Fleas, like most bloodsucking insects, inject a chemical into your skin which acts as an anesthetic so that you will not be able to detect him biting, although sometimes you will still be able to feel the bite.
Mostly bites are felt by the victim sometime after the initial bite, however, when the injected chemicals cause an allergic reaction in the body of the victim.
This allergic reaction manifests itself as a raised red welt with a puncture wound in the center, much like a mosquito bite. Some people are very sensitive to flea bites and their symptoms will be quite extensive, while others may have a minimal or no allergic reaction at all.
The bites usually then become very itchy, an urge which should be avoided because itching the flea bite can cause the skin to break and expose the victim to a risk of infection.
Instead, you may use calamine lotion or hydro-cortisone cream to reduce aging in the rotation of the bites. They should go away on their own in a few days.