Microbiology and Immunology

WORLD OF MICROBIOLOGY AND IMMUNOLOGY Anaphylaxis

17

•

era Peptostreptococcus, Streptococcus, and Staphylococcus.

Rod-shaped bacteria that stain Gram-negative include

Bacteroides, Campylobacter, and Fusobacterium. Finally,

Gram-negative spherical bacteria are represented by the genus

Veillonella.

The word anaerobic means “life without air.” In the

human body, regions that can be devoid of oxygen include the

interior of dental plaquethat grows on the surface of teeth and

gums, the gastrointestinal tract, and even on the surface of the

skin. Normally the anaerobic bacteria growing in these envi-

ronments are benign and can even contribute to the body’s

operation. Most of the bacteria in the body are anaerobes.

However, if access to underlying tissues is provided due to

injury or surgery, the bacteria can invade the new territory and

establish an infection. Such bacteria are described as being

opportunistic pathogens. That is, given the opportunity and the

appropriate conditions, they are capable of causing an infec-

tion. Typically, anaerobic bacteria cause from five to ten per

cent of all clinical infections.

Anaerobic infections tend to have several features in

common. The infection is usually accompanied by a foul-

smelling gas or pus. The infections tend to be located close to

membranes, particularly mucosal membranes, as the infection

typically begins by the invasion of a region that is bounded by

a membrane. Anaerobic infections tend to involve the destruc-

tion of tissue, either because of bacterial digestion or because

of destructive enzymesthat are elaborated by the bacteria.

This type of tissue damage is known as tissue necrosis. The

tissue damage also frequently includes the production of gas

or a fluid.

There are several sites in the body that are prone to

infection by anaerobic bacteria. Infections in the abdomen can

produce the inflammationof the appendix that is known as

appendicitis. Lung infections can result in pneumonia, infec-

tion of the lining of the lung (empyema) or constriction of the

small air tubes known as bronchi (bronchiectasis). In females,

pelvic infections can inflame the lining of the uterus

(endometritis). Mouth infections can involve the root canals or

gums (gingivitis). Infections of the central nervous system can

lead to brain and spinal cord infections. Infection of the skin,

via bites and other routes of entry, causes open sores on the

skin and tissue destruction. An example is that massive and

potentially lethal tissue degradation, which is known as necro-

tizing fascitis, and which is caused by group A b-hemolytic

Streptococcus. Finally, infection of the bloodstream (bac-

teremia) can prelude the infection of the heart (endocarditis).

The diagnosis of anaerobic infections is usually based

on the symptoms, site of the infection and, if the infection is

visible, on both the appearance and smell of the infected area.

Most of the bacteria responsible for infection are susceptible

to one or more antibiotics. Treatment can be prolonged, how-

ever, since the bacteria are often growing slowly and since

antibiotics rely on bacterial growthto exert their lethal effect.

In the case of infections that create tissue destruction, the

removal of the affected tissue is an option to prevent the

spread of the infection. Amputation of limbs is a frequent

means of dealing with necrotizing fascitis, an infection that is

inside of tissue (and so protected from antibiotics and the

host’s immune response) and is exceptional in that it can

swiftly spread.

See alsoBacteria and bacterial infections

ANAPHYLACTIC SHOCK•seeIMMUNITY: ACTIVE,

PASSIVE, AND DELAYED

AAnaphylaxisNAPHYLAXIS

Anaphylaxis is a severe allergic reaction. The symptoms

appear rapidly and can be life threatening.

The symptoms of anaphylaxis include the increased out-

put of fluid from mucous membranes (e.g., passages lining the

nose, mouth, and throat), skin rash (e.g., hives), itching of the

eyes, gastrointestinal cramping, and stiffening of the muscles lin-

ing the throat and trachea. As a result of the latter, breathing can

become difficult. These symptoms do not appear in every case.

However, some sort of skin reaction is nearly always evident.

Anaphylaxis results from the exposure to an antigen

with which the individual has had previous contact, and has

developed a heightened sensitivity to the antigen. Such an

antigen is also known as an allergen. The allergen binds to the

specific immune cell (e.g., immunoglobulin E, also known as

IgE) that was formed in response to the initial antigen expo-

sure. IgE is also associated with other specific cells of the

immune systemthat are called basophils and mast cells. The

basophils and mast cells react to the binding of the allergen-

IgE complex by releasing compounds that are known as medi-

ators (e.g, histamine, prostaglandin D2, trypase). Release of

mediators does not occur when IgE alone binds to the

basophils or mast cells.

The release of the mediators triggers the physiological

reactions. For example, blood vessels dilate (become larger in

diameter) and fluid can pass across the blood vessel wall more

easily. Because the immune system is sensitized to the partic-

ular allergen, and because of the potent effect of mediators, the

development of symptoms can be sudden and severe. A condi-

tion called anaphylactic shock can ensue, in which the body’s

physiology is so altered that failure of functions such as the

circulatory system and breathing can occur. For example, in

those who are susceptible, a bee sting, administration of a

penicillin-type of antibiotic, or the ingestion of peanuts can

trigger symptoms that can be fatal if not addressed immedi-

ately. Those who are allergic to bee stings often carry medica-

tion with them on hikes.

Anaphylaxis occurs with equal frequency in males and

females. No racial predisposition towards anaphylaxis is

known. The exact number of cases is unknown, because many

cases of anaphylaxis are mistaken for other conditions (e.g.,

food poisoning). However, at least 100 people die annually in

the United States from anaphylactic shock.

See alsoAllergies; Immunoglobulins and immunoglobulin

deficiency syndromes

womi_A 5/6/03 1:06 PM Page 17