Introduction to Immunology

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Introduction to Immunology
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I. General Introduction
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A. Definitions

• Immunity the state of protection from
  infectious disease involving specific and
  non-specific elements
• Specific immunity (aka aquired immunity) employs
  components of the immune response that
  specifically recognize and selectively eliminate
  microorganisms and molecules perceived as foreign
  by the host

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• Nonspecific immunity (aka innate immunity)
  utilizes the basic resistance to disease that a
  species possesses and makes up the first line of
  defense
• Antigen is anything that the adaptive immune
  response (IR) can recognize (antibody generating)

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B. The Origin of Immunology

• Edward Jenner (1796) used the cowpox virus
  (vaccinia) to confer induced protection fro human
  small pox
• Vaccination term now used to explain how
  healthy subjects are inoculated with attenuated
  (weakened) strains of pathogens to induce
  ACQUIRED protection (Active immunization)
• Small pox successfully eradicated in 1979
  Announced by the WHO

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• Robert Koch (1843-1910) proved that bacteria were
  responsible for causing anthrax and tuberculosis.
• He developed Kochs postulates
• A set of criteria to be used when establishing a
  causative link between a particular microorganism
  and a particular disease
• Kochs postulates are still followed today to
  show that pathology (disease) is caused by one of
  the four major groups of pathogens
• Viruses
• Bacteria
• Pathogenic fungi
• Parasites

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• Louis Pasteur (1822-1895) developed vaccines
  against cholera (Vibrio cholerae) and rabies
  (Rhabdovirus Negribodies RNA virus)
• Emil von Behring (1845-1917) and Shibasaburo
  Kitasato (1852-1931) using diphtheria toxin,
  identified that serum of vaccinated individuals
  contained antibodies that specifically interact
  with the immunogen (antisera, passive
  immunization when injected with immune serum)

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• Eli Metchnikoff (1845-1916) reported the
  engulfment and degradation of microorganisms by a
  type of phagocytic cell he called macrophages
• First line of defense
• Innate (non-adaptive) immunity
• Receptor-mediated endocytosis/phagocytosis

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C. Major cells of Innate and Acquired Immune
Responses

• Involve action of white blood cells called
  leukocytes (lymphocytes, polymorphonuclear
  leukocytes and monocytes) and dendritic cells
• Innate immunity
• Involves mainly granulocytes
• Granules in cytoplasm
• Different cell types many are phagocytic
• Most are polymorphonuclear leukocytes
  neutrophils (multilobed nuclei and cytoplasmic
  granules), eosinophils, basophils, mast cells
  (PMNLs)
• Also involves monocytes (mononuclear) ?
  Macrophages in tissues
• Acquired (adaptive) immunity
• Involves lymphocytes (B cells, T cells, Natural
  Killer Cells)
• Lifelong immunity established through generation
  of memory cells (B and T cells have memory)

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The Components of the Immune System
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A. Hematopoietic Stem Cells Give Rise to White
and Red Blood Cells

• All circulating blood components originate in the
  bone marrow
• The same precursor cell or progenitor gives rise
  to all of the various lineages Pluripotent
  Hematopoietic Stem Cell
• Differentiate or mature into two main progenitor
  populations
• Lymphoid Lineage B, T, and NK cells
• Myeloid Lineage PMNL (Basophils, Mast cells,
  Eosinopils, Neutrophils), Monocytes/Macrophages
• Erythroid Lineage (Megakaryocyte, Platelets and
  Erythrocytes)
• Hierarachy of Cell Maturation Pluripotent Stem
  Cells ? Committed Progenitor Cells ? Terminally
  Differentiated Cells

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B. Maturation of Lymphocytes

• Resting B and T cells
• Small, inactive, heterochromatin,scantycytoplasm
  containing few organelles
• No functional significance

