FRYEAN MYTHOS: *drama*
CAMPBELLIAN FUNCTION: *cosmological, sociological*
"Immunity Syndrome" is another well-made "white-knuckle" thriller, without many deeper resonances, aside from the fact that the "syndrome" of the title applies to humankind itself, as explained by Doctor McCoy:
Now, isn't that a thought? Here we are, antibodies of our own galaxy, attacking an invading germ. Be ironic indeed if that were our sole destiny, wouldn't it?
The cosmological theme of the story involves the Enterprise combating a gargantuan amoeba-like organism that has annihilated billions of lives in a nearby star-system and which is drawing the ship toward it like Charybdis trying to engulf the ship of Odysseus. Refreshingly, Robert Sabaroff's script doesn't try to make the danger relatable to audiences by stressing what will happen when the organism reaches Earth: it's implied that the giant cell is a danger to the entire universe, not just the HQ of the Federation. Indeed, within the scope of the episode the creature's first victims are Vulcans.
The episode makes an interesting contrast to "Obsession." In that episode, Kirk wants to destroy the gas-creature because it killed someone important to him and made Kirk feel impotent to strike back. Spock doesn't know any of the Vulcans killed, though in a near-mystical sense he empathizes with them far more than with the humans he lives with. Philosophically Mister Spock abjures revenge for its own sake. However, he argues with fierce logic, as the Enterprise is being drawn into the creature's midst, that he alone must be the one to pilot a shuttlecraft ahead of the main ship, plunging past the cell's membrane to gather intelligence on its nature so that they can destroy it. It's a little vague as to how much Spock's intelligence contributes, for the solution simply pops into Kirk's head at one point: to destroy the monster with anti-matter taken from the ship;s engines (another idea possibly cadged from "Obsession.")
Since the giant cell in itself doesn't generate any interpersonal conflict, the Sabaroff script picks up on the quarrelsome relationship of Spock and McCoy and ratchets it up into intellectual competitiveness: McCoy, never a big research-guy, suddenly wants to pilot the shuttle so that he can get up close and study the big cell even as they're trying to find a way to destroy it. Happily future episodes ignored McCoy's uncharacteristic portrait, and despite its problematic nature actors Nimoy and Kelley sell the dramatics for *more* than they're worth-- particularly with the telling line, "Tell Doctor McCoy-- he should have wished me luck."
An interesting side-note that at one point in Sabaroff's script, the creature's malign tendency to suck the energy out of organisms is termed "anti-life" by one of the characters. A few years later, comics-artist Jack Kirby used the same term in his "New Gods" saga, applying it to a somewhat different, though still inimical, quasi-mystical power, a "One Ring" being sought by the evildoers of the narrative.