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.

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