Biology 112
Chapter 14: Lymphatic System & Immunity
Introduction/Functions of the Lymphatic System
- draining excess interstitial fluid: lymphatic vessels drain excess
fluid from tissue spaces & return it to the blood
- transporting dietary lipids: lymphatic vessels transport lipids & lipid-soluble
vitamins (A,D,E & K) absorbed by GI tract to the blood
- carrying out immune responses: lymphatic tissue initiates specific
immune responses to microbes or abnormal cells
Lymphatic Pathways: lymphatic capillaries ® lymphatic
vessels ® subclavian veins
- lymphatic capillaries: microscopic closed-ended tubes
that extend into interstitial spaces & receive lymph through
their walls
- lymphatic vessels: walls similar to veins (thinner) with
valves to prevent backflow
- large lymphatic vessels pass through lymph nodes & merge into lymphatic
trunks
- lymphatic trunks lead to 2 collecting ducts:
thoracic duct & right lymphatic duct
- collecting ducts join subclavian veins
Tissue Fluid & Lymph
- tissue fluid is formed from blood plasma; includes smaller proteins (excludes
large)
- increased protein concentration in tissue fluid increases colloid
osmotic pressure
- pressure increase forces some tissue fluid into lymphatic capillaries ® lymph
- lymph returns proteins with fluid to bloodstream & transports
foreign particles to lymph nodes
- movement of lymph is generally slow due to low pressure; contraction of
skeletal muscles & pressure due to breathing accelerates lymph movement
Lymphoid Organs: Thymus & Spleen
Thymus: located in mediastinum above heart
- composed of lymphatic tissue subdivided into lobules
- size is large at birth; shrinks during aging following puberty
- T lymphocytes mature (become fully active) in the thymus; the mature T
cells then may migrate to other lymphoid tissues to provide immunity
Spleen: located in upper left quadrant of abdominopelvic
cavity; under diaphragm & posterolateral to stomach
- resembles large lymph node subdivided into lobules
- islands of white pulp contains lymphocytes; surrounding red
pulp contains red blood cells & some leukocytes
- sinuses within splenic lobules filled with blood;
spleen is primary site for red blood cell breakdown
- macrophages in spleen filter foreign particles & begin
breakdown of damaged RBCs
Body Defenses Against Infection: innate (nonspecific) & adaptive
(specific)
- pathogens (bacteria, viruses, fungi, parasites, etc.)
cause infection
Innate (Nonspecific) Defenses: includes species resistance,
mechanical & chemical barriers, fever, inflammation & phagocytosis
- species resistance: each species is resistant to diseases
that affect other species
- mechanical barriers: skin & mucous membranes; block
entry of pathogens
- chemical barriers: enzymes & acid (low pH) in gastric
juice kill pathogens; antimicrobials (lysozyme) in tears kills some pathogens
- interferons: proteins released by virus-infected cells
that stimulate uninfected cells to synthesize antiviral proteins
- stimulates phagocytosis, blocks viral replication, enhances WBC activity & slows
tumor growth
- fever: increase in body temperature due to infection
- decreases blood iron level (slows bacterial growth) & increases phagocyte
activity
- inflammation: tissue response to injury or infection
- 4 signs: localized redness, swelling, heat & pain
- chemicals released by damaged tissues attract WBCs to site
- inflammatory mediators: histamine from mast
cells & basophils & other chemicals that cause vasodilation & increased
blood vessel permeability
- fibrous CT may form a sac around injured tissue & block pathogen
spread
- phagocytosis: neutrophils & macrophages destroy
bacteria & foreign particles
- neutrophils enter infected tissue from blood; monocytes exit bloodstream & transform
into macrophages that stay in tissues
- phagocytes associated with lining of blood vessels, bone marrow, liver,
spleen, lungs & lymph nodes
Adaptive (Specific) Defenses: immune responses to specific
pathogens & unwanted cells by antibodies from B
lymphocytes & T lymphocytes
- antigens: molecules (especially proteins) that provoke
an immune response
- during development, body learns to distinguish self antigens from nonself
- nonself antigens combine with surface receptors on B cells & T cells
and stimulate an immune reaction
- haptens: small molecules that need carrier
(larger molecule) to become antigenic
- lymphocytes are produced in red bone marrow & released
into blood
- T cells mature in the thymus
- B cells mature in the bone marrow
- lymphatic tissues contain both T cells & B cells
- T cells interact with antigens directly ® cellular
immune response
- T cells secrete cytokines (interleukins)
