An interrogative pronoun is a pronoun used in order to ask a question.
Often it has no antecedent because the antecedent is unknown. That is why the question is being asked!
In modern English there are five interrogative pronouns:
what, which, who, whom, and whose.Note that all five words may also be used as relative pronouns. A relative pronoun may be found in a question; an interrogative pronoun is found only in a question.
In addition, these pronouns may take the suffixes -ever and -soever.
Examples (interrogative pronoun in italics):
What did you say?
Who said that?