How does the human immune system fight infection

The human immune system  has two divisions or arms  that help fight off infection namely the innate or non - specific immunity and the acquired immune system also called  specific immunity.

The non - specific immunity arm is the first line of defense against pathogens. It incorporates physical barriers which include the skin and mucous membranes. Pathogens usually cannot penetrate into the body unless the skin is broken. The mucus membranes trap foreign particles using the mucus they secrete and the intruders are expelled from the body.

The specific immunity arm is the second line of defense and attacks pathogens that have breached the first line of defense.  It works by making memory cells, special lymphocytes  that store memory of a pathogen when  encountered for the first time. If the same type of pathogen  invades the body a second time, the memory cells remember it and trigger a more rapid and vigorous response specific to that pathogen, neutralizing it through antibody production and destroying it.

fumikage isn't really a bird, and tsuyu does have teeth. tsuyu has a transformation quirk which still allows her to be partially human. otherwise we'd have a large frog running around. fumikage has teeth because again, he isn't a bird. his head was a quirk mixture if i'm correct.