Sick and sicker: immunity differences in identical twins

Jonathan and Matthew developed colds a while back.
Jonathan is on the mend now, racing the dog from the dining room through the kitchen, eating like a teenager and jumping from the sofa to the floor over and over and over again, ignoring my demands that he stop.
He's driving me crazy.
Matthew is curled up on another sofa, covered with a blanket and watching TV through half-opened eyes. It breaks my heart to watch him hold his ribs when hacking coughs overtake his body. I can't wait for the antibiotics to do their stuff.
He has walking pneumonia.
This medical inequity is nothing new.
A few weeks ago, Jonathan developed a fever that lasted for two days. Matthew caught the same virus, but his fever continued for seven days. A stomach bug that left Jonathan slightly dehydrated for a day as a baby left Matthew with bleeding ulcers and a month's prescription of Zantac.
When Jonathan develops an ear infection, Matthew often gets it in both.
My immediate reaction was to surmise that somehow, when the egg split, Matthew lost an immunity gene to Jonathan. It made sense. Matthew's always been sicker and he was born lighter, slighter, a bit more frail than his brother.
But I came to a different conclusion after doing a little research.
Recent studies are finding that epigenetics -- or the way in which genes express themselves in different environments -- is likely responsible for many differences that develop in identical twins, particularly when it comes to immunity.
Sometime after conception, either in the womb or outside it, one of the twins was likely exposed to a virus or bacteria that missed the other. In fighting off the intruder, he either gained any army (Jonathan), strengthening his physical fortress, or lost one (Matthew), weakening his defenses.
Their identical genes learned to express themselves in different ways when confronted by bacterial infections or viruses, resulting in permanently different immune systems.
Amanda Carpenter, a virology student, writes about an excellent example here. Identical twins born in 1983 were exposed to HIV, the virus that causes AIDS. Fifteen years later, one was relatively healthy and strong while the other was sickly and ill.
After studying blood samples from the pair, researchers concluded that environment, which led to a depressed immunity system in one twin, was likely to blame for their different reactions. Their shared DNA did not ensure a shared prognosis.
My hope for Matthew is that someday this will turn itself around -- that someday he'll be exposed to something that makes him stronger instead or weaker -- and that Jonathan's immunity will remain unchanged.
Who knows?
Maybe this is the one.
Maybe all that hacking and coughing that kept me up last night, worried that he will choke or stop breathing, is the just influence his genes needs.

Ear aches and fevers

I was wrong when I guessed that Matthew had a weaker immune system than his twin brother. This time Jonathan is the more sickly one.
Both boys have ear infections (in the same ear), but Jonathan has more congestion than Matthew along with a fever of 102 degrees. Matthew has no fever.
Matthew has regained his weight from his earlier illnesses. At the doctor's Monday, he weighed just two ounces less than Jonathan at 26 pounds, 14 ounces.

Genetically identical can sometimes mean genetically different

Health issues were a big factor behind our decision to find out whether our boys were identical. If one had allergies, it made sense that his identical twin would have allergies. The same logic applied to genetic diseases, or so we thought.
This recent difference in immunity between the boys, though slight, inspired me to do a little research. What I found was that identical twins are an exciting mystery to the world of genetic disease research: a mystery because it would make sense that shared genes would mean shared genetic diseases. Yet that is not always the case. Sometimes, one identical twin develops a genetically influenced disease when the other does not.
The research is exciting because studies of identical twins can provide the key to cures for such diseases as Rheumatoid arthritis, leukemia and myopia. No laboratory can create specimens as perfect for such research as identical twins.
As for my boys, Matthew is making up for lost calories. He is shoveling food in his mouth faster than I can put it on his tray. The antibiotics have give him the edge on Jonathan, who is recovering from their illness, but not quite as quickly. Jonathan starts throwing food off his tray after the first five minutes.
But even runny noses and hacking coughs cannot slow these two guys down. Yesterday, they learned how to open the laundry chute. They scrape each other with the metal door as they fight over who gets to play with it. So for now, my laundry chute is taped shut and I have to peel the sticky stuff off every time I want to toss a bib into the hamper.

Immunity

The differences between the boys are becoming more apparent.
Unfortunately, it doesn't bode well for Matthew.
Both boys caught a stomach virus a few weeks ago. Matthew threw up longer and more often than his brother. At their one-year appointment a week later, Matthew was a pound lighter than Jonathan.
Now, they both have colds. It started with runny noses nine days ago. Then Matthew developed a chesty, mucusy cough. I took him to the doctor's when I noticed that he would sometimes stop playing and scream for a minute.
The doctor checked both boys.
Jonathan is fine. Still no cough. Still just a runny nose.
Matthew has an ear infection and this time, he was a one pound, 10 ounces lighter than his brother. He still has the nasty cough, but his chest is clear.
The antibiotics should kick in tomorrow.
Though Matthew apparently has a lower resistance to illness than Jonathan, he proved last night that he is stronger in other ways: he took his first independent steps.
So Matthew will get better soon, probably just a few days after his brother.
And Jonathan will be walking soon, probably just a few days after his brother.