanuts. Other countries have different allergies depending on the nut they most commonly eat.
- Chinese people aren't usually allergic to peanuts even though it is a big part of their diet. One factor may be how they prepare it. Americans tend to dry roast their peanuts while the Chinese fry or boil them. Studies have shown that the higher temperature of dry roasting makes the peanuts more allergenic