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THE IMMUNE SYSTEM
Chapter 43
All animals have some form of
immunity (resistance to disease).
Relies on ability of animal’s immune
system to identify “foreign” antigens.
Antigen - any molecule
that elicits an immune
response (usually a
carbohydrate or protein)
 Innate
 rapid
immunity is inborn.
 nonspecific
 Adaptive
 slow
immunity develops after birth.
(first encounter)
 highly specific
 has memory (rapid response in subsequent
encounters)
Invertebrates possess innate immunity.
Vertebrates possess innate & adaptive
immunity.
A. Innate Defenses of Humans
Consist of physical barriers, phagocytes,
inflammation & antimicrobial proteins.
1. Physical Barriers
Body’s first line of defense; prevent
microbes from entering body.
 skin
& mucus membranes
 antimicrobial secretions
 nose hairs & respiratory cilia
 earwax
2. Phagocytes
White blood cells that engulf & digest
microbes managing to penetrate the
skin & mucus membranes.
 macrophages - large cells derived from
monocytes; engulf bacteria & cellular
debris.
 free
macrophages move through tissues
 fixed macrophages are anchored in a
particular organ.
 neutrophils
- most abundant WBC;
engulf bacteria & cellular debris.
 eosinophils - destroy parasitic worms
(tapeworms, flukes, pinworms, hookworms).
Dead phagocytes are a component of pus.
3. Inflammation
Localized response to tissue injury;
creates an environment hostile to
microbes.
4. Antimicrobial Proteins
Produced & released in response to
microbial invasion.
 defensins - released by neutrophils; lyse
bacteria.
 complement
system - group of ~ 20
plasma proteins; enhance phagocytosis,
intensify inflammation & lyse bacteria.
MAC (membrane
attack complex)
 cytokines - proteins synthesized in certain
activated cells; stimulate or inhibit the
activity of other cells.
 interferons
- released by virus-infected
cells; protect adjacent uninfected cells
from infection & activate macrophages.
 interleukins
 pyrogens
- stimulate WBC production.
- released by WBCs &
macrophages; causes fever (resets body’s
thermostat in hypothalamus).
B. Adaptive Defenses of Humans
Consist of macrophages, B lymphocytes
(B cells), and T lymphocytes (T cells).
Interact with
components of
innate defenses.
Adaptive immunity distinguishes self
from nonself.
 Molecules
called the Major
Histocompatibility Complex (MHC)
identify a cell as “self”.
 Anything with something different is
identified as “foreign”.
 Foreign invaders are vigorously
attacked.
 The system “REMEMBERS”.
All WBCs are produced in bone marrow.
Monocytes enter bloodstream, then exit &
enlarge to form macrophages.
 Most lymphocytes enter bloodstream &
travel to thymus gland (develop into T cells).
In thymus, each T cell is genetically
programmed to respond to one specific kind
of “foreign” antigen.

T cell antigen receptors

Some lymphocytes remain in bone marrow
(develop into B cells).
In bone marrow, each B cell is genetically
programmed to respond to one specific kind
of “foreign” antigen.
B cell antigen receptors
(antibodies)
Once programmed, B & T cells migrate to
lymphoid tissues.
1. Macrophages
Function in adaptive immunity as
antigen-presenting cells (APCs).
Macrophage ingests
bacterium.
 Displays “foreign”
antigen on its MHC “self”
protein.
 Certain helper T cells recognize & bind to
antigen-MHC protein complex.
 Activated helper-T cells secrete interleukin-2.

2. B Lymphocytes (B cells)
B cells are responsible for humoral
immunity (antibodies are used to fight
bacteria & viruses in body fluids).
B cells are
activated when:
 they recognize &
bind to a foreign
antigen, AND
 are exposed to
interleukin-2
Interleukin-2 (from activated
helper T cell)
Plasma cells secrete antibodies that
circulate in blood or lymph.
(plasma cell)
(plasma cell)
Antibodies bind to
the same foreign
antigen that
triggered their
production.
(plasma cell)
Antibodies mark antigens for destruction
by macrophages or complement.
Memory B cells remain dormant in body
fluids; function in the secondary
immune response.
Primary immune response - occurs when B
cells are first exposed to a particular
antigen; antibody levels rise slowly & decline
rapidly.
Secondary immune response - occurs when
memory B cells encounter the same antigen
in the future; antibody levels rise rapidly &
remain high for a long period.
Vaccinations produce memory B cells.
3. T lymphocytes (T cells)
T cells are responsible for cellmediated immunity (T cells destroy body
cells infected with bacteria & viruses).
Types of T cells
 helper T cells (CD4 / T4 cells)
 activated by antigen presenting cells
(macrophages or B cells displaying foreign
antigens)
 produce cytokines (interleukins,
interferons & tumor necrosis factor)
 cytotoxic
T cells (CD8 / T8 cells)
 activated by interleukin-2
 bind to body cells displaying foreign
antigens (virus- or bacteria-infected
cells, cancer cells, transplanted or
transfused cells)
 release
perforin (causes cell lysis)
C. Rh Incompatibility During
Pregnancy
Rh antigen is one of many “self” antigens
found on the surface of red blood cells.
 Individuals
are Rh+
whose RBCs possess Rh antigen
 Individuals
whose RBCs do not have Rh
antigen are Rh-
Rh- individual produces Rh antibodies only
if exposed to the Rh antigen.
The Rh antigen causes problems during
pregnancy when an Rh- woman carries
an Rh+ fetus.
Newborns have temporary immunity
(passive immunity):
 some antibodies (IgG) pass from
mother to fetus through placenta.
 some antibodies (IgA) pass from
mother to infant through breast
milk.
Newborns begin producing their own
antibodies (active immunity) by 6 months.
D. Immune System Malfunction
1. Immunodeficiency Disorders
Result from breakdown of the immune
system.
 HIV (human immunodeficiency virus)
 acquired
through infection
 virus initially attacks helper T cells
 virus
subsequently attacks cytotoxic T
cells & macrophages (triggers apoptosis)
 opportunistic
infections of AIDS develop
(Kaposi’s sarcoma, Pneumocystis pneumonia)
 SCID (severe combined immune deficiency)
 inherited
B
& T cells are nonfunctional
 individuals have little or no protection
against pathogens
 treatments include bone marrow transplant
& gene therapy
2. Autoimmune Disorders
Occur when the immune system attacks
“self” antigens.
 Type
I (juvenile) diabetes mellitus -
antibodies target beta cells in pancreas.
 Rheumatoid
arthritis - antibodies target
 Myasthenia
gravis - antibodies target
cells lining joints.
neurotransmitter receptors on skeletal
muscle cells.
 Grave’s
disease - antibodies target
thyroid gland.
3. Allergies
Occur when immune system is overly
sensitive & responds to a normally
harmless substance (allergen).
Common allergens: foods, dust mites,
drugs (penicillin), pollen, fur, insect
venom.
Initial exposure:
allergen activates overly sensitive B cells
 B cells produce clone of plasma cells that
release IgE antibodies
 IgE antibodies attach to mast cells

Subsequent exposure:
if allergen ever encountered again, will
attach to IgE antibodies on mast cells
 mast cells release inflammatory chemicals
(histamine) - cause allergy symptoms
 anaphylactic shock may occur