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• Receptors to recognize specific antigen
• B cell receptors
• Membrane-bound antibody surface immunoglobulin
  (Ig)
• Following B cell activation ? Differentiation
  into plasma cell ? Cytoplasm enlarges, ER
  expands, active transcription, increase in
  organelles ? Antibody secretion
• T cell receptors
• Related to Ig but distinctive
• Following T cell activation (in peripheral
  lymphoid organ)
• Cytotoxic T cells (Tcyt) ? Kill infected target
  cells
• Inflammatory (TH1) and Helper (TH2) T cells ?
  Activation of other cells (macrophages and B
  cells, respectively)

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• Sites of Maturation ? Central/Primary Lymphoid
  Organs
• B cells mature in the bone marrow (or Bursa of
  Fabricius in birds)
• T cells mature in the thymus
• After maturation, lymphocytes are transported to
  the bloodstream and then traffic ? to the
  Peripheral/Secondary Lymphoid Organs
• This is where antigen is encountered
• This is where lymphocytes divide (clonal
  expansion)

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Adaptive IRs Occur in Peripheral Lymphoid Organs

• Peripheral Lymphoid Organs
• Lymph nodes
• Spleen
• Mucosal associated lymphoid tissue (MALT)
• Bronchial associated lymphoid tissue (BALT)
• Gut associated lymphoid tissue (GALT)
• Tonsils
• Adenoids
• Appendix
• Peyers patches
• Function to trap antigen from sites of
  infection and present it to circulating, resting
  lymphocytes to induce adaptive immune responses

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• Lymph nodes (LN)
• The afferent lymphatic vessel delivers lymph
  draining the extracellular spaces of the body to
  the L.N. (e.g. in interstitial spaces of
  tissues)
• Antigen (Ag) becomes trapped in the L.N.
• The efferent lymphatic vessel takes lymph away
  from the L.N. medulla region

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• Anatomy of a lymph node
• Post-capillary venules deliver naïve lymphocytes
• Cortex
• Outer cortex B lymphocytes in 1o follicles and
  2o follicles (germinal centers, where
  proliferation occurs if B cells are responsive to
  Ag)
• Paracortex T lymphocytes and dendritic cells
• Medulla
• Medullary cords macrophages and plasma cells
• Region where lymph leaves
• Similar organization in spleen and Peyers
  patches. Important for TB cooperation.

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• Spleen (Largest peripheral lymphoid organ)
• Collects and traps Ag from the blood (via splenic
  artery)
• Important for systemic infections
• Not supplied by afferent lymphatics
• Final stop for dying (senescent) RBC
• Red pulp major area and site of RBC disposal by
  splenic macrophages
• White pulp forms around a central arteriole
• Periarteriolar lymphoid sheath (PALS) Mainly T
  cells
• B Cell corona and germina center
• Arterioles ? vascular sinusoids ? splenic vein

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D. Continuous Recirculation of Lymphocytes

• Naïve lymphocytes are circulating continuously
  between blood and peripheral lymphoid regions
• Homing to 2o lymphoid tissue
• Involves binding to adhesion molecules on
  lymphocytes called L-Selectin, to its ligand
  called mucin-like vascular addressin (CD34
  GlyCAM-1 in L.N. MAdCAM-1 in MALT) on high
  endothelial venules (HEV capillaries delivering
  cells)

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If Ag IS Encountered

• 1. Ag enters L.N. through the afferent lymph
  (often via phagocytic cells)
• 2. Ag is trapped by professional antigen
  presenting cells (APC)
• 3. Ag is displayed to naïve lymphocytes
• 4. Lymphocytes, which have a specific cell
  surface receptor that recognizes Ag, remain in
  peripheral lymphoid organ, proliferate, and then
  differentiate into effector cells
• 5. Effector cells leave L.N. through efferent
  lymphatic vessel ? return to blood via thoracic
  duct
• 6. Emigration of WBC out of the bloodstream to
  sites of infection using adhesion molecules
  called integrins (extravasation)
• T cells express increased levels of LFA-1, and
  begin to express VLA-4 once activated (binds
  ICAM-1 and VCAM-1, respectively)
• Macrophages express MAC-1 (binds ICAM-1)

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If Ag IS NOT Encountered

• Lymphocyte leaves through efferent lymphatic
  vessel of 2o lymphoid
• Returns to bloodstream via thoracic duct

