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

What: Shares of Ariad Pharmaceuticals (NASDAQ: ARIA) climbed 6% today after Stifel upgraded the biotechnology company from "hold" to "buy."

So what: Along with the upgrade, analyst Joel Sendek planted a price target on the stock of $7.00, representing about 60% worth of upside to yesterday's close. Ariad shares plummeted 65% in early October after the Food and Drug Administration halted all new trials of its leukemia treatment Iclusig, but physician enthusiasm over the drug at the American Society of Hematology's annual meeting suggests to Stifel that the problem is temporary.

Now what: According to Stifel, Ariad's risk/reward trade-off is particularly attractive at this point.

"Our discussions with physicians as ASH were universally positive regarding the risk/benefit for Iclusig and indicating continuing demand for the drug as a 3rd line agent for CML," noted Stifel. "We forecast resumption of commercial distribution in the US in 2Q14 with a black box warning label indicating increased risk of CV events, in line with the November EMA recommendation of risk factor assessment before treatment and monitoring during treatment."

So while Ariad is still too speculative for conservative investors, Iclusig's seemingly improved prospects, coupled with the stock's still-beaten down price (off more than 80% from its 52-week highs), make it an intriguing opportunity for biotech-savvy Fools.