Immunity: Environment can have big impact

When germs enter the body, a powerful defense system may turn on. Known as the immune system, it calls in cells that serve as combat troops, snipers and medics. Some of these cells take on invaders. Others clean out sick and dead cells. Still more trigger healing. Genes hold instructions for making many of those immune troops and their weapons. But a study now finds that the environment may play a bigger role than genes do in revving up the immune system and shutting it down when the battle is over. 

Genes tell a cell what to do. Those instructions get passed down, in DNA, from parents to their offspring. But things around us — such as bacteria and viruses — also can influence that offspring’s development. The new study found that those environmental factors can influence immunity more than our genes do. And as people age, the environment’s impact on immunity grows stronger, the scientists now report.

The immune system is supposed to protect the body from invaders. However, many serious diseases can develop when the immune system turns against the body it is supposed to guard. This creates what are called autoimmune diseases. One of the better known is rheumatoid arthritis. Here the immune system attacks a person’s joints.

Studies like the new one suggest that genes alone may not be responsible for such autoimmune diseases, says Janko Nikolich-Zugich. He did not work on the new study. He studies aging and immunity at the University of Arizona in Tucson. The immune system seems to need a trigger — like an infection — to develop many immune disorders, he told Science News.

In the new study, scientists studied 105 pairs of twins. About three-quarters were identical twins. The rest were fraternal twins. Identical twins come from a single fertilized egg that splits in two; this creates two babies. Because each came from the same egg, these twins share nearly all of the same genes. Fraternal twins come from two separate eggs. They share only about half of their genes. This makes them no more similar than any other two siblings. After birth, twins go on to live different lives. This can expose them to quite different environments. (If life is like a board game, the pieces are the genes a person is born with — and the moves of the game are environmental factors.)

The scientists measured 204 different parts of the immune system. They counted a person’s immune cells, for example. They also measured various chemicals in the blood. They even studied the twins’ responses to flu vaccines (the medicine used to trigger a protective response to some disease-causing virus).

If an immune response was more similar within identical twins, genes probably played a major role. But some reactions were about the same in both types of twins. In this case, the environment likely had a bigger role. Overall, about 58 percent of the difference in responses could be traced to the environment — not genes, the new study found. Mark Davis of Stanford University in California and his coworkers reported their findings January 15 in Cell.

The new findings confirm “that genetics can play a big role,” Davis told Science News. However, he added, his team’s new findings also indicate that “inherited influences — genes — are not the whole story.”

For instance, a single virus changed 119 of the 204 reactions studied. That cytomegalovirus infects more than half of American adults. Most of those people will never show symptoms. In babies or people with a weak immune system, though, the virus can make people very ill.

But even where people have no symptoms, “that virus single-handedly changes your immune parameters,” Nikolich-Zugich told Science News.

The scale of the new study is “absolutely stupendous,” says Stephen Kingsmore. This scientist directs the Center for Pediatric Genomic Medicine at Children’s Mercy Hospital-Kansas City in Missouri.  

But the new twin study does have its limits. For instance, he notes, Davis’ group only studied people with healthy immune systems. Earlier studies suggested many immune diseases run in families and result when people inherit a faulty gene. So in those people, genes may be more important than the environment in deciding whether they develop a disease.

Power Words

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allergen  A substance that causes an allergic reaction.

allergy  The inappropriate reaction by the body’s immune system to a normally harmless substance. Untreated, a particularly severe reaction can lead to death.

arthritis  A disease that causes painful inflammation in the joints.

autoimmunity    A process whereby the immune system turns against its host. This inappropriate reaction can cause disease instead of curing it. Autoimmune diseases can be quite severe and hard for doctors to treat. They include rheumatoid arthritis (affecting joints, such as knees), multiple sclerosis (targeting nerves and muscles), Crohn’s disease (affecting the gut), psoriasis and lupus (affecting skin) and the type of diabetes that typically develops in young children. In all of these cases, the immune system generates out-of-control inflammation.

bacterium (plural bacteria)  A single-celled organism. These dwell nearly everywhere on Earth, from the bottom of the sea to inside animals.

cell   The smallest structural and functional unit of an organism. Typically too small to see with the naked eye,it consists of watery fluid surrounded by a membrane or wall. Animals are made of anywhere from thousands to trillions of cells, depending on their size.

cytomegalovirus  A virus related to the germs that cause chickenpox and infectious mononucleosis (also known as mono). At least half of American adults will have become infected by age 40. There is no cure. The germ spreads through close contact with another person’s bodily fluids, such as saliva, blood, urine and breast milk. People who become infected as adults usually develop no symptoms and therefore don’t realize they are infectious to others. But in babies and people with weak immune systems, this virus can cause fever, pneumonia, seizures, a rash or blotchy skin, blindness or coma.

disorder   (in medicine) A condition where the body does not work appropriately, leading to what might be viewed as an illness. The term can sometimes be used interchangeably with disease.

fertilize   (in biology) The merging of a male and a female reproductive cell (egg and sperm) to set in create a new, independent organism.

flu (see influenza)

fraternal   Of our relating to brothers, or others with whom people develop close friendships and affection. (in genetics) The term for a type of twin birth where each baby comes from a separate fertilized egg. This is in contrast to identical twins, which come from a single fertilized egg (creating two separate but nearly identical babies).

gene   (adj. genetic) A segment of DNA that codes, or holds instructions, for producing a protein. Offspring inherit genes from their parents. Genes influence how an organism looks and behaves. Scientists who work in this field are known as geneticists.

immune system  The collection of cells and their responses that help the body fight off infection.

immunity  The ability of an organism to resist a particular infection or poison by producing and releasing special protective cells.

immunology     The field of biomedicine that deals with the immune system. Scientists and doctors who work in this field are known as immunologists.

parameter   A conditions of some situation to be studied or defined that can be quantified or in some way measured.

influenza (or flu)  A highly contagious viral infection of the respiratory passages causing fever and severe aching. It often occurs as an epidemic.

sibling    A brother or sister.

vaccine  A biological mixture that resembles a disease-causing agent. It is given to help the body create immunity to a particular disease. The injections used to administer most vaccines are known as vaccinations.

virus  Tiny infectious particles consisting of RNA or DNA surrounded by protein. Viruses can reproduce only by injecting their genetic material into the cells of living creatures. Although scientists frequently refer to viruses as live or dead, in fact no virus is truly alive. It doesn’t eat like animals do, or make its own food the way plants do. It must hijack the cellular machinery of a living cell in order to survive.