HUMAN BIOLOGY

ImmunIty and dIsease 171

allergen Substance that

causes an allergic reaction.

allergy Immune response

caused by a normally harm-

less substance.

anaphylactic shock

Whole-body allergic

reaction.

autoimmunity Immune

response against normal

(self) body cells or proteins.

immunodeficiency

Weakened or absent

immune responses.

immunological tolerance

Lack of an immune

response against normal

body cells.

What are some immune system malfunctions?

• Allergies are immune responses to a normally harmless
  substance.


• Autoimmune disorders reflect an immune response against
  normal body cells or proteins.


• A lack of properly functioning lymphocytes leads to deficient
  or absent immune responses.

takE-homE mEssaGE

immune responses can be deficient

Immunodeficiency is a lack of

properly functioning lymphocytes—

and therefore the inability to mount

normal immune responses. Both

T and B cells are in short supply

in the disorder known as severe

combined immune deficiency

(SCID). SCID usually is inherited,

and infants born with it may die early

in life. Lacking adequate immune

responses, they are extremely vul-

nerable to infections that are not life-

threatening to other people.

The human immune deficiency

virus (HIV) disables several kinds

of white blood cells. The result is

acquired immune deficiency syn-

drome (AIDS).

autoimmune disorders attack “self”

Normally, a B or T cell does not have receptors that
can “see” a body cell’s MHC self tags as antigens of an
invader. This immunological tolerance is what protects
the body’s own cells from attack by the immune sys-
tem. In fact, a newly forming B or T cell that recognizes
healthy body cells as foreign undergoes apoptosis and
dies. This weeding out of self-reactive B and T cells goes
on throughout a person’s life. When it goes awry, the
result is autoimmunity, in which the immune system
generates so-called autoantibodies—antibodies against
normal body cells or proteins (Table 9.4). An example is
rheumatoid arthritis (RA). People with RA are geneti-
cally predisposed to the disease. Their macrophages and
T and B cells become activated by IgG antibodies asso-
ciated with the joints. Immune responses are mounted
against their body’s collagen molecules and also anti-
bodies that have bound to an (as yet unknown) antigen.
Inflam mation, the complement system, and malfunction-
ing repair mechanisms damage joint tissues further (Fig-
ure 9.19). Eventually the af fected joints become immobile.
Another autoimmune disease is type 1 diabetes. This
is a type of diabetes mellitus in which the pancreas does
not secrete enough of the hormone insulin for proper
absorption of glucose from the blood. In type 1 diabetes,
the immune system attacks and destroys the insulin-
secreting cells. A viral infection may trigger the disor-
der. Chapter 15 looks at the various forms of diabetes in
more detail.
A symptom of systemic lupus erythematosus
(SLE) is a “butterfly” rash on the cheeks that extends
across the nose. The rash is one sign that the affected per-
son has developed antibodies to her or his own DNA and
other “self” components. Antigen–antibody complexes
accumulate in joints, blood vessel walls, the skin, and the
kidneys. Other symptoms include fatigue, painful arthri-
tis, and in some cases a breakdown of kidney function.
Medicines can help relieve many SLE symptoms, but there
is no cure.

Figure 9.19 This person’s hand is crippled by

rheumatoid arthritis.

James Stevenson/Science Source

Disorder Autoantibody Target Affected Area

Crohn’s disease Proteins in neutrophils Digestive tract

Diabetes type 1 Insulin or insulin-

making cells

Pancreas

Graves’ disease Receptors for thyroid

hormone

Thyroid gland

Hashimoto’s disease Proteins involved in

synthesis of thyroid

hormone

Thyroid gland

Lupus erythematosus DNA, proteins in cell

nucleus

Connective tissue

Multiple sclerosis Proteins in myelin sheath

of motor neurons

Brain and spinal

cord (central

nervous system)

Pernicious anemia Proteins of cells that

produce stomach acid

Stomach

epithelium

Rheumatoid arthritis IgG antibodies Joints

Table 9.4 Examples of Autoimmune Disorders

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