Mass. Gen. Laws ch. 176B § 14
Every medical service corporation is hereby declared to be a charitable corporation. No such corporation shall be liable for injuries resulting from negligence or malpractice on the part of any participating physician, participating chiropractor or any other participating provider of health services under the laws of the commonwealth or of any of its employees, nor shall it be liable for the cost of medical services, chiropractic services or other health services under the laws of the commonwealth to which the subscriber or covered dependent may be entitled under the provisions of chapter 152. Every such corporation shall be exempt from all provisions of the insurance laws of the commonwealth, except as otherwise provided in this chapter. The property of every such corporation shall, except as hereinafter provided, be exempt from all state and local taxes.
Mass. Gen. Laws ch. 176B, § 14