Before delving too far into antihistamine
s, one should be aware of what a histamine
is for an antihistamine to be anti-
of. A histamine is a compound that is part of many venom
s and sting
s, it is also produced naturally by the immune system
as a response to tissue
damage. When histamines are released they produce an immediate allergic
response that often includes inflammation
of the tissue and a host of other actions (such as increased gastric acid
release). The release of a histamine also changes the size of blood vessels (known as vasodilation
) that allows the cells that handle whatever happened to get there faster.
Inflammation caused by histamines happens when the histamine binds to a receptor on a smooth muscle cell that lines blood vessels causing them to dilate. However, in the digestive system and lungs the histamine causes the smooth muscles to contract (this is why allergic reactions make it hard to breath).
An antihistamine is a drug that blocks the receptor on the smooth muscle known as H1 for histamine, type 1. There are drugs that also block the H2 receptors - these are often used to treat ulcers by stopping the gastric acid release (for example, the drug Tagamet). The antihistamine 'competes' with the histamines for receptors and thus reduces the level of an allergic reaction. When it comes down to numbers, the antihistamines win - outnumbering the histamines by several orders of magnitude.
Antihistamines are often taken in the form of a pill which is absorbed in the gut and metabolized in the liver. This liver action is part of the reason for the warning of don't consume alcohol - which is also metabolized in the liver. This may cause alcohol poisoning because the alcohol didn't get metabolized from a poison to something less deadly soon enough.
The dopy nature of antihistamines that comes from the fact that histamines are also neurotransmitters. In this case, the histamine keeps the brain awake and alert. Newer antihistamines are targeted to the body but not the brain (this actually is not that difficult - most medications don't get to the brain).