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have a lowered resistance to disease, thus making them more susceptible to

infections. Many of these patients are said to be “compromised hosts”—they

have breaks in their skin due to accidental or surgical wounds (lesions, bedsores,

or burns). Others may have compromised mucous membranes that line the res-

piratory tract, the digestive tract, or the urinary and reproductive system, mak-

ing them more susceptible to disease-causing pathogens.

Prevention and Control

of Nosocomial Infections

Most hospitals and clinical facilities have control measures and procedures

aimed at preventing nosocomial infections. For a hospital to be accredited, it

must have a designated person responsible for developing and implementing

policies and procedures that would control infections and communicable dis-

eases. This person can be a registered nurse or an epidemiologist.

PUBLIC HEALTH ORGANIZATIONS:

THE CENTERS FOR DISEASE CONTROL

Public health agencies have been created in cities, countries, states, and at the

federal level. The United States has recognized the importance of identifying

and controlling infectious diseases since the eighteenth century. The Centers for

Disease Control and Prevention (CDC) is a branch of the United States Public

Health Service (USPHS). The CDC is located in Atlanta, Georgia, and is the

central source of epidemiological study and information. The CDC is responsi-

ble for the control and prevention of disease, public education, and occupational

health and safety.

Quiz

1. The number of people infected by a disease at any point in time is called

(a) incidence

(b) prevalence

(d) prevalence rate

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2. The total number of old and new cases of a disease is called

(a) incidence

(b) prevalence

(d) prevalence rate

3. The number of people in a given population who are ill is called

(a) incidence

(b) prevalence

(d) prevalence rate

4. What is a disease called when there is a worldwide distribution of the

disease?

(a) Pandemic

(b) Sporadic disease

(d) Propagated epidemic

5. What is it called when a disease arises from person-to-person contact?

(a) Pandemic

(b) Sporadic disease

(d) Propagated epidemic

6. What is it called when large numbers of a population are infected suddenly

from a common source?

(a) Pandemic

(b) Sporadic disease

(d) Propagated epidemic

7. What is it called when there are small isolated occurrences of a disease

reported?

(a) Pandemic

(b) Sporadic disease

(d) Propagated epidemic

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8. What is the period of time called when a disease is contagious and com-

municable before showing signs and symptoms and during recovery?

(a) Symptomatic

(b) Incubation period

(d) Carrier period

9. Incubatory carriers are individuals who transmit the disease before becom-

ing symptomatic.

(a) True

(b) False

10. Convalescent carriers are individuals who develop chronic infections and

transmit the infection for long periods of time.

(a) True

(b) False

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CHAPTER

211

Immunity

There’s a battle going on beneath your skin. Thousands of microbes crawl into

every nook and cranny of your body in a daily invasion. Yet you rarely notice

because an army of B cells, T cells, natural killer cells, and other parts of your

immune system counterattack, ripping most microbes to pieces. Those that sur-

vive give you a runny nose, cough, and feverish feeling, and, on rare occasions,

more serious illness.

The immune system fights invading bacteria, viruses, and pollen and venom

from bees. Anything foreign that enters your body is surrounded and either

neutralized or destroyed by your immune system before it can cause damage to

your body.

In this chapter, you’ll learn the parts of the immune system and how each

functions to give you protection days and years from now.

What Is Immunity?

An immune system is an organism’s protection from invading organisms and for-

eign substances, such as bacteria, viruses, fungi, helminths, protozoa, pollen,

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transplanted tissues, and insect venom. Parts of these substances are called anti-

gens, or immunogens. Antigens can be polysaccharides or proteins and provoke

a specific immune response in organisms. Immunity is a specific defensive

response of a host when a foreign substance or organism invades it.

The body reacts to the foreign substance (antigen) by forming antibodies.

Antibodies are proteins that are made by the body in response to an antigen and

can combine specifically with that antigen.

The immune system recognizes a body or substance within the organism as

self or nonself. Self is any body or substance that belongs to the organism. Non-

self is any body or organism that doesn’t belong to the organism. An antigen is

recognized as nonself by the immune system.

