Benadryl contains the antihistamine diphenhydramine. The brain cells that are most concerned with wakefulness are the ones that release histamine, and these are the same brain cells that are targeted by antihistamines.
Histamine has many effects in the body. It is a neurotransmitter in the brain. It causes the passageways in the lungs to constrict. It dilates blood vessels, allowing more blood to flow and lowering blood pressure. It is part of the immune system, and is the cause of many of the symptoms you get when fighting off disease.
To help fight infections, histamine makes the small blood vessels leaky, so that white blood cells and antibodies can leak from the blood into the tissues to fight bacteria and viruses.
A side effect of leaky blood vessels is water leaking into tissues from the blood, causing runny noses and watery eyes. The enlarging of the small blood vessels causes swelling of tissue, in places like the nasal passages, which then become blocked.
But histamine’s other action, as a neurotransmitter, causes side effects when you have a cold. The nerves that detect sensation in your nose get stimulated, and you sneeze.
The action that makes the cells in the blood vessels loosen and leak can also affect skin cells, causing hives. The pain and itching of insect bites is due to histamine’s effects on the skin and blood vessels as the body recognizes foreign proteins and attacks them.
To combat the symptoms of colds, allergies, and insect bites, we use antihistamines. But since histamine is a neurotransmitter that regulates wakefulness, a side effect of relieving cold symptoms is drowsiness.
There are four different types of histamine receptors in the body, and they control different reactions to histamine. Some new antihistamines target only one or two of these receptors, so they can relieve allergy symptoms without causing drowsiness.