legal basis for the immunization of HMOs from being sued for malpractice

Consider such things as:
• the relationship of the HMO with its member physicians,
• the HMO organizational structure (IPA versus PPO versus staff model and so forth),
• the principle of direct liability, enterprise liability, indemnification, ERISA preemption, ostensible agency, and anything else that you believe would fall under an HMO's vulnerability to being sued.

An historical perspective would enhance the strength of your paper. 2-3 pages.