Americans have a lot to be grateful for on Thanksgiving. The Four Horsemen of the Apocalypse have harried human beings since we dropped down from the trees, but three of the four — War, Famine and Pestilence — have essentially been banished from our shores. We are so rich that we spend our days worrying about getting fat and sedentary (particularly as we tuck into that second helping of pie).

The fourth horseman, Death, is still with us. But organ donors manage to cheat the Grim Reaper by using their deaths to give others life — and giving many of the rest of us something to be extra-thankful for.

In 2017, The Post ran Laura Lefler Herzog’s beautiful reflection on the lives her brother Trey saved after a fatal car accident. Trey had checked the organ donor box on his license, and also made it clear to his girlfriend that if he was killed, he wanted to help as many people as possible: “I’ll be with the Big Guy,” Trey said. “Give it all.”

I read Herzog’s story when we published it, but I read it again Friday morning because I have a friend who is waiting for a heart-lung transplant. More than 100,000 other people are also waiting on America’s transplant lists, and suffering terribly, because you don’t get to take your place in line for a donated organ unless you are already very, very sick.

Unfortunately, we have a terrible shortage of transplantable organs in the United States. Most organs can only be donated after death, and only a tiny fraction of the people who die in America each year suffer the kind of head trauma that causes brain death but leaves the organs still healthy enough for transplant doctors to harvest. Of those people, some will not be healthy enough to make good organ donors. So even if every eligible donor organ were used, we’d still have a shortage in many critical areas.

But we also have a shortage because not every eligible donor organ can be harvested. Only 54 percent of Americans have registered as donors. My friend, and everyone else on a transplant list, desperately need us to make that same decision to “give it all” — and not just by checking the box on our driver’s license that opts us in to the program. (Dare I suggest that you could reach over to your wallet right now and make sure that you’ve done so?) Though there’s been a fair amount of discussion in recent years about moving the United States to an “opt out” system where people are presumed to want to donate their organs unless they specifically check a box saying “don’t,” it’s still up to all of us to indicate our wishes.

And even if the United States does make that switch, it turns out that it actually doesn’t matter that much, because all organ donation systems, whether opt-out or opt-in, give the final say to the families. In fact, opt-out systems might in some ways be worse, because they deprive those families of an important signal from their loved ones that yes, they really wanted to give this last, momentous gift.

Even in opt-in systems, families have the right to ignore that signal. And sometimes they do, because that moment of grief and terror when they’re told there’s no longer any hope is not the best time to be deciding whether their beloved sister, husband or child really meant it when they checked the box.

Which means that if you actually do want to “give it all,” you need to do more than check the box; you need to make it crystal clear to your family that if the worst happens, they should tell the transplant doctors to go ahead. It would be a good idea not just to write that wish down, but to say it out loud to as many relatives as possible, so that they can support each other, rather than second-guessing the decision. In fact, one might say that the day after Thanksgiving is the perfect time to mention it, when everyone is still there, and still filled with postprandial bonhomie.

I’m not suggesting you weigh the holiday down with an extended meditation on the brevity of life. I’m just suggesting you could say to your family what I just said to my husband when he wished me good morning: “Hey, I was just reading a beautiful piece on an organ donor who saved five lives, and if anything ever happens to me, I want you to donate as many of my organs as possible to anyone who can use them.”

If the time comes, hopefully they’ll remember. And if the time comes, hopefully so will a lot of grateful families who then get to sit down with their loved one next Thanksgiving, and for many Thanksgivings to come.

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