The most memorable element of this episode, which otherwise serves as a template for any number of other later episodes where the given crew must pull their ship through a given cosmic crisis (so vague you otherwise know it's fairly basic, generic Star Trek storytelling), is a Starfleet ship with an entirely Vulcan crew.
|via Space Doubt|
While the second season as a whole did a great deal of work establishing firm continuity for the series (and later franchise), this Vulcan crew remains an anomaly that was never really explained. Are we to believe that Starfleet has ships populated by different member species of the Federation? So much had always been made of Spock's singular status as a Vulcan in Starfleet, but this episode suggests that it's more that he serves among humans rather than among his own kind. Who knows? Best not to think about?
Other than that, "The Immunity Syndrome" is an episode that presents a giant crisis our crew just happens to be able to solve. Not exactly subtle storytelling, nor highlight of the season or series. But it's also very typical of the series, and indeed franchise. But maybe don't think of it in that way. If you can. Basically the template for technobabble episodes to come.
four quarter analysis
franchise * series * essential * character