Essentials of Anatomy and Physiology

protection provided by the vaccine may be incom-
plete.

IMMUNITY

Immunitymay be defined as the ability to destroy
pathogens or other foreign material and to prevent
further cases of certain infectious diseases. This ability
is of vital importance because the body is exposed to
pathogens from the moment of birth.

Antigensare chemical markers that identify cells.

Human cells have their own antigens that identify all

the cells in an individual as “self ” (recall the HLA

types mentioned in Chapter 11). When antigens are

foreign, or “non-self,” they may be recognized as such

and destroyed. Bacteria, viruses, fungi, and protozoa

are all foreign antigens that activate immune re-

sponses, as are cell products such as bacterial toxins.

Malignant cells, which may be formed within the

body as a result of mutations of normal cells, are also

recognized as foreign and are usually destroyed before

they can establish themselves and cause cancer. Unfor-

tunately, organ transplants are also foreign tissue, and

the immune system may reject (destroy) a transplanted

kidney or heart. Sometimes the immune system mis-

takenly reacts to part of the body itself and causes

an autoimmune disease; several of these were men-

tioned in previous chapters. Most often, however, the

immune mechanisms function to protect the body

from the microorganisms around us and within us.

Immunity has two main components: innate immu-

nity and adaptive immunity. Before we describe each

component, a brief general comparison may be help-

ful. Innate immunity may be called nonspecific, does

not create memory, and its responses are always the

same regardless of the target. Adaptive immunity is

very specific as to its target, may involve antibodies,

does create memory, and may become more efficient.

Both kinds of immunity work together to prevent

damage and disease.

INNATE IMMUNITY

Innate immunity has several aspects: anatomic and

physiological barriers, phagocytic and other defensive

cells, and chemical secretions and reactions, including

inflammation. These are not separate and distinct;

rather there is a great deal of overlap among them, as

you will see. The innate immune responses are always

the same, and their degree of efficiency does not

increase with repeated exposure.

Barriers

The stratum corneum of the epidermis of the skin is

non-living, and when unbroken is an excellent barrier

to pathogens of all kinds. The fatty acids in sebum

help limit the growth of bacteria on the skin. The liv-

ing cells of the epidermis produce defensins, which are

antimicrobial chemicals. The mucous membranes of

the respiratory, digestive, urinary, and reproductive

tracts are living tissue, yet still a good barrier. The

The Lymphatic System and Immunity 327

BOX14–1 HODGKIN’S DISEASE

Hodgkin’s diseaseis a malignant disorder of

the lymph nodes; the cause is not known. The

first symptom is usually a swollen but painless

lymph node, often in the cervical region. The

individual is prompted to seek medical attention

because of other symptoms: chronic fever,

fatigue, and weight loss. The diagnosis involves

biopsy of the lymph node and the finding of

characteristic cells.

Treatment of Hodgkin’s disease requires

chemotherapy, radiation, or both. With early

diagnosis and proper treatment, this malignancy

is very often curable.

Trachea

Clavicle

First

rib

Thymus

gland

Figure 14–5. Location of the thymus in a young child.
QUESTION:Which blood cells mature in the thymus?