An antigen causes the organism to form antibodies and specialized lympho-

cytes that target the specific antigen. If the antigen invades again, these antibod-

ies and specialized lymphocytes attack the antigen, making the antigen inactive

or destroyed. This is called an immune response against an antigen.

Cells that become cancerous are recognized as foreign and can be destroyed.

Cancerous cells, once established as a tumor, might have a dreadful effect on the

organism because the immune system is not be able to fight them.

ACQUIRED IMMUNITY

The development of antibodies and specialized lymphocytes is called acquired

immunity because it is acquired over an organism’s life through naturally

acquired immunity and artificially acquired immunity means.

Acquired immunity is the protective defense mechanism an organism devel-

ops against foreign substances and microorganisms. This type of immunity is

established throughout an individual’s life.

There are two types of acquired immunity. These are active and passive.

When an individual is exposed to a disease-causing microorganism or foreign

substance, the person’s own immune system responds by making its own anti-

bodies and lymphocytes.

In passive immunity, already-made antibodies are introduced or passed on

to an individual. The individual does not make its own antibodies.

Naturally acquired active immunity occurs when an individual is exposed to

an infectious disease. The individual’s immune system responds by making its

own antibodies and lymphocytes (T cells and B cells) on its own.

Naturally acquired passive immunity occurs when antibodies (IgG) are made

by a mother and passed on to the fetus through the placenta. IgA antibodies are

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also passed to the baby in the first secretion of breast milk, called colostrum,

during breast-feeding.

Artificially acquired active immunity occurs when an individual is given a

vaccine. A vaccine is a substance that contains the weakened or dead organisms.

These antigens stimulate the immune response, but do not cause major sickness.

The body remembers the antigen with memory cells the next time there is expo-

sure to the antigen.

Artificially acquired passive immunity occurs when antibodies are developed

outside the individual and intravenously injected into the body. This form of

immunity helps the body’s own defenses in combating infection.

Serum and Antibodies

Serum is the fluid that remains after the blood has clotted and the serum is sep-

arated from the blood cells and clotted material. Electrophoresis is a laboratory

technique that introduces electrical current into a serum placed in a gel. This

causes proteins within the serum to move across the gel at different rates, which

represents different globulins.

There are three globulins. These are called alpha, beta, and gamma globulins.

Gamma globulin contains the most antibodies. A serum rich in antibodies is

called either gamma globulin or serum globulin.

Gamma globulin can be taken from a person who is immune to an antigen and

injected into a person who lacks the antigen, who then immediately becomes

immune from the antigen. However, the immunity lasts for about three weeks,

at which time the antibodies degrade.

Types of Immunity

ANTIBODY-MEDIATED IMMUNITY

Antibody-mediated immunity, which is also known as humoral immunity, uses

antibodies in extracellular fluids, such as mucus secretions, blood plasma, and

lymph, to combat antigens. These antibodies, produced from B cells, which are

also known as B lymphocytes, primarily attack bacteria, bacterial toxins, and

viruses that invade body fluids. They also attack transplanted tissues.

Antibody-mediated immunity was discovered by German scientist Emil von

Behring at the turn of the twentieth century in his quest to create an immuniza-

tion against diphtheria. Behring called this humoral immunity because the med-

ical community called body fluids humors.

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CELL-MEDIATED IMMUNITY

Cell-mediated immunity involves specialized lymphocytes called T cells, also

known as T lymphocytes, to attack foreign organisms rather than using antibod-

ies. T cells are also effective against helminths, fungi, and protozoa. In addition,

T cells regulate aspects of the immune system.

Cell-mediated immunity was explained by Russian biologist, Elie

Metchnikoff, who in the early 1900’s noticed that phagocytic cells were much

more effective in animals that were immunized. This immunity was used in the

mid-twentieth century to protect people against tuberculosis.

A Closer Look at Antigens

An antibody identifies its corresponding antigen by one or more regions on the

antigen known as the epitopes, which are also called antigenic determinants. The

epitope must be the right size, shape, and chemical structure for the antibody to

bind to the epitope and then proceed to disable or destroy the antigen.

Antigens tend to have a molecular weight of 10,000 or more yet some foreign

substances might have a lower molecular weight and are not antigens. They are

called haptens and must attach themselves to a large carrier molecule in order to

become antigenic. Antibodies only attack the hapten and not the carrier molecule.

