In a sense it sounds a bit like immunotherapy for allergies. That is typically about injecting pathogens into your body (e.g. under your tongue, under your skin, or in your lymph nodes), and then over time teaching your body that these pathogens are harmless.

As far as _why_ that works instead of sending someone into anaphylactic shock... ¯\_(ツ)_/¯ The immune system is weird and complex, and I guess that's why they want a medical specialist around in case you're one of the unlucky ones. (Only the first time with sublingual tablets as far as I know)