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Chapter 43
THE IMMUNE SYSTEM
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 immunity is inborn. rapid nonspecific
Adaptive immunity develops after birth. slow (first encounter) highly specific has memory (rapid response in
subsequent encounters)
Invertebrates possess innate immunity.Vertebrates possess innate & adaptive
immunity.
A. Innate Defenses of HumansConsist of physical barriers,
phagocytes, inflammation & antimicrobial proteins.
1. Physical BarriersBody’s first line of defense; prevent
microbes from entering body. skin & mucus membranes antimicrobial secretions nose hairs & respiratory cilia earwax
2. PhagocytesWhite 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. InflammationLocalized response to tissue injury;
creates an environment hostile to microbes.
4. Antimicrobial ProteinsProduced & 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 - stimulate WBC production.
pyrogens - released by WBCs & macrophages; causes fever (resets body’s thermostat in hypothalamus).
B. Adaptive Defenses of HumansConsist 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.
Once programmed, B & T cells migrate to lymphoid tissues.
B cell antigen receptors (antibodies)
1. MacrophagesFunction 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.
Antibodies bind to the same foreign antigen that triggered their production.
(plasma cell)
(plasma cell)(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 cell-
mediated 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-2bind 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 whose RBCs possess Rh
antigen are Rh+
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 Malfunction1. 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 DisordersOccur when the immune system attacks
“self” antigens. Type I (juvenile) diabetes mellitus -
antibodies target beta cells in pancreas. Rheumatoid arthritis - antibodies
target cells lining joints. Myasthenia gravis - antibodies target
neurotransmitter receptors on skeletal muscle cells.
Grave’s disease - antibodies target thyroid gland.
3. AllergiesOccur 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
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