Leaving aside "interjections", we recognise eight (8) word categories (parts of speech). Six (6) of them can function as the head of corresponding phrases: the other two can’t. The very small coordinator and subordinator classes do not function as head but serve as markers of coordination and subordination: there is no such thing as 'coordinator phrases' or 'subordinator phrases'.
 Noun (NP) “this clear case of dedication to duty”
 Verb (VP) “saw something in there”
 Adjective (AdjP) “very eager for further news”
 Determinative (DP) “almost every”
 Adverb (AdvP) “quite separately from this issue”
 Preposition (PP) “right out of the area”
 Coordinator (no phrasal category)
 Subordinator (no phrasal category)
Note: pronouns are not treated as a distinct part of speech, but as a subclass of noun.