Ticks inject anesthetics so that you don't feel anything.
Ticks inject anticoagulants so that the blood will keep flowing where they are biting and it doesn't clog.
Ticks inject antihistamines so that you don't feel the itch when they bite.
Ticks inject molecules that fight your immune system so you are immune-suppressed where they bite you.
For more information about ticks, please read this http://www.cnn.com/2017/09/15/health/tick-museum/index.html