Clue: Important substance in immune development

Answer: DIRT

Length: 4

---

Similar Clues:

• International Association for the Development of ___ (organization that oversees the sport of freediving) (APNEA with 5 letters)
• Important loan info, for short (APR with 3 letters)
• Important info at JFK (ARR with 3 letters)
• Valley girl's "yeah, right" [IMPORTANT: SEE THE 'NOTE' (above) for key info about this week's puzzle and SEE THE LAST CLUE DOWN for the explanation of last week's puzzle.] (ASIF with 4 letters)
• Intro to "mania" [IMPORTANT: SEE NOTE ABOVE.] (BEATLE with 6 letters)
• Important West Indies crop (CACAO with 5 letters)
• Important initials for Cronkite (CBS with 3 letters)
• Important interval (ERA with 3 letters)
• Important info to txt to your ride from the airport (ETA with 3 letters)
• Important info for making connections (ETA with 3 letters)