What causes teething to affect the immune system?

It develops it: Teething create a bacteremia, essentially inoculating the body systemically with bacteria from the mouth and challenging the immune system in a good way to develop and refine its system to future attacks.

Bacteria & swelling: As a tooth cuts through the gum it opens a pathway for the normal bacteria found in the mouth to enter the tissues. This will activate the body's defensive system called the immune system. You will see redness and swelling. Fever and/or diarrhea can occur as well.

Questionable: As far as immune system health experts disagree on whether or not a link exists between teething and illness, such as diarrhea or fever. Let your own intuition guide you.