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 analyst upgrades and downgrades -- just in case their reasoning behind the call makes sense.

What: Shares of Mellanox Technologies, Ltd. (NASDAQ:MLNX) rallied more than 4% in premarket trading Wednesday after Jefferies upgraded the fabless semiconductor company from hold to buy.

So what: Along with the upgrade, analyst James Kisner boosted his price target to $50 (from $37), representing about 40% worth of upside to yesterday's close. So while momentum traders might be turned off by Mellanox's price weakness over the past year, Kisner's call could reflect a sense on Wall Street that its growth prospects are becoming too cheap to pass up.

Now what: Jefferies lowered its 2014 EPS for Mellanox from $0.95 to $0.76 but raised its 2015 outlook from $1.85 to $2.00. "We're upgrading MLNX to Buy as our checks suggest the high-performance Interconnect specialist is likely to benefit from an upgrade cycle driven by the launch of Intel's Grantley server platform," said Kisner. "While we think light Q3 guidance is likely, we believe confidence in the shares remains quite low into the print and on balance would be owners." Given Jefferies' solid stock-picking track record -- currently ranked in the top 10% of our CAPS community -- Fools might want to take a closer look at Mellanox.