that enhance cellular responses to antigens
- T cells may also secrete toxic substances that destroy target cells
- B cells interact with antigens indirectly ® humoral
immune response
- T cells & B cells can form millions of different varieties to react
against a wide variety of possible antigens
- each individual variety can interact with only one specific antigen
- members of each variety form a clone
- T cells & cellular immune response
- T cells activate when an antigen-presenting cell (APC) displays
a foreign antigen
- macrophages phagocytize pathogen or large antigen, partially digests
proteins & displays antigens on cell surface in association with major
histocompatibility complex (MHC or HLA) protein
- helper T cell: activated by binding antigen/HLA complex
on APC
- when helper T cell binds antigen/HLA complex on B cell, it releases cytokines that
stimulate B cell & T cell proliferation & attracts macrophages
for phagocytosis
- cytotoxic T cells recognize antigen/HLA complexes on tumor
cells or virus-infected cells & destroy these cells with perforin proteins
- memory T cells respond quickly to subsequent antigen
exposure
- B cells & humoral immune response
- a B cell becomes activated when antigen binds
to cell surface receptors; activation may be enhanced by helper T cell
stimulation
- an activated B cell generates a clone that produces memory
B cells & plasma cells that
secrete antibodies
- antibodies bind to & generally inactivate the antigen
- B cells & their antibodies can defend against millions of different
antigens by forming different antigen-binding sites on antibodies
- 5 classes of antibodies (immunoglobulins)
- antibodies are soluble proteins consisting of 4 polypeptide
chains (2 heavy chains, 2 light chains) that form a Y-shaped structure
- each antibody has 2 antigen-binding sites at variable regions
of chains
- IgD: B cell antigen receptor
- IgM: pentamer in blood; first Ig class secreted; agglutinating
agent; activates complement
- IgG: most abundant antibody in blood; protects against
bacteria, viruses & toxins; activates complement; crosses placenta
- IgA: dimer found in secretions (mucus, saliva, sweat
intestinal juice, milk), helps to prevent pathogens from entering body
- IgE: normally rare in blood (increase during allergic
reaction), binds to mast cells & basophils & causes release of
histamine & other mediators of inflammation
- antibody actions
- antibodies bind to antigens, activate complement, & stimulate tissue
inflammation
- antigen binding results in agglutination, precipitation or neutralization
- activated complement attracts phagocytes, makes foreign
cells more susceptible to phagocytosis & ruptures foreign cell membranes
(lysis)
- primary & secondary immune responses
- primary immune response is body’s first
response to antigen; antibodies are produced
for several weeks & long-lived memory cells are
produced
- secondary immune response uses memory cells
to react quickly to subsequent exposure to previously encountered antigen
- practical classification of immunity
- active immunity: produce own antibodies; passive
immunity: given antibodies
- naturally acquired active immunity: produced
through antigens from infections
- artificially acquired active immunity: produced
through antigens from vaccines
- naturally acquired passive immunity: antibodies
pass from mother to child
- artificially acquired passive immunity: antibodies
acquired from immune serum
- allergic reactions
- allergic reactions are excessive immune responses
to antigens (allergens) that may damage tissue
- repeated exposure to antigens can result in delayed hypersensitivity that
can cause skin inflammation
- immediate hypersensitivity results from overproduction
of IgE against allergen
- allergy results from mast cells releasing histamine, prostaglandin & leukotrienes that
cause tissue inflammation, contraction of bronchial & intestinal
smooth muscles & increased mucus production
- allergy symptoms include hives, hay fever, asthma, eczema & gastric
disturbance
- transplantation & tissue rejection
- tissue rejection occurs when immune system
of transplant recipient reacts against donated tissue
- matching donor & recipient tissue antigens (MHC antigens
(HLA in humans) on cell surface of tissue cells) & immunosuppression can
minimize tissue rejection
- immunosuppressive drugs may, however, increase likelihood of infection
- autoimmunity
- in autoimmune disorders, the immune system
produces autoantibodies that attack a person’s
own body tissues
- autoimmunity may result from a previous viral infection, faulty T cell
development or a nonself antigen that resembles self antigen