Facebook on Wednesday began rolling out a tool that helps inform Americans that nearby blood banks need donations.

If you sign up for it, you'll be alerted when one of Facebook's blood bank partners in your area needs a specific kind of blood donor.

The tool launched in Bangladesh, Brazil, India and Pakistan in 2017 and is rolling out now in Chicago, New York City, the San Francisco Bay Area, Baltimore and Washington, D.C., before it's available nationwide "in the coming months," Facebook said.

You have to opt in to the program, which means Facebook won't send you notifications out of the blue. But if you like to donate blood anyway, this can make it easier to locate blood banks that need your blood type. Facebook's blood donation donor terms page, which you should read first, says it's just connecting you and blood banks.

"Facebook does not guarantee that you will be able to donate blood," the page says. "No contract, express or implied, is created hereby among any parties using the Feature, or with Facebook."

"Facebook is not responsible for assessing the licensure, accreditation, or reputation of a Blood Bank," it adds. "Donors are responsible for assessing the fitness of a Blood Bank prior to donating blood." It also recommends that you receive medical advice from a qualified medical professional before you donate.

Current partners include America's Blood Centers, the American Red Cross, Inova, New York Blood Center, Rock River Valley Blood Center, Stanford Blood Center, Versiti and Vitalant, Facebook said.

Here's what you do if that sounds good and if you want to sign up and you're in one of the aforementioned locations.

Visit Facebook's blood donation page.

Tap "Sign up."

Agree to the program terms by clicking sign up again.

You can invite friends or share that you've signed up.

That's it. Now Facebook says it will "notify you when nearby blood banks need your help." There's no obligation to donate, but you'll know if someone needs you.

If you want to stop getting these notifications, do this:

Open your profile on Facebook.

Tap "About."

Choose "Details about you" on the left bar."

Tap the "X" next to "Blood Donor on Facebook."

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