While Fools should generally take the opinion of Wall Street with a grain of salt, it's not a bad idea to take a look at particularly stock-shaking upgrades and downgrades -- just in case their reasoning behind the call makes sense.

What: Shares of Celldex Therapeutics (CLDX -2.78%) popped 10% today after Leerink Swann raised its price target on the stock by a whopping 60%.  

So what: Leerink maintained its "outperform" rating on Celldex, but a new price target of $45 per share (from $28) implies a valuation of more than $1 billion and represents 55% worth of upside to Friday's closing price. Analyst Howard Liang believes that Celldex's experimental cancer immunotherapy, CDX-1127, has huge potential as a co-stimulatory agonist, but given the fact that there's still no human data on the drug, Fools shouldn't read too much into the call.

Now what: Celldex expects to report its first phase 1 patient data in early November.

"As a combination partner for immune checkpoint inhibitors such as anti-PD-1 agents, we believe the efficacy bar is not high based on our analysis of early clinical data on other IO agents, and we believe any demonstration of single-agent activity coupled with a clean safety profile could position CDX-1127 as one of the leading candidates for PD-1 combination," Liang wrote in a note to clients.

Of course, when you consider just how far CDX-1127 still has to go, the risk/reward trade-off on Celldex shares -- which have already run 380% over the past year -- might be as attractive as Leerink would have you believe.