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 Pfizer (PFE) gained over 2% this morning after Jefferies upgraded the pharmaceutical giant from hold to buy.

So what: Along with the upgrade, analyst Jeffrey Holford boosted his price target to $38 (from $33), representing about 24% worth of upside to yesterday's close. While momentum traders might be turned off by the stock's sluggish action in recent months, Holford thinks Pfizer is too juicy to pass up given its growth and margin enhancement opportunities.

Now what: Jefferies raised its 2014 earnings-per-share outlook for Pfizer from $2.24 to $2.27 and its 2015 view from $2.33 to $2.37. "Oncology and restructuring are key themes for the Pharma sector," noted Holford. "Pfizer has both with [breast cancer drug] palbociclib looking like it could launch before year end, while management look to give increased visibility on a new organizational structure from Q1'14." When you couple those solid prospects with Pfizer's peer-lagging forward P/E of 13, it's tough to disagree with Jefferies' bullish stance.