Some people do not react to the bite at all, and are not aware they’ve been bitten and bled. But if you do notice the bite, it could be the day after you were bitten, or a few days later. The itch factor could range from mild itchiness to more severe reactions, which are known to occur but are quite uncommon. The degree depends on how your own personal body chemistry responds to the protein in the bug’s saliva.
Minor reactions could be a redness and a small raised bump at the bite site. Larger welts could form if you’re more sensitive. Rashes and blistering can happen if you’re very sensitive. It’s rare, but some people are severely allergic to bed bug bites (as they are to other insects), and can experience life-threatening anaphylactic shock.
Itching and other effects could last from a few days to several weeks. Scratching the area can prolong and intensify the itching, and even cause infection. (Of course, we all know that, but it’s so hard not to scratch!)