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VT 106
Comparative Anatomy and Physiology
Lymphatic and Immune System
Immunity – ability to ward off disease with defense mechanisms
2 forms – nonspecific immunity, specific immunity
Susceptibility – lack of adequate immunity
NONSPECIFIC IMMUNITY
not specific – responds the same way to any challenge
1) physical barriers – skin and mucous membranes
stratified squamous epithelium and secretions
prevent foreign materials from entering the body
normal bacterial fauna – inhibits bad microbes
2) phagocytes
neutrophils and monocytes in blood
macrophages – in tissues (derived from monocytes)
3) immunological surveillance – seeking out and destroying abnormal cells
natural killer cells (nonspecific lymphocytes)
wander throughout body and recognize and bind to most abnormal
surface molecules (eg. virus-infected cells, tumors)
cytolysis – release perforins which form holes in target cell’s
membrane and destroy it
4) antimicrobial proteins
interferons – produced by virus-infected cells and some WBCs
induce unifected cells to produce antiviral proteins
virus can enter cells, but can’t reproduce
complement system – cascade of plasma enzymes activated by microbes
or antibodies
promote inflammation and chemotaxis of WBC
opsinization – bind to target and promote phagocytosis
cytolysis – form holes in target’s cell membrane
5) inflammation – same response to any tissue damage
damaged cells release tissue factors (cytokines) that cause:
vasodilation & increased capillary permeability
mast cell degranulation – release histamine, heparin, and other tissue factors
redness, heat, swelling, and pain
chemotaxis of WBC
many neutrophils arrive and begin phagocytosis
monocytes arrive and differentiate into more macrophages
pus – dead cells and fluid
abscess – accumulation of trapped pus
tissue factors stimulate hemostasis, granulation, and regeneration
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6) fever – increased body temperature due to resetting of thermostat in
hypothalamus
inhibits growth of some microbes
increases metabolic rate = faster repair functions
SPECIFIC IMMUNITY
properties of specific resistance:
specificity – each immune cell responds to one specific antigen
versatility – huge variety of immune cells can respond to most any foreign
antigen
tolerance – immune cells don’t normally attack normal body cells
memory – faster, stronger response to a second exposure to the same antigen
Lymphatic System
lymph – excess interstitial fluid, proteins, and lipids collected from tissues
content depends on tissue region
lymphatic vessels
system of vessels that collect lymph and return it to the bloodstream
transport dietary lipids and lipid-soluble vitamins from tissues of GI
tract to blood (chyle – creamy white lymph)
thoracic duct – largest lymphatic vessel, in thoracic cavity
empties lymph into a major vein, returning it to the bloodstream
obstruction of lymphatic vessels causes edema in tissues
lymphatic tissues & organs – specialized reticular connective tissue structures
containing many lymphocytes
lymphocytes – immune cells responsible for specific immunity
B cells – develop in bone marrow (Bursa of Fabricius in birds)
T cells – mature in thymus
LYMPHATIC TISSUES AND ORGANS
Thymus
lobular organ, ventral to trachea and heart base
site of T cell maturation
promoted by thymic hormones
mature T cells enter bloodstream and lymphatic vessels
thymus atrophies (becomes smaller) with age
Lymph nodes – many bean-shaped organs located along lymphatic vessels
reticular connective tissue with clusters of mature B cells and T cells
lymph flows into node and through reticular tissue
reticular fibers filter out abnormal materials
antigen-presenting cells (APCs) – macrophages and other cells remove
abnormal materials and present antigens to immune cells
immune cells that recognize antigens produce immune responses
clean lymph, immune cells, and their products flow out and enter bloodstream
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Lymphatic nodules – clusters of lymphocytes without a capsule
MALT (mucosa-associated lymphatic tissues) – within mucous membranes
tonsils – pharyngeal region
Peyer’s patches (GALT) – intestine
Spleen – largest lymphatic organ
connective tissue and smooth muscle capsule
trabeculae – branches of capsule extending into the pulp of the spleen
pulp – soft inner tissues with a fine reticular framework
red pulp – many capillary sinusoids full of blood (stores extra blood)
contraction of trabeculae squeezes blood out into bloodstream
during stress
macrophages phagocytize worn-out RBCs and abnormal materials
and present antigens to lymphocytes
white pulp – contains B cells and T cells producing immune responses
FUNCTION OF LYMPHOCYTES
Antigens – specific surface molecular arrangements recognized by immune cells
specificity – lock & key binding of immune cell to antigen
