This pair of bed bug bite photos below show the immediate (left) and next day (right) reaction after feeding a colony of bedbugs from a container in the lab. (That's why they are contained in a small circular area). The hive-like immediate symptoms are replace by dark red rash type reaction the following day with inflammation and less severe redness radiating out to a larger area.
The size of bed bug bites varies with a number of different factors. Bed bugs inject an anti-coagulant along with their saliva when they pierce the skin to take a blood meal. This anti-coagulant is mostly responsible for how a person reacts to the bite and determines the size of the bug’s bite. Since people will have various sensitivities to the bed bug’s bite, the size of the bite will vary, as well. Another factor that influences the size of a bed bug reaction is the number of times a person is bitten. Bite reactions of people bitten many times are also variable, and their response may be either more or less intense as the number of bites increases.