Can you refuse to answer a question in court based on your answering might get you killed?

Most Americans are familiar with the 5th Amendment that guarantees your right to refuse to answer a question in court on the grounds that it might incriminate you. That right does not apply if you have been granted immunity since the possibility of self incrimination does not exist. Is there also the right to refuse to answer a question on the grounds that your answer might get you killed? For example, let's say you've been granted immunity and you're asked something that you know will infuriate a mobster who in turn will have you murdered. The police have offered you protection, but you have good reason to believe that this protection is not adequate or has been infiltrated by the mobster. Can you legally refuse to answer in court in order to save your life?