NEW YORK -- Traders placed long-term bets on Celldex Therapeutics (CLDX - Get Report) on Monday, bucking the broad market downturn. 

OptionMonster's Heat Seeker tracking system shows that almost 3,500 January 2016 20 calls were purchased in the session, mostly for $5 and $5.10. The volume was above open interest, which indicates that new positions were initiated. 

These long calls lock in the price where the stock can be bought through early 2016 no matter how far it might climb. They could be sold sooner at a profit if premiums rise with a rally before then, providing potentially significant leverage, but the contracts will expire worthless if shares remain below $20. 

CLDX rose 3.04% to $18.97 on Monday. The biopharmaceutical company exploded higher in mid-November after reporting positive preliminary results for its rindopepimut therapy, which combats recurring brain tumors. The company called those results "exploratory" at that time and said it anticipated final data in three to six months. 

The stock has found support along its rising 50-day moving average since and is back near the top of its recent range. Short interest reportedly fell 5.6% in December but still accounts for an estimated 22.5% of the float. 

Monday's overall option volume in the name was six times its daily average for the last month. Calls accounted for a bullish 80% of the total.

-- Written by By Mike Yamamoto of  OptionMonster

Yamamoto has no positions in CLDX.