Antigen receptors – membrane proteins on T cells and B cells that recognize
and bind specific antigens
a billion different receptors are produced by genetic recombination
Major Histocompatibility Complex (MHC) Proteins
self antigens on membranes of all body cells except RBCs
genetically determined – unique to individuals (except identical twins)
self tolerance – selection process during lymphocyte development
destroys cells with receptors reactive to the body’s own antigens
immune cells recognize self and non-self
Antigen Presentation
1) macrophages phagocytize antigens and attach them to their MHCs
2) infected or abnormal cells attach abnormal antigens to their MHCs
Costimulation – a second signal besides antigen is required to activate most
T cells and B cells
cytokines released by macrophages, infected cells, or helper T cells
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CELL-MEDIATED IMMUNITY – T CELLS
1) APC presents antigen-MHC complex
2) T cell receptor recognizes and binds specific antigen-MHC complex
3) costimulation
T cell is activated
4) activated T cell divides and differentiates
helper T cells – secrete cytokines
costimulators for B cells and other T cells
chemotaxis and stimulation of macrophages and NK cells
cytotoxic T cells – seek and destroy cells with the initiating antigen
kill target cells by 3 methods:
cytolysis – release perforin proteins, which forms holes in
target cell membrane
lymphotoxin – toxic secretion that kills cell
apotosis – programs target cell DNA to kill cell
memory T cells – clone cells remaining in body for months to years
initiate faster, stronger second response to the same antigen
suppressor T cells – moderate T and B cell function by negative feedback
HUMORAL (ANTIBODY-MEDIATED) IMMUNITY – B CELLS
1) B cells produce specific antibodies attached to their cell membranes
2) B cell membrane antibodies recognize and bind specific antigen
3) B cell ingests antigen and presents it with MHC proteins
4) B cell antigen-MHC presented to helper T cells
5) helper T cells produce cytokines which costimulate B cell
6) activated B cell divides and differentiates
memory B cells
plasma cells – secrete antibodies into blood
antibodies seek out initiating antigen
antigen-antibody complexes – antibodies bind to initiating antigen
Functions of Antibodies
neutralize antigens – binding to antigen alters function of a microbe or
a chemical it produces
eg. neutralizes a bacterial toxin, prevents virus entering a cell,
immobilizes bacteria by binding to its cilia or flagella
enhance nonspecific immunity
agglutination – antigens clump together, making phagocytosis
more likely
activate complement system – stimulates phagocytes
opsonization – enhances phagocytosis
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Antibodies
immunoglobulins – large proteins composed of 4 polypeptide chains
variable segment – antigen-binding site, specific for antigen
constant segment – 5 similar structures
5 classes of immunoglobulins – based on constant segment
IgM – made at first exposure; level rises slowly
IgG – little produced at first expose
produced very rapidly at second exposure – prevents illness
IgA – can enter tissues and secretions
protects body surfaces and cavities
IgE – involved in allergies (found on basophils and mast cells)
IgD – involved in B cell activation (found on B cells)
ACTIVE IMMUNITY – animal develops memory T and B cells following exposure to
a specific antigen
second exposure – much faster, stronger immune response usually prevents the
animal from getting sick
vaccination – vaccine exposes animal to a specific antigen so it will develop
active immunity
attenuated antigen – antigen is weakened so it cannot cause disease, but it
still provokes an immune response
killed vaccine – organism is dead
live vaccine – organism cannot cause disease
PASSIVE IMMUNITY – giving a susceptible animal antibodies from another animal to
prevent or fight disease
passive transfer – fetus or newborn gets antibodies from mother (mainly IgG)
transplacental – fetus absorbs antibodies across placenta
colostrum – first mammary secretions (usually within the first 24 hours)
passive transfer lasts weeks to months, then young must develop its own
active immunity
transfer of immunity – antibodies can be harvested from one animal’s blood and
given to a susceptible animal
tetanus antitoxin – antibodies against toxin produced by tetanus bacterium
antivenins for snakebites
IMMUNE DISORDERS
Autoimmune disorders – immune responses against normal body tissues
Allergies – inappropriate or excessive immune responses
Immunodeficiency diseases
failure to develop normal lymphatic system
immunosuppressive diseases – AIDS, Cushings syndrome
immunosuppressive agents – drugs (corticosteroids), radiation
failure of passive transfer – newborn fails to receive adequate colostrum
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