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Innate versus Adaptive Immunity
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A. Distinction Between Innate and Adaptive
Immune Responses

• Innate immunity is non-adaptive and helps to
  initiate adaptive immune responses ( first line
  of defense but LIMITED)
• Immediate (0-4 hours)
• Adaptive immunity provides a more universal line
  of defense and has long-lived memory to provide
  protection upon re-infection
• Second line of defense
• Generation of Ag-specific effector cells
• Early (4-96 hours)
• Late (gt96 hours)

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B. Innate Immune Responses

• Innate defense is present in al individuals and
  can operate at various locations in the body

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• Seven types of defensive barriers
• 1 Anatomical Barriers
• Skin epidermis, dermis, keratin, sebum and
  other epithelial surfaces
• Mucous membrane surfaces saliva, tears, mucous
  secretions
• 2 Physiological barriers
• Temperature, pH, O2 tension, soluble factors
  lysozyme, interferons, acute phase proteins,
  complement, digestive enzymes, cytokines,
  chemokines, monokines (IL-1, IL-6, TNF alpha)

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Three Functions of Interferons

• 1. Induce resistance to viral replication by
  activating cellular genes that
• Destroy viral mRNA
• Inhibit translation of viral proteins
• 2. Increase Major Histocompatibility Complex
  (MHC) Class I expression universally
• Increases level of Ag presentation to Tcyt (CD8)
  cytotoxic T cells (aka killer T cells)
• Increase resistance of uninfected cells to NK
  cell attack (more later)
• 3. Activate NK cells to kill virus-infected cells

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Acute Phase Proteins in Humans Act as Opsonins

• Hepatocytes in liver produce APP in response to
  IL-1, IL-6 and TNF alpha
• 1. Mannose binding protein (MBP)
• Binds mannose residues on bacterial cells
• Acts as an opsonin (enhances receptor mediated
  endocytosis by phagocytes)
• Activates complement (lectin complement pathway)
• Mimics activity of antibodies by acting as an
  opsonin and activating complement

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• 2. C-Reactive Protein (CRP)
• Binds bacterial phosphorylcholine
• Mimics activity of Ab by acting as opsonin and
  activating complement (classical pathway)
• 3. Fibrinogen
• Also an APP made by hepatocytes
• Definition of opsonization Alteration of the
  surface of a pathogen enabling its ingestion by
  phagocytic cells (neutrophils and macrophages)
  through RME. Examples Ab, C, CRP, MBP

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• 3 Endocytic and Phagocytic Barriers
• Endocytosis Pinocytosis or receptor-mediated
  endocytosis or macromolecules (non-specific
  versus specific, respectively), followed by
  fusion with primary lysosomes where they are
  digested and processed (eliminated)
• Phagocytosis Ingestions of particulate material
  aided by microfilaments which fuse with lysosomes
  ? phagolysosomes

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• 4 Inflammatory Barriers
• Major events
• Vasodilation
• Increased capillary permeability
• Influx of phagocytic cells
• Vasodilation results in reduced blood flow
  velocity allowing leukocytes to move out of
  capillaries to vascular endothelium where they
  penetrate, resulting in accumulation of fluid
  (swelling, pain)

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• Macrophages produce MONOKINES which recruit more
  phagocytic cells and effector molecules to site
  of infection
• IL-1, IL-6, IL-8, IL-12, TNF-alpha
• Can also produce harmful, systemic effects
• Systemic Shock Disseminated Intravasclar
  Coagulation (DIC) ? Organ Failure
• Have a variety of effects at different locations

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Inflammation cont.

• Other inflammatory mediators released/generated
  by macrophages and neutrophils include
• Plasminogen activator
• Prostaglandins
• Phospholipase
• Platelet activating factor
• Leukotrienes
• Respiratory burst molecules Nitric oxide,
  hydrogen peroxide, superoxide anion (toxic to
  bacteria, generated in phagolysosome)

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• Inflammatory mediators induce the expression of
  adhesion molecules that bind monocytes and PMNLs
  and aid in their recruitment to sites of
  infection in tissues ? EXRAVASATION (4 steps)
• 1. Rolling adhesion
• 2. Tight binding
• 3. Diapedesis (crossing the vascular endothelial
  wall)
• 4. Migration
• These 4 steps will now be reviewed.

