@Daxar you're not wrong, it does use the term antigen as well, in its proper usage. Antigens are what the leukocytes react to, alerting them to the presence of an unwelcome germ, be it bacterial, fungal, etc. So when their hats go off, they do exclaim they detect an antigen then go off to stab something to death.
@Daxar: Was it using Crunchyroll's subtitles (which would include the version offered by HorribleSubs)? Because those make the same mistake that I pointed out profusely.
That said, @TheGTF has it right. The leukocytes saying "Antigen detected!" is sensible because that's how they operate: by detecting unfamiliar antigens. Their antigen-sense does not, however, tell them what exactly is it that they detected; in particular, neutrophils (such as our WBC protag in the anime and main manga) need to eat whatever it is that bears the antigens in order to identify it (which is first shown in the episode/chapter about allergens IIRC).
@Arcara Apparently. From what I can tell, the Ch 26 main series translation floating around the internet erroneously calls that one "Backward cap-kun", but I have seen the raw and even my limited ability can read it clearly says -chan. If there's other context from U1146, Leader, Megakaryocyte, etc's phrasings to indicate it's a boy, since -chan could still be applied to a boy in some contexts, that is beyond my skill level- I can just look at the character's name and confirm its the name as in this spinoff.
@TheGTF@MarqFJA87 Thanks for the clarification! Will update when I get the chance. I don't have a medical degree, so let me know if I get anything else wrong.
@Arcara simplified answer: we dunno. I'm going with "she" pronouns because genderless pronouns just sound clunky in English, particularly with how the wording in this chapter worked out.