No person shall be excused from testifying or from producing any papers, books, accounts or documents in any investigation or inquiry or upon any hearing ordered by the Commission, when ordered to do so by the Commission or any commissioner or administrative law judge, upon the ground that the testimony or evidence, documentary or otherwise, may tend to incriminate him or subject him to a penalty or forfeiture. But no person shall be prosecuted or subjected to any penalty or forfeiture for or on account of any transaction, matter or thing concerning which he may testify or produce evidence, documentary or otherwise, before the Commission or a commissioner or administrative law judge: Provided, that such immunity shall extend only to a natural person, who in obedience to a subpoena, gives testimony under oath or produces evidence, documentary or otherwise under oath. No person so testifying shall be exempt from prosecution and punishment for perjury committed in so testifying. The Commission or a commissioner or administrative law judge may, on the motion of a party or on its own motion, strike, in whole or in part, the testimony of a person who is not reasonably prepared to respond to questions under cross-examination intending to elicit information directly related to matters raised by that person in his testimony.