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Step 1 Rolling Adhesion

• Endothelium is induced by inflammatory mediators
  to express SELECTINS
• P-selectin induced by leukotriene B4, C5a or
  histamine
• Appears immediately
• Stored in endothelial granules called
  Weibel-Palade Bodies
• E-selectin induced by TNF-alpha
  Lipopolysaccharide (LPS) ? appears after a few
  hours
• Selectin ligand glycoprotein sialyly-Lewisx on
  monocytes and neutrophils
• Reversible binding ? rolls along endothelium

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Step 2 Tight Binding

• ICAM-1 on endothelium induced by TNF-alpha
• ICAM-1 binds integrins LFA-1 MAC-1 (CR-3)
• Tight binding induced by IL-8 (or other
  chemokines) ? changes conformation of LFA-1
  MAC-1
• Binds better
• Increases adhesion
• Rolling stops

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Step 3 Diapedesis

• Crossing of endothelial wall Extravasation
  (diapedesis)
• Leukocytes squeeze through
• Penetration of basement membrane (ECM)

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Step 4 Migration

• Migration through tissues via chemokines
• Recruitment to appropriate location to enable
  phagocytosis/antigen processing

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• 5 Normal Microbiological Flora (Microbiota)
• Non-pathogenic organisms associated with
  epithelial surfaces compete with invading
  organisms for attachment sites
• Compete for nutrients
• Flora can produce anti-microbial substances (e.g.
  colicins made by Escherichia coli, oleic acid
  made by Propionibacterium acnes)
• Often displaced by antibiotics enabling
  colonization by opportunistic bacteria

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• 6 Alternative pathway of complement activation
• Does not require antibody
• Acts immediately
• Activates the terminal complement components
  which destroy bacteria by creating holes (pores)
  in the bacterial membrane ? Membrane Attack
  Complex
• Opsonization also enhanced (C3b ? binds to CR1)

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• 7 Lymphoid lineages involved in non-adaptive
  responses
• Natural killer (NK) cells (aka large granular
  lymphocytes LGL)
• Defend host against virus-infected cells
• Kill sensitized targets
• Activated by IL-12, alpha-interferon and
  beta-interferon
• MHC class I involved
• Present ? Negative signal overrides activity of
  killing receptors

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• Intraepithelial gammadelta T cells (gd)
• A subset of T cells that are produced early
  during embryogenesis in waves
• Homogeneous T cell receptors within any
  epithelium location
• Do not recirculate
• May reorganize alterations on the surfaces of
  epithelial cells as a result of infection
• Exact function still unclear

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• CD5 B cells (aka B-1 B cells)
• Also arise early in embryogenesis
• Limited rearrangement of V genes (ab genes),
  mainly IgM
• Present as major lymphocyte in the peritoneum
• Respond t polysaccharide antigens (TI-2 type
  repeating subunit structure)
• Exact function still debatable
• Once IgM is bound, can activate complement

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C. Adaptive Immune Responses

• Clonal selection of lymphocytes
• Lymphocytes express receptors with only one
  specificity
• The specificity of each lymphocyte is unique
• The body contains a pool of lymphocytes with a
  HUGE repertoire of different specificities
• Lymphocytes with useful receptors are selected to
  survive
• Most lymphocytes with self-reactive receptors are
  deleted (apoptosis) or rendered non-responsive
  (anergy)

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• Clonal expansion of lymphocytes
• Because of huge variety of different receptors,
  the actual number of lymphocytes that can respond
  to a particular antigen is quite small
• Lymphocytes will proliferate after activation
  prior to differentiating into effector cells