Penicillin is a common hapten. Penicillin does not have an antigen effect in

most people. However, when penicillin attaches to serum proteins an allergic

reaction results in some people. These people are said to be allergic to penicillin.

An allergic reaction is a typical immune response.

Antigens can be proteins, large polysaccharides, lipids, or nucleic acids.

However, antigens that are lipids and nucleic acids must be combined with pro-

teins and polysaccharides; otherwise they are not antigens.

A Closer Look at Binding

Antibodies are known as immunoglobulins (Igs), which is a group of soluble

proteins. An antigen can cause the production of different antibodies if the anti-

gen has several epitopes. Epitopes or antigenic determinants, are known as antigen-

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binding sites. The number of antigen-binding sites is called the antibody’s val-

ence. There are at least two antigen-binding sites on each antigen where a human

antibody can bind. This is referred to as a bivalent antibody because the anti-

body’s valence value is two.

The structure of a bivalent antibody is called a monomer and consists of four

protein chains that are named after their relative molecular weights. These are

two light (L) chains and two heavy (H) chains.

These protein chains are joined together to form a Y-shaped molecule that is

flexible enough to form a T-shaped molecule. There are two regions of the pro-

tein chain. These are the variable (V) region and the constant (C) region.

The variable region is located at the ends of the arms of the Y. These are the

sites where the antibody binds with an antigen. The variable region is a three-

dimensional structure of amino acid sequences whose structure reflects the epi-

topes of the antigen.

The constant region is the stem of the Y and is called the Fc region. The Fc

region binds adjacent complementary antibodies if both antigen-binding sites

are attached to an antigen; otherwise the adjacent complementary antibodies are

free to attach and react with antigen.

There are five types of constant regions each associated with the five classes

of immunoglobulin.

IgG

IgG immunoglobulin neutralizes bacterial toxins and attacks circulating bacteria

and viruses by enhancing the effectiveness of phyagocytic cells. Nearly 80 per-

cent of the antibodies in serum are IgG. IgG can cross blood vessel walls and the

placenta and can enter tissue fluids.

IgM

IgM immunoglobulin is the first antibody to respond to an antigen or initial infec-

tion and makes up about 10 percent of antibodies in serum. IgM is relatively large

in size and has a pentamer structure of five monomers bonded together by a join-

ing (J) chain. This chain is a polypeptide. Because of its size, IgM remains in

blood vessels and not in tissue fluids. IgM responds to the ABO blood group anti-

gens and enhances the effectiveness of phyagocytic cells. When initial exposure

to an antigen occurs, IgM antibodies are the first to appear.

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IgA

IgA immunoglobulin is the most common antibody in body secretions and in

mucous membranes. It consists of about 15 percent of the antibodies in a serum.

IgA protects infants from gastrointestinal infections and fights antigens that

affect the respiratory tract. Plasma cells in the mucous membrane form secretory

IgA, which is then passed through the mucosal cell and attacks antigens on the

mucosal surface such as bacteria and viruses. IgA is short-lived

IgD

IgD immunoglobulin is found in blood and lymph fluid and is an antigen re-

ceptor on the surface of B cells. This also provides protection against para-

sitic worms.

IgE

IgE immunoglobulin binds to basophil cells and mast cells that release chemical

mediators, such as histamine, that cause an allergic reaction. When pollen is the

antigen, the allergic reaction is referred to as hay fever. IgE is less than one per-

cent of serum antibodies. This provides protection against parasitic worms.

B Cells

B cells are cells that develop from stem cells in the bone marrow and the liver

of fetuses. They are transported to the lymph nodes and spleen where they use

antigen receptors, also known as antigen-binding sites, on the cell’s surface to

seek out antigens.

Once an antigen is detected, the B cell with T cells activates a special group

of lymphocytes that produces antibodies used in the antibody-mediated immu-

nity response. T cells do not make antibodies. When B cells come in contact with

extracellular antigens, the B cell transforms into plasma cells, that produce anti-

bodies at about 2,000 antibodies per second to combat that antigen.

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