Ticks remain largely undetected while feeding on a human host due to:
Pain Killers
Ticks secrete local anesthetics into the skin during the attachment process. This numbs the area, preventing the host from feeling the pain or irritation of the mouthparts piercing the skin.
(Daniel Cameron)
Anti-Inflammation
Immune Modulation
The saliva contains substances that dampen the host’s immune response at the bite site. This prevents the typical itching, redness, and inflammation that would usually alert a host to the presence of an intruder.
pmc.ncbi.nlm.nih.gov
Anticoagulants
To ensure a steady flow of blood, ticks inject anticoagulants that prevent the host’s blood from clotting. This allows them to create a pool of blood under the surface of the skin, which they then feed on continuously without the interruption of clotting or the host’s immune system reacting to the injury.
npr.org
By combining these tools ticks can remain attached and feed for several days without the host realizing they are there.
pmc.ncbi.nlm.nih.gov