Moody’s Investors Service has raised the ratings of a U.S. health insurer.

Moody’s, New York, has increased the issuer rating on senior unsecured debt issued by Health Care Service Corp., Chicago, to A1, from A2.

Moody’s also has increased the company’s insurance financial strength rating to Aa3, from A1.

Health Care Service is the parent of the Blue Cross and Blue Shield plans in Illinois, New Mexico, Oklahoma and Texas.

Moody’s is increasing Health Care Service Financial’s credit rating because the company now ranks fourth among U.S. health carriers in terms of enrollment, and it is very profitable, Moody’s says.

Possible risks at Health Care Service Financial include vulnerability to changes in the regulatory environment in Illinois and in other states in which the company does business, Moody’s says.