McDonald's iconic Big Mac turns 50 this year.

To celebrate, the chain is giving away MacCoins on Thursday. MacCoins are a "global currency" that can be redeemed for a Big Mac.

The Big Mac's massive reach is part of its power, but it also makes it difficult for the chain to make any changes, like adding a chicken or veggie version to the menu. In an interview with Business Insider, CEO Steve Easterbrook called the burger "sacrosanct."

It's a big year for the Big Mac.

This week, McDonald's is kicking off its iconic burger's 50th anniversary celebrations.

"It's not often that any food item is as relevant today as it was 50 years ago," McDonald's CEO Steve Easterbrook told Business Insider in a recent interview.

On Sunday, the chain unveiled the MacCoin, a "global currency" that can be redeemed for a free burger at McDonald's locations around the world. Customers can receive a MacCoin with the purchase of a Big Mac on Thursday, August 2, to be redeemed starting the next day at McDonald's in more than 50 countries around the world.

The deal was inspired by the Big Mac Index, an annual report that uses Big Mac prices at countries around the world to see whether currencies are properly valued.

While Easterbrook admits it may not always be the perfect approximation, it is one that reveals the international power of the Big Mac. There are few other items — in or out of the fast-food industry — with the global reach of the Big Mac, with McDonald's serving millions of theoretically identical burgers at more than 36,000 locations around the world.

McDonald's Grand Mac and Mac Jr. have been available for limited periods. Hollis Johnson

That reach and the burger's 50 years of history gives the Big Mac a certain power that few other fast-food items can hope to grasp. That's great in many ways (Easterbrook says the company sold 1.3 billion Big Macs around the world last year), but it can also force the company to tread carefully when it comes to tweaking the burger.

McDonald's has made very few adjustments to the Big Mac over the years, with one of the few changes being the removal of high-fructose corn syrup from its buns in 2016, something the company was careful to do without impacting the burgers' flavor.

McDonald's has found success with offering different sizes in the form of the Grand Mac and Mac Jr. for limited periods. However, Easterbrook says the chain is cautious of making big changes like swapping the patty for a veggie burger or chicken.

"We think the Big Mac needs to be sacrosanct," Easterbrook said.

Another thing McDonald's won't get the Big Mac mixed up in: cryptocurrency.

Despite the MacCoin's phonetic similarities to Bitcoin, Easterbrook assured Business Insider that the "currency" was purposefully created to be "tangible," in keeping with the Big Mac's meaty tradition.