Section VI. ANTIHISTAMINES
In order to understand the role of the histamine-antagonizing agents
(antihistamines), you must be familiar with histamine and its effects on the body.
Histamine, a basic amine, may be found wherever protein is broken down into its
component amino acids in the presence of putrefactive organisms; thus, it is found in
the intestines, and in the putrefaction of meats. It occurs as well in all body tissues and
is present in the highest concentration in the lungs. Histamine is not normally found in
the bloodstream. It is released at the time of their injury.
4-25. HISTAMINE POISONING
When injected into the body, histamine causes swelling and itching. The
affected area may be very sensitive to pressure. Headache and congestion of the head
occur. Breathing becomes difficult. The body systems are affected as follows:
a. Circulatory. After histamine is injected, there is a short initial rise in blood
pressure due to direct action on the smooth muscle, followed by a marked fall in
pressure because of dilation of the capillaries. The walls of the capillaries lose tone
and become distended with blood. The client "bleeds" into his own capillaries, and
shock and collapse follow.
b. Respiratory. Histamine causes constriction of smooth muscle (with the
exception of the blood vessels). The resulting constriction of the bronchioles makes
respiration labored and difficult. This smooth muscle action is not antagonized by
atropine, because histamine acts directly on the cells of the muscles rather than on
their nerve innervation.
c. Stomach and Intestines; Uterus. Histamine causes the stomach,
intestines, and uterus to contract more powerfully. Spasm may occur.
4-26. ALLERGY AND ANAPHYLAXIS WITH HISTAMINE
Histamine has been demonstrated to have an important role in allergic reactions.
Reactions produced by the injection of histamine and those seen with anaphylactic
shock and allergic reaction seem identical, that is, contraction of the bronchioles, low
blood pressure, increased gastrointestinal motility, increased permeability of the
capillaries resulting in edema in the skin and mucous membranes, increase in lacrimal
fluid, nasal discharge and fluid in the lungs. There is a nerve-ending reaction in the
skin, which produces pain and itching. Since histamine is believed to cause allergic
reactions, or at least many of the symptoms, antihistaminic substances are used in the
control of hay fever and other allergic manifestations.