Encyclopedia of Health and Medicine

How These Medications Work
Antihistamines work by blocking the ability of histamine
to bind with histamine receptors on the
surfaces of cell membranes. Antihistamines target
the two types of histamine receptors involved
with the immune response: H1 and H3. H1 receptors
regulate arteriole dilation and capillary permeability.
Histamine causes dilation of the
arterioles, the body’s tiniest arteries, to increase
blood flow to the tissues and increases the flow of
plasma out from the capillaries into the interstitial
spaces, to flood the tissues with antibodies,
cytokines, PROSTAGLANDINS, and other molecules
essential to the immune response.
When taken at the onset of symptoms (while
histamine release is still taking place) or prophylactically
(to prevent histamine release, such as to
treat seasonal allergies), antihistamine medications
relieve the common symptoms of ALLERGY
such as itching and sneezing. However, antihistamines
cannot reverse the effects of histamine
release after they occur or reduce inflammation
that has already developed. Antihistamine medications
to treat hypersensitivity reaction are available
in oral, topical, inhalation, and injection
preparations. Doctors may also prescribe antihistamines
to relieve NAUSEA, particularly that of
motion sickness, and mild anxiety.
First-generation antihistamines, long the mainstay
of treatment for allergies, are nonselective.
They block both H1 and H3 receptors. H1 blocking
subdues symptoms of hypersensitivity reaction.
H3 receptors signal BRAIN neurotransmitters in the
HYPOTHALAMUS that regulate alertness and the nauantihistamine
medications 247
lymphocytes (PLASMA cells) generate antibodies to
further target the antigen.
Antigens and Cancer
The antigens on the surface of cancer cells have
become a focus of much research into early diagnosis
and new treatments for cancer. Cancer cells
begin as normal cells in the body, bearing self-cell
antigens. As the cells change and become cancerous
they develop additional antigens. Blood tests
can detect some of these antigens, such as PROSTATE
SPECIFIC ANTIGEN (PSA) on PROSTATE CANCER cells.
Cancer researchers believe the immune response
fails to recognize the mixed antigen population on
cancer cells as nonself, which allows the cancer to
grow.
For further discussion of antigens within the
context of the structures and functions of the
immune system, please see the overview section
“The Immune System and Allergies.”
See also ANTIBODY; B-CELL LYMPHOCYTE; BLOOD
TRANSFUSION; CELL STRUCTURE AND FUNCTION; CLUSTERS
OF DIFFERENTIATION; COMPLEMENT CASCADE; CYTOKINES;
ERYTHROCYTE; HUMAN LEUKOCYTE ANTIGENS (HLAS);
IMMUNOGLOBULIN; IMMUNOTHERAPY; MONOCYTE; TUMOR
MARKERS.
antihistamine medications Medications that
block the action of HISTAMINE, a chemical that acts
on the BLOOD vessels during an IMMUNE RESPONSE to
allow fluid to flood the tissues. The resulting
INFLAMMATION is part of the body’s means of delivering
INFECTION-fighting agents to the site (such as
T-lymphocytes, CYTOKINES, and antibodies). However,
this response is exaggerated in a HYPERSENSI-
TIVITY REACTION, during which histamine secretion
is excessive or continues after the immune
response has neutralized the triggering ANTIGEN.
Large, granulated leukocytes (white blood cells)
called mast cells produce, store, and release histamine
(as well as serotonin and other chemicals).
Mast cells reside in the mucous membranes of the
respiratory tract (nasal passages, nasal sinuses, TRA-
CHEA, and bronchi) and the gastrointestinal tract
(primarily the STOMACH). Mast cells also store and
release serotonin. Histamine is primarily responsible
for the inflammatory changes that result in
hypersensitivity reaction (allergic reaction).
STABILIZING MAST CELLS
TO PREVENT HISTAMINE RELEASE
A different therapeutic approach to managing the
HISTAMINE cascade in HYPERSENSITIVITY REACTION is
to regulate histamine release at the level of the
mast cells, which is most effective as a treatment
for ALLERGIC ASTHMA (hypersensitivity reaction of
the airways). Drugs called MAST-CELL stabilizers
work by preventing the granules in mast cells
from releasing histamine (called degranulation)
as part of the IMMUNE RESPONSE. Mast-cell stabilizers
to treat allergic ASTHMA include cromolyn and
nedocromil.