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• Stages of Clonal Expansion
• Ag trapped in 2o lymphoid tissue is displayed to
  circulating naïve lymphocytes
• Ag is recognized by a lymphocyte bearing a
  receptor with correct specificity for that Ag
• Lymphocyte enlarges ? Lymphoblast
• Chromatin is less dense
• Nucleoli appear
• Cytoplasm increases
• Transcription and translation begin

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• Cell division (2-4x every 24 hours for 3-5 days)
• Can get up to 1000 daughter cells from one parent
  cell
• Clones of daughter cells all have the same
  specificity for Ag as original activated cell
• Differentiation into effector cells
• B cells secrete Antibody (Ab) ? Plasma cells
• T cells destroy infected cells or help other
  cells to become activated

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• Some effector cells persist and develop into
  memory cells (more rapid 2o recall responses)
• Lymphocytes with receptors that recognize host
  proteins (self) are deleted early in ontogeny and
  do not appear in the mature lymphocyte repertoire
  TOLERANCE

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• Combinatorial diversity
• Susumu Tonegawa (1976) demonstrated that Ig genes
  are a set of multiple gene segments that together
  encode the VARIABLE region of the antibody
  molecule (Nobel Price 1987 Gene Rearrangement
  in Ab Synthesis)
• Gene segments are joined together differently in
  each cell, generating a unique gene for the
  variable region (same process occurs in T cells)

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• Limited number of gene segments can give rise to
  large, diverse sets of products
• Cells express unique Ag-receptors ? huge
  repertoire of specificities
• Genomic DNA recombined, changes are permanent ?
  Somatic gene rearrangement (all daughter cells
  will have the same rearrangement)
• 108 different lymphocytes in our bodies each
  with unique specificity

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IV. Antigen Presenting Cells
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Types of antigen presenting cells (APC)

• Langerhans cells of epidermis (in sin and
  squamous epithelia) Bone Marrow Derived
• Phagocytic dendritic cells (DC) of tissues
• Migrate to LN as veiled cells via afferent
  lymph to paracortical regions (T rich) ? Now
  referred to as Interdigitating Dendritic Cells
  (ICD)
• Most potent stimulators of T cell responses
• MHC class II positive, B7 positive in LN

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• Follicular dendritic cells (FDC)
• Highly branched network in lymphoid follicle
  stromal areas (B rich)
• Non-migratory, origin uncertain
• Contain non-endocytic Fc receptors and complement
  receptors that hold AbAg and CAg complexes in
  place for long periods of time (months to years)
• Play important role in B cell circulation,
  mortality, and memory
• MHC class II negative

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• Germinal Center Dendritic Cells (GCDCs)
• Migratory, Class II
• Interact with T cells in germinal center areas
• Thymic Interdigitating Dendritic Cells (IDCs)
• Migratory, Class II
• Abundant in medulla and at cortico-medullary
  junction of thymus
• Important in deletion of self-reactive T cells
• Interstitial Dendritic Cells
• Migratory, Class II
• Populate most organs (heart, lung, liver, kidney)

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• B cells as APC
• MHC class II
• Present Ag to T cells (TH2)
• Use Ig and receptor-mediated endocytosis (very
  effective at low Ag).
• Macrophages
• MHC class II after stimulated (e.g. infected)
• Present Ag to TH1
• Use variety of cell surface receptors for
  receptor-mediated endocytosis (Fc receptor, CR-1)

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• Non-professional APCs
• On-immune, somatic cells
• Normaly MHC class II negative, but can be induced
  to express class II inappropriately (gamma IFN,
  TNF-alpha)
• Include keratinocytes, thyroid epithelium,
  endothelium)
• Ag presentation can result in autoimmunity and
  prolonged (chronic) inflammation

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B. Two Signals Needed for Full Activation of
Lymphocytes

• B cells usually receive 2nd signal from T cells
• 1st signal binding to Ag using cell-bound Ig
• Cytokines (e.g. IL-4)
• Cell surface molecules (CD40) (T cells express
  CD40Ligand CD40L- when activated)
• Important in isotype switching
• Hyper IgM syndrome ? CD40L deficiency
• No help to B cells, Unable to isotype switch,
  Respond to T-independent Ags only

