@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).