chapter 20 – the lymphoid system and immunity
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Chapter 20 – The Lymphoid System and Immunity . Functions of lymphoid system . Production (along with red bone marrow) and distribution of lymphocytes Fluid balance Cardiovascular system delivers more fluid to peripheral tissues (via capillaries) than it drains away - PowerPoint PPT PresentationTRANSCRIPT
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Chapter 20 – The Lymphoid System and Immunity
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Functions of lymphoid system
• Production (along with red bone marrow) and distribution of lymphocytes
• Fluid balance – Cardiovascular system delivers more fluid to
peripheral tissues (via capillaries) than it drains away • Excess fluid enters lymphatic system, and is then
returned to cardiovascular system
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Components of lymphoid system • Lymph – Has lower
concentration of proteins than plasma
• Lymphatic vessels – Collect fluid from
tissues and return to venous system
• Lymphoid tissues and organs
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Lymphatic capillaries • Smallest vessel type • Originate as pockets (not
continuous as in cardiovascular system)
• Endothelial cells overlap and form valves – Allows entry into lymphatic
system, but prevents re-entry into intercellular spaces
• Lacteals are large capillaries in small intestine – Involved with lipid transport
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Small lymphatic vessels • Lead toward body’s
trunk • Similar in structure
to veins – Have valves, which
form bulges in the vessel
– Valves are closer together than in veins
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Major vessels
• Superficial lymphatics– Dermal layer of skin; areolar layer of GI tract;
respiratory, urinary, and reproductive systems • Deep lymphatics – Accompany arteries and veins of skeletal muscles,
deeper organs• Superficial and deep converge to form
lymphatic trunks
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Major vessels cont • Trunks converge to form:
– Thoracic duct • Collects lymph from body inferior to diaphragm and left side of body superior to
diaphragm – Cisterna chyli – sac-like area at base of thoracic duct
» Receives lymph from lower regions
– Right lymphatic duct • Collects lymph from right side of body superior to diaphragm
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Lymphoid organs and tissue
• Organs– Lymph nodes – Thymus – Spleen
• Tissues – MALT (mucosa-associated lymphoid tissue)– Tonsils
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Lymph nodes • Kidney bean shape • Afferent lymphatic carries
lymph into nodes; efferent lymphatic carries out
• Purifies lymph before returning to venous circulation– 99% of foreign antigens are
removed – Macrophages in walls
remove debris • Serve as antigen-presenting
cells
• Largest nodes are cervical, axillary, and inguinal
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Thymus • Largest (relative to body
size) during young childhood (1 – 2 yrs)
• After puberty, starts to become fibrous– Involution
• Two lobes – Septa divide each into
smaller, multiple lobules – Outer region = cortex;
inner region = medulla• Site of T cell maturation
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Spleen • Removes abnormal blood
cells from phagocytosis • Stores iron (recycled from
blood cells)• Initiation of immune
responses by T and B cells to antigens in circulating blood
• Reticular connective tissue– Red pulp – contains RBCs
• All circulating components of blood, and macrophages
– White pulp – contains lymphocytes (stains purple)
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Lymphoid tissues • MALT (mucosa-
associated lymphoid tissue)– Peyers patches in
intestinal walls; walls in vermiform appendix
• Tonsils – Pharyngeal – singular
(“adenoid”)– Palatine – paired – Lingual – paired
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Innate defenses
• First line of defense against infection – Bacteria, viruses, other pathogens
• Present before an exposure occurs
• Non-specific
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Innate defenses cont
• Skin – Lysozyme – digests bacterial cell walls
• Mucous membranes – Traps microbes; stomach acids kill most bacteria
• Neutrophils/macrophages – Engulf microbes by phagocytosis
• Natural killer cells – Attack cancer cell and viral-infected cells by releasing
chemicals to promote apoptosis
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Innate defenses cont
• Interferons – Produced by viral-infects cells that help other cells
resist infection • Complement – ~30 proteins in plasma – Circulate in an inactive form – activated by
exposure to microbes – Leads to lysis of invader and an inflammatory
response
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Innate defenses cont • Inflammatory response – Histamine released by damages tissue causes nearby blood