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• T cells require 2nd signal from APC
• 1st signal recognition of Ag- binding using T
  cell receptor (TCR)
• 2nd signal Cytokines (e.g. IL-2 T cell growth
  factor) (often autocrine effect)
• 2nd signal co-stimulatory molecule B7 (binds
  CD28 on T cells)

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V. Recognition and Effector Mechanisms of
Adaptive Immunity
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A. Humoral Immunity

• Antibody-mediated immunity by B cells
• Involves interaction with innate and adaptive
  mechanisms and complement cascade
• Five different Ab classes called ISOTYPES have
  different effector functions (IgG, IgA, IgM, IgE
  and IgD) (GAMED)

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• Elimination of Ab-coated Ag
• Cross-linking Ag forms clusters that bind to
  phagocytic cells via Fc receptors OPSONIZATION
• Coating bacterial toxins or viral particles and
  inhibiting binding to host cell NEUTRALIZATION
• Activation of COMPLEMENT resulting in lysis of
  invading organism and activation of phagocytes

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B. Cell-Mediated Immunity

• Involves the association of T cells with APC
• Controls intracellular infections
• Activates B cells or Macrophages, or destroys
  infected, tumor, or transplanted cells
• Provides help to B cells (T-dependent B cell
  resonses)(TH2)

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• Cytotoxic T cells Tcyt
• Recognize virus-infected cells
• Kill infected cells directly by inducing
  apoptosis (programmed cell death cell suicide)
• CD8, MHC class I-restricted

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• Inflammatory T cells TH1
• Activate macrophages
• Effective in eradicating bacteria-infected cells
  (e.g. Mycobacterium tuberculosis in macrophages)
• CD4, MHC class II-restricted

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• Helper T cells (TH2)
• Help to eradicate extracellular pathogens
• Provide help to B cells (2nd signal)
  (T-dependent B cells responses
• CD4, MHC class II-restricted

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C. T Cells Recognize Ag as Peptide Associated
with MHC Molecules

• T cells recognize short peptide fragments
  associated with membrane-bound, glycoproteins
  encoded by the Major Histocompatibility Complex
  (MHC)
• MHC/peptide complexes are used by APC to present
  Ag to T cells

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D. The Two Classes Of MHC Molecules

• MHC Class I
• Presents Ag derived from intracellular
  sourc/cytosol
• CD8 T cells interact with MHC class I
  (cytotoxic T cells ? kill target)

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• MHC Class II
• Present Ag derived from extracellular or
  cell-bound source
• CD4 T cells interact with MHC class II
  (inflammatory T cells and helper T cells)

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VI. When Things Go Wrong
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A. Immunodeficiency Diseases

• When some unit or the immune response does not
  function effectively
• Can be life threatening
• Often associated with recurrent infections
• Acquired Immune Deficiency Syndrome (AIDS)
• TH1 and TH2 subsets of T cells destroyed
• Caused by human immunodeficiency virus (HIV)
• Individual suffers from multitude of infections,
  including those normally controlled by macrophages

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B. Allergies, Autoimmune Diseases and Graft
Rejection

• Response mounted against Ags in the absence of
  infectious disease
• Allergy Ag innocuous foreign substance (e.g.
  pollen)
• Autoimmunity Ag self Ag (not tolerant)
• Graft rejection Ag foreign cell
• Therapy Ag-specific suppression and general
  immunosuppression

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How to Exploit the Immune System
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A. Vaccination

• Adaptive IR is specifically triggered and long
  lasting memory is established
• Examples of successful vaccination programs
• Diphtheria, Polio, Tetanus, Pertussis, Measles,
  Mumps, Rubella, Haemophilus influenzae B,
  Smallpox
• Many diseases for which vaccination does not
  exist or is not effective
• Malaria, Schistosomiasis, AIDS, Tuberculosis..

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B. Tumor Surveillance

• Tumor cells express proteins capable of inducing
  immune responses
• Vaccination lends potent cancer prevention
  strategy