Affiliates of UnitedHealth Group Inc. the nation’s largest health insurer, have taken their first shot at knocking out a class action lawsuit claiming they are illegally underpaying reimbursements for mental health services.

Lawyers at Zuckerman Spaeder and Psych-Appeal, Inc. sued United Healthcare Insurance Co. and United Behavioral Health in October claiming that the companies imposed arbitrary reimbursement penalties for certain psychotherapy services provided by psychologists and master’s-level counselors rather than doctors. Their client, an anonymous Pennsylvania woman, claimed UnitedHealth reduced the “eligible expense” of charges from her counselor by 35 percent, even though there would have been no reduction for identical services had they been provided by a physician.