When a mite ingests living blood, the blood mounts an immune response from inside the mite’s gut. When any parasite ingests blood, it leaves behind proteins in the host.

Mites have evolved to leave behind a protein that suppresses the immune system of the host, so the mite won’t be attacked by its food.

One effect is that bite marks are less pronounced (or nonexistent) when a host has been bitten by mites for months.