Do twins become allergic to the same things?

If one of a pair of twins is allergic to something, does that mean the other twin will be sensitive too?

Allergies, whether spring sneezes due to pollen or trouble breathing triggered by a certain food, are caused by a combination of someone’s genes and the environment they live in. The more of these things two people share, the higher their chances of being allergic to the same things. Twins are more likely to share allergies because of everything they have in common, but the story doesn’t end there.

Allergies are really complex and a lot of factors play a role in who gets them and who doesn’t.

Your immune system makes defence proteins, antibodies, whose job is to keep watch and attack any invading germs or other dangerous substances that get inside your body before they can make you sick.

An allergy happens when your body mistakes some usually harmless substance for a harmful intruder. These trigger molecules are allergens. The antibodies stick like suction cups to the allergens, setting off an immune system reaction. That process leads to common allergy symptoms: sneezing, a runny or stuffy nose, itchy, watery eyes, and a cough. These symptoms can be annoying but minor.

Allergies can also cause a life-threatening reaction, anaphylaxis, which requires immediate medical attention. For example, if someone ate a food they were allergic to, and then had throat swelling and a rash, that would be considered anaphylaxis.

The traditional treatment for anaphylaxis is a shot of the hormone epinephrine (also known as adrenaline) into the leg muscle. Allergy sufferers can also carry an auto-injector to give themselves an emergency shot in case of a life-threatening case of anaphylaxis. An epinephrine nasal spray is now available, too, which also works very quickly.

A person can be allergic to things outdoors, like grass or tree pollen and bee stings, or indoors, like pets and tiny bugs called dust mites that hang out in carpets and mattresses.

A person can also be allergic to foods. Food allergies affect 4% to 5% of the population. The most common are to cow’s milk, eggs, wheat, soy, peanuts, tree nuts, fish, shellfish and sesame. Sometimes people grow out of allergies, and sometimes they are lifelong.

Each antibody has a specific target, which is why some people may only be allergic to one thing.

The antibodies responsible for allergies also take care of cleaning up any parasites that your body encounters. Thanks to modern medicine, people in the United States rarely deal with parasites. Those antibodies are still ready to fight, though, and sometimes they misfire at silly things, like pollen or food.

Hygiene and the environment around you can also play a role in how likely it is you’ll develop allergies. The more different kinds of bacteria that you’re exposed to earlier in life, the less likely you are to develop allergies. Studies have even shown that children who grow up on farms, children who have pets before the age of five, and children who have a lot of siblings are less likely to develop allergies. Being breastfed as a baby can also protect against having allergies.

Children who grow up in cities are more likely to develop allergies, probably due to air pollution, as are children who are often around people who smoke.

Children are less likely to develop food allergies if they try foods early in life rather than waiting until they are older. Sometimes a certain job can contribute to an adult developing environmental allergies. For example, hairdressers, bakers and car mechanics can develop allergies due to chemicals they work with.

Genetics can also play a huge role in why some people develop allergies. If a parent has environmental or food allergies, their child is more likely to have allergies. Specifically for peanut allergies, if your parent or sibling is allergic to peanuts, you are seven times more likely to be allergic to peanuts.

Back to the idea of twins: yes, they can be allergic to the same things, but not always.

Researchers in Australia found that 60% to 70% of twins in one study both had environmental allergies, and identical twins were more likely to share allergies than fraternal (nonidentical) twins. Identical twins share 100% of their genes, while fraternal twins only share about 50% of their genes, the same as any pair of siblings.

A lot more research has been done on the genetics of food allergies. One peanut allergy study found that identical twins were more likely to both be allergic to peanuts than fraternal twins were.

So, twins can be allergic to the same things, and it’s more likely that they will be, based on their shared genetics and growing up together. But twins aren’t automatically allergic to the exact same things.

Imagine if two twins are separated at birth and raised in different homes: one on a farm with pets and one in the inner city. What if one’s parents smoke, and the others don’t? What if one lives with a lot of siblings and the other is an only child? They certainly could develop different allergies, or maybe not develop allergies at all.

Scientists like me are continuing to research allergies, and we hope to have more answers in the future.

Breanne Hayes Haney is an immunologist specialising in allergies at West Virginia University in the US.

* This article is adapted from a piece thatoriginally appeared in The Conversation’s Curious Kids series, where experts answer questions from children, and is republished under a Creative Commons licence.

-BBC