HUMAN BIOLOGY

170 Chapter 9

immune system malfunctions

in allergies, harmless substances

provoke an immune attack

In some people, normally harmless substances

can provoke immune responses. These sub-

stances are allergens (al-ur-jenz) and the

response to them is an allergy (Figure 9.17).

Common allergens are pollen, various foods

and drugs, dust mites, insect venom, and

ingredients in cosmetics. Some responses start

within minutes; others are delayed. Either way,

the allergens trigger mild to severe inflamma-

tion of mucous membranes and in some cases

other tissues as well.

Some people are genetically predisposed to allergies.

Infections, stress, or changes in air temperature also may

cause the reactions. The first time an allergic person is

exposed to a triggering allergen, a primary response

sensitizes B cells that produce IgE antibodies to the trig-

ger. The antibodies attach to mast cells—which, recall,

contain granules containing histamine. The next time the

allergen enters the body, a secondary response occurs. IgE

antibodies on mast cells bind it and the mast cells secrete

prostaglandins, histamine, and other substances that fan

inflammation (Figure 9.18). They also cause an affected per-

son’s airways to constrict, and sometimes trigger the itchy

raised welts called hives. In hay fever, the allergic response

produces stuffed sinuses, a drippy nose, and sneezing

(Figure 9.17A). But not all allergies are triggered by antibod-

ies. Skin rashes and other contact allergies involve a cell-

mediated response to an allergen (Figure 9.17B).

In food allergies, the immune system registers a par-

ticular food as an “invader.” The most common culprits are

shellfish, eggs, and wheat. Depending on the person and

the food involved, symptoms typically include diarrhea,

vomiting, and sometimes swelling or tingling of mucous

membranes.

Some allergies, such as to peanuts or bee and wasp

stings, can trigger potentially deadly anaphylactic

shock—a whole-body allergic response. Within moments

air passages to the lungs close almost completely. Fluid

gushes from dilated blood vessels throughout the body.

Blood pressure plummets, which can cause the cardiovas-

cular system to collapse. One emergency treatment is an

injection of epinephrine. People who know they are at risk

can keep injectable epinephrine on hand.

As their name suggests, antihistamines are anti-

inflammatory drugs that counteract the histamine released

by basophils and mast cells. Many people use them to

relieve short-term allergy symptoms. In some cases a suf-

ferer may undergo a desensi tization program in which

inflammatory responses to one or more allergens are

blocked by treatment that stimulates the patient’s body to

make IgG instead of IgE.

F i g u r e 9.18 This diagram shows the basic steps leading to

an allergic response. (© Cengage Learning)

PRIMARY RESPONSE

IgE antibodies attach

to mast cells

Plasma cells

(effector B cells) produce

IgE antibodies to the

allergen

Allergin binds

B cell receptors

mast cell

histamine

granule

histamine

released

SECONDARY RESPONSE (allergy)

9.9

F i g u r e 9.17 Allergies are misguided immune responses.

Colin Hawkins/Cultura/Getty Images Biophoto Associates/Science Source

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