Another Blue Cross health insurer could compete for Western Pennsylvanians’ business under a historic merger agreement announced Friday, lining up a potential new alternative to Highmark and UPMC Health Plan.

Indianapolis-based Anthem Inc. said it would buy Cigna Corp. of Bloomfield, Conn., for about $54.2 billion, a record deal that would create the largest U.S. health insurer by membership. Executives said their national plan should save billions of dollars in corporate expenses and enable them to contain prices for their policies, which would cover more than 53 million people — about 17 percent of the country.

That scale could help the merged company dent Highmark and UPMC’s dominance in the Pittsburgh region, where the Downtown-based nonprofits control nearly 80 percent of the commercial health insurance market, said Dennis Olmstead, chief strategy officer at the Pennsylvania Medical Society.

“If the consolidation results in lower premiums for the residents of Pennsylvania and providers are paid a fair rate, it’s probably not a bad thing. We don’t know how all of this is going to play out,” Mr. Olmstead said. The society represents about 20,000 physicians and medical students statewide.

An Anthem-Cigna company would start small in Pennsylvania, where Cigna customers make up about 3 percent of the health insurance market, according to state records. Anthem has no presence, state Insurance Department spokeswoman Ali Fogarty said.

Still, with policyholders across 14 states, for-profit Anthem is the biggest member of the Blue Cross Blue Shield Association, a Chicago-based federation of 36 health insurance groups. Three of them are rooted in Pennsylvania: Highmark in the west, Capital in the Harrisburg area and Independence in Philadelphia.

Although the Blues generally avoid one another’s turf, industry analysts said, nuances in association standards might let Anthem use Cigna’s footprint to offer policies in Western Pennsylvania. Any Anthem policies in the region probably could not carry Blue Cross or Blue Shield branding under those rules, Mr. Olmstead said.

Highmark declined to comment, calling it too early in the process to speculate. Anthem said it has already “integrated non-Blue-branded business in states where we do not operate the local Blue Cross and Blue Shield plan.”

“While we do not have specifics to offer at this point, there are multiple ways in which that can be accomplished with regard to this transaction, and we are committed to working collaboratively with [the association] regarding any potential compliance issues,” Anthem spokeswoman Kristin Binns said in an email message.

The association said it routinely reviews mergers to make sure member companies are following the rules. It shared little else in a brief statement, emphasizing that the government review and approval processes should take months.

At UPMC Health Plan, spokeswoman Gina Pferdehirt said only that the group “remains committed to continue to offer high-quality coverage options at competitive prices regardless of who our competitors are.”

Anthem and Cigna said their deal should close in the second half of 2016. They unveiled the acquisition three weeks after Aetna Inc. agreed to buy Humana Inc. for $37 billion, another arrangement that regulators will scrutinize for monopoly risks and other threats to the public.

Physician and consumer advocacy groups urge caution over the deals, which they say could inflate prices for patients and jeopardize negotiating power for health care providers. The mergers together would slim the top five health insurers nationwide to three. The No. 1 carrier, UnitedHealth Group Inc., counted nearly 46 million members as of June 30.

“Stronger insurers could result in lower health care costs. The question is whether those lower costs get passed along to consumers and employers” who pay for the coverage, said Larry Levitt, a senior vice president at the Kaiser Family Foundation in Menlo Park, Calif.

Evolving insurance markets under the federal Affordable Care Act and consolidation among hospitals are helping fuel the mergers, which might give insurers more leverage over providers, Mr. Levitt said.

Yet a strengthened Anthem could have a tough time breaking into Western Pennsylvania, where Highmark maintains the entrenched Blue Cross Blue Shield presence with patients, insurance brokers and physicians, he said.

“It’s not impossible. The market can shift,” Mr. Levitt said. “But those changes happen slowly.”