Dear Claire,

I’ve been allergic to fleas ever since I was a kitten. Flea bites give me an itchy, red rash.

I talked about why that happens with my friend Bevan Briggs. He’s a nurse practitioner and professor at Washington State University. Nurse practitioners are nurses with advanced training. They diagnose illnesses, order tests and prescribe medicine.

Briggs told me that often rashes happen when the immune system gets turned on. The immune system is the body’s defense system.

“It’s the way our body tries to protect us from germs and poisons,” he said. “Rashes happen because your immune system identifies something as foreign—either an infective agent or some kind of toxin.”

When a flea bites me, it punctures my skin with its sharp mouthparts. Then, it feeds on my blood. A little bit of the flea’s saliva drips into my body.

My immune system knows that flea saliva isn’t part of my body. It’s an invader—and invaders could be dangerous. So, my immune system sends out special cells and chemicals to deal with it.

Some of the chemicals my immune system activates are called histamines. They do a couple things. They send signals to rev up my immune response. It’s like screaming, “Over here!” Histamines also cause my blood vessels to become a little bit leaky. Both those things help my immune system get special cells and chemicals to the right place. They also cause symptoms like my rash.

So, my rash isn’t caused by fleas directly. It happens when my immune system responds to the flea saliva.

Head and neck of orange tabby cat scratching its tilted head with its right paw
Image by wirestock on Freepik


Sometimes invaders—like some viruses—damage cells before the immune system gets there. That can cause a rash, too. The immune system will sense the damage. It will send immune cells and chemicals to fight the virus and fix the damage.

Most of the time, the immune system works well. Without it, we would be sick all the time.

But sometimes it gets a little overeager. That’s like my flea allergy. Flea saliva isn’t going to kill me. But my immune system launches a huge response anyway. That can happen if someone is allergic to a medicine, too.

Sometimes the immune system gets confused. It may recognize the body’s healthy cells as invaders and try to fight them. That’s called autoimmunity. It can also cause rashes.

Briggs told me that rashes often get better on their own—but it’s a good idea to talk to a nurse practitioner or other healthcare provider if you have a rash. They’ll help you figure out what’s causing the rash and how to treat it.

That’s awe-flea good news if you’re itchy.


Dr. Universe