Microbiology and Immunology

WORLD OF MICROBIOLOGY AND IMMUNOLOGY Amebic dysentery

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Dust and the house dust mite constitute another major

cause of allergies. While the mite itself is too large to be

inhaled, its feces are about the size of pollen grains and can

lead to allergic rhinitis. Other types of allergy can be traced to

the fur of animals and pets, food, drugs, insect bites, and skin

contact with chemical substances or odors. In the United

States, there are about 12 million people who are allergic to a

variety of chemicals. In some cases an allergic reaction to an

insect sting or a drug reaction can cause sudden death. Serious

asthma attacks are sometimes associated with seasonal rhinitis

and other allergies. About nine million people in the United

States suffer from asthma.

Some people are allergic to a wide range of allergens,

while others are allergic to only a few or none. The reasons for

these differences can be found in the makeup of an individ-

ual’s immune system. The immune system is the body’s

defense against substances that it recognizes as dangerous to

the body. Lymphocytes, a type of white blood cell, fight

viruses, bacteria, and other antigens by producing antibodies.

When an allergen first enters the body, the lymphocytes pro-

duce an antibodycalled immunoglobulin E (IgE). The IgE

antibodies attach to mast cells, large cells that are found in

connective tissue and contain histamines along with a number

of other chemical substances.

Studies show that allergy sufferers produce an excessive

amount of IgE, indicating a hereditary factor for their allergic

responses. How individuals adjust over time to allergens in

their environments also determines their degree of susceptibil-

ity to allergic disorders.

The second time any given allergen enters the body, it

becomes attached to the newly formed Y-shaped IgE antibod-

ies. These antibodies, in turn, stimulate the mast cells to dis-

charge its histamines and other anti-allergen substances. There

are two types of histamine: H 1 and H 2. H 1 histamines travel to

receptor sites located in the nasal passages, respiratory system,

and skin, dilating smaller blood vessels and constricting air-

ways. The H 2 histamines, which constrict the larger blood ves-

sels, travel to the receptor sites found in the salivary and tear

glands and in the stomach’s mucosal lining. H 2 histamines

play a role in stimulating the release of stomach acid, thus

contributing to a seasonal stomach ulcer condition.

The simplest form of treatment is the avoidance of the

allergic substance, but that is not always possible. In such

cases, desensitization to the allergen is sometimes attempted

by exposing the patient to slight amounts of the allergen at

regular intervals.

Antihistamines, which are now prescribed and sold over

the counter as a rhinitis remedy, were discovered in the 1940s.

There are a number of different antihistamines, and they either

inhibit the production of histamine or block them at receptor

sites. After the administration of antihistamines, IgE receptor

sites on the mast cells are blocked, thereby preventing the

release of the histamines that cause the allergic reactions. The

allergens are still there, but the body’s “protective” actions are

suspended for the period of time that the antihistamines are

active. Antihistamines also constrict the smaller blood vessels

and capillaries, thereby removing excess fluids. Recent

research has identified specific receptor sites on the mast cells

for the IgE. This knowledge makes it possible to develop med-

icines that will be more effective in reducing the symptoms of

various allergies.

Corticosteroids are sometimes prescribed to allergy

sufferers as anti-inflammatories. Decongestants can also bring

relief, but these can be used for a short time only, since their

continued use can set up a rebound effect and intensify the

allergic reaction.

See alsoAntibody and antigen; Antibody-antigen, biochemi-

cal and molecular reactions; Antibody formation and kinetics;

Antigenic mimicry; Immunology

AAmebic dysenteryMEBIC DYSENTERY

Amebic (or amoebic) dysentery, which is also referred to as

amebiasis or amoebiasis, is an inflammationof the intestine

caused by the parasite Entamoeba histolytica. The severe form

of the malady is characterized by the formation of localized

lesions, called ulcers, in the intestine, especially in the region

known as the colon, abscesses in the liver and the brain, and

by vomiting, severe diarrhea with fluid loss leading to dehy-

dration, and abdominal pain.

Amebic dysentery is one of the two most common

causes of intestinal inflammation worldwide. The other is

infection with bacteriaof the Shigellagroup.

Amebiasis is contracted mainly by ingesting the para-

site in contaminated food or water. Person–to–person trans-

mission is less likely, but can occur. The disease is thus most

common where sanitation is poor, in the developing world.

The disease is especially prevalent in regions where untreated

human waste is used as fertilizer. Run–off from fields can

contaminate wells contaminating the drinking water.

Amebiasis can occur anywhere in the world in almost any cli-

mate, excluding polar areas and mountainous high altitudes.

Even now, approximately 500 cases are reported each year in

New York State.

Hayfever allergy triggered by oilseed rape plants.

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