vessels to dilate and become ‘leaky’– Complement attracts phagocytes to area – Clotting proteins and platelets form local clot to seal off
infected area
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Acquired immunity • Fully develops AFTER exposure to a pathogen – Can amplify non-specific responses (inflammation,
complement)• Responds to the presence of foreign antigens– Increase the number of cells that fight invader, and
produce defense proteins called antibodies • Has memory – A second exposure to a previously encountered antigen
is faster and stronger • Acquired by:– Natural exposure – Vaccination/immunization
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Types of acquired immunity • Active – Person’s own immune system actively produces
antibodies – Either due to natural exposure or artificial (vaccine)
• Passive – Receive pre-made antibodies • Maternal antibodies to fetus through placenta• Breast milk to babies • Shot of antibodies after high-risk exposure
– Lasts only as long as antibody “lifespan”• Weeks to months
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Lymphocytes – B cells • B cells differentiate in
bone marrow • Humoral immunity – Secrete antibodies which
circulate in blood and lymph
– Defend against bacteria and viruses in body fluids
– Antibodies join with antigen • Complex recognized by
macrophages
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Antigens • Have specific regions
where antibodies bind to them
• Antigenic determinant or epitope – Small region on antigen’s
surface– Recognized by antibody – Single antigen may be
recognized by different antibodies at different sites
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Antibody structure
• 4 polypeptide chains • Each arm of Y has 2
variable regions that form a specific antigen recognition site
• C region sequences determine class of antibody
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Classes of antibodies • IgG– Most abundant– Only one that can cross the placenta – Fights bacteria, viruses, toxins; activates complement
• IgM– First type produced by newborn – Antibodies for ABO antigens; activates complement
• IgA– Found in saliva, tears, colostrum, mucous membranes – Primary defense against local infections of mucous membranes
• Prevents attachment of viruses and bacteria to epithelial cells
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Classes of antibodies cont
• IgD– Cell membranes of B lymphocytes – Antigen receptor for initiation of B cell differentiation
• IgE– Involved in inflammation, allergic responses,
parasitic infections – Binds to mast cells and basophils
• Triggers release of histamine and other inflammation & allergy mediators
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Antibodies mark antigens for elimination
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Clonal selection
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Primary and secondary immune response
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Lymphocytes – T cells
• Differentiate in thymus • Cell-mediated
immunity – Circulate in blood and
attack cells that are infected
– Also protects against fungal and protist invaders
– Aid in elimination of cancerous cells
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T cells
• Helper T cells (CD4)– Interact with other WBCs that function as antigen-
presenting cells • Activation of helper T stimulates other cells of
immunity
• Cytotoxic T cells (CD8)– Attack body cells that are infected by pathogens
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Hypersensitivity
• Excessive or inappropriate activation of the immune response
• The body is damaged by the immune response, rather than by the antigen
• Type I hypersensitivity – Commonly called ‘allergic reactions’– Mediated by IgE
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Anaphylaxis
• Systemic response to the inflammatory mediators released in type I hypersensitivity – Histamine, acetylcholine, kinins, leukotrienes, and
prostaglandins all cause vasodilation – Acteylcholine, kinins, leukotrienes, and
prostaglandins all can cause bronchoconstriction
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Autoimmune diseases
• Immune system attacks self-antigens • Normally, self-reactive immune cells are killed in
the lymphoid organs or suppressed by regulatory T cells
• In autoimmunity, this self-tolerance breaks down • Immune system destroys body tissues • Anti-tissue antibodies appear in blood (ex: anti-
thyroid antibodies)
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HIV – human immunodeficiency virus
• HIV is a retrovirus – Has RNA instead of DNA
as its genetic material – Uses the enzyme reverse
transcriptase to make DNA from RNA
• Binds to CD4 receptors
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HIV cont • New viral particles can be
shipped from cell via exocytosis
• Mutation then causes HIV to destroy helper T cells (cells lyse)
• AIDS – helper T cell population is dangerously low– Affects humoral and cell
mediated immunity