Anaphylaxis.

When a specific allergen is injected

directly into the circulation, the allergen can react with

basophils of the blood and mast cells in the tissues

located immediately outside the small blood vessels

if the basophils and mast cells have been sensitized

by attachment of IgE reagins.Therefore, a widespread

allergic reaction occurs throughout the vascular

system and closely associated tissues. This is called

anaphylaxis. Histamine is released into the circulation

and causes body-wide vasodilation as well as increased

permeability of the capillaries with resultant marked

loss of plasma from the circulation. An occasional

person who experiences this reaction dies of circulatory

shock within a few minutes unless treated with

epinephrine to oppose the effects of the histamine.

Also released from the activated basophils and mast

cells is a mixture of leukotrienes called slow-reacting

substance of anaphylaxis.These leukotrienes can cause

spasm of the smooth muscle of the bronchioles, eliciting

an asthma-like attack, sometimes causing death by

suffocation.

Urticaria. Urticaria results from antigen entering specific

skin areas and causing localized anaphylactoid

reactions. Histamine released locally causes (1) vasodilation

that induces an immediate red flare and (2)

increased local permeability of the capillaries that

leads to local circumscribed areas of swelling of the

skin within another few minutes. The swellings are

commonly called hives. Administration of antihistamine

drugs to a person before exposure will prevent

the hives.

Hay Fever. In hay fever, the allergen-reagin reaction

occurs in the nose. Histamine released in response to

the reaction causes local intranasal vascular dilation,

with resultant increased capillary pressure as well as

increased capillary permeability. Both these effects

cause rapid fluid leakage into the nasal cavities and

into associated deeper tissues of the nose; and the

nasal linings become swollen and secretory. Here

again, use of antihistamine drugs can prevent this

swelling reaction. But other products of the allergenreagin

reaction can still cause irritation of the nose,

eliciting the typical sneezing syndrome.

Asthma. Asthma often occurs in the “allergic” type of

person. In such a person, the allergen-reagin reaction

occurs in the bronchioles of the lungs. Here, an important

product released from the mast cells is believed

to be the slow-reacting substance of anaphylaxis, which

causes spasm of the bronchiolar smooth muscle. Consequently,

the person has difficulty breathing until the

reactive products of the allergic reaction have been

removed. Administration of antihistaminics has less

effect on the course of asthma because histamine does

not appear to be the major factor eliciting the asthmatic

reaction.