IMMUNITY MEDIATED BY B LYMPHOCYTES AND ANTIBODIES

*B lymphocytes recognize extracellular pathogens and toxins transported to secondary lymphoid tissues *Recognition stimulates proliferation and differentiation into *Plasma cells and memory B cells *B lymphocytes generally require help from activated T lymphocytes for differentiation into plasma cells *Plasma cells produce antibodies

ACTIVATION OF B LYMPHOCYTES *Begins with antigen binding by receptors resulting in crosslinking of receptors *Clustering and aggregation of receptors activates *Tyrosine kinases *Tyrosine kinases phosphorylates Ig-alpha and Ig-beta proteins initiating intracellular signaling *Additional signals are required and provided by *B cell co-receptor *CD4 TH2 lymphocytes

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SIGNAL ENHANCEMENT BY B CELL CO-RECEPTOR *B cell co-receptor is complex of 3 proteins *CD21 [Complement receptor 2 (CR2)] *Binds to complement on pathogen *CD19 *CD81 *Signal enhancement results from juxtaposition of receptor and co-receptor *1,000 to 10,000 X *Additional signals required depending on nature of antigen and provided by CD4 TH2 cells

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FINAL OUTCOME OF B CELL ACTIVATION *Proliferation and differentiation of B cells into antibody producing plasma cells *Morphology of B cells and plasma cells *B cells *Large nucleus and small cytoplasm *Plasma cells *Large nucleus and large cytoplasm packed with ER

THE NATURE OF ANTIGENS AND THE ANTIBODY RESPONSE *Thymus independent antigens (TI antigens) *Activate naïve B cells without help from CD4 cells *Classification into *TI-1 antigens (Lipopolysaccharides) *TI-2 antigens (Polysaccharides) *Minority of antibody production *Thymus dependent antigens (TD antigens) *Activation of naïve B cells requires help from CD4 cells *Majority of antibody production

THYMUS INDEPENDENT ANTIGENS AND ANTIBODY PRODUCTION *TI-1 antigens *Lipopolysaccharide of gram-negative bacteria *Stimulate production of IgM only *LPS specific activation *LPS non-specific co-activation *Repeating epitopes not required *TI-2 antigens *Polysaccharides and proteins of bacteria *Stimulate production predominately of IgM *Repeating epitopes required

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MECHANISM OF CD4 T CELL AND B CELL INTERACTION TO TD ANTIGENS *TD antigens transported to secondary lymphoid tissues for meeting with CD4 T cells and B cells *CD4 T cells are activated in T cell zone by APC *B cells enter T cell zone and bind same antigen *Antigen bound to B cell is internalized by *Receptor mediated endocytosis

MECHANISM OF CD4 T CELL AND B CELL INTERACTION TO TD ANTIGENS *Antigen is processed and presented on B cell surface with MHC class II molecules *T cell and B cell interact via CD40L and CD40 *T cell produce cytokine (interleukin-4) which activates B cell *Cognate interaction *Interaction of B and T cells specific for same antigen

COGNATE INTERACTION OF CD4 TH2 CELLS AND B CELLS *CI results in primary focus of B lymphoblasts in T cell area *Some B lymphoblasts move to medullary cords and differentiate into plasma cells *IL-5 and IL-6 from TH2 cells *Some B lymphoblasts move to primary follicles and differentiate into centroblasts *Centroblasts proliferate and follicle changes morphology *Germinal center

GERMINAL CENTERS IN SECONDARY LYMPHOID TISSUES *Mantle zone *Resting B cells present in follicle prior to arrival of activated B cells and T cells *Light zone *Non-dividing centrocytes associated with *Follicular dentritic cells *Stromal cells of lymphoid follicles *Dark zone *Proliferating centroblasts

GERMINAL CENTERS IN SECONDARY LYMPHOID TISSUES *Site for somatic hypermutation and affinity maturation *Initiated by cytokines of T cells *Begins with centroblasts in dark zone *Results in centrocytes with mutated receptors in light zone *B cells (centrocytes) which undergo somatic hypermutation *Produce receptor with range of affinities *Highest affinity receptors are selected *Must bind antigen or face apoptosis *Antigen provided by follicular dentritic cells

PRESENTATION OF ANTIGEN BY FDC’S *Follicular dentritic cells (FDC’s) *Bind antigen in form of immune complexes *Bound immune complexes are not internalized and become clustered as *Iccosomes (Immune complex coated bodies) * Iccosomes are shed from FDC’s and taken up by centrocytes *Centrocytes must obtain, internalize and present antigen for differentiation into plasma cells

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COMPARISON OF RESTING B CELLS AND PLASMA CELLS *Differentiation based on intrinsic and inducible properties *Intrinsic *Surface IG *Surface MHC class II molecules *High rate of antibody secretion *Inducible *Growth *Somatic hypermutation *Isotype switching

ISOTYPE SWITCHING IN B CELLS *Takes place primarily in germinal centers *Determined by *Cognate interaction with CD4 T cells *Induction requires CD40L to CD40 *T cell cytokines *Induction or inhibition of isotypes *Hyper-IgM Syndrome * Genetic immunodeficiency from no CD40L * B cells cannot switch isotypes * No response to TD antigens

GENERAL EFFECTOR FUNCTIONS OF ANTIBODIES *IgM *Protection of blood *IgG and IgA (monomeric) *Protection of blood and extracellular fluids *IgA (dimeric) *Protection mucous membranes and secretions *IgE *Protection of connective tissues

PROTECTION OF BLOOD BY IgM ANTIBODIES *Primary function *Early protection against blood-borne pathogens *Characteristics *First antibody produced *Secreted form is pentamer with 10 binding sites *Penetration of tissue fluids is limited *Phagocytic cells have no IgM Fc receptors *Fc region can bind complement

PROTECTION OF MUCOUS MEMBRANES BY DIMERIC IgA *Dimeric IgA made by plasma cells in mucosal-associated lymphoid tissues *GALT and BALT *Dimeric IgA transfer to epithelial surface *Transcytosis *Receptor mediated transport of macromolecules across epithelial cells *Mechanism of transcytosis *Binding to poly-Ig receptor on basolateral epithelium *Endocytotic vesicle transport to apical epithelium *Protease cleavage of receptor to secretory piece

IgG TRANSPORT FROM BLOOD TO EXTRACELLULAR FLUIDS *Transport mediated by endothelial receptor *Brambell receptor (FcRB) *Structure of receptor *Similar to MHC class I molecule *Mechanism *Binding to FcRB on apical endothelium *Endocytotic vesicle transport to basolateral endothelium

ANTIBODIES PROTECTING FETUS AND NEWBORN *IgG *Protects fetus and newborn *Maternal circulation to fetal circulation *Transfer across placenta mediated by FcRB *IgG levels similar in mothers and newborns *IgA *Protects GI tract of newborn *Transfer by breast milk *Dimeric form (IgA2 subclass)

DISTRIBUTION OF ANTIBODIES IN HUMAN BODY *Plasma *IgM, IgG and IgA (monomeric) *Extracellular fluids *IgG and IgA (monomeric) *Mucous membranes and secretions *IgA (dimeric) *Connective tissues *IgE

ANTIBODIES PROTECT AGAINST BACTERIAL EXOTOXINS *Number of bacteria cause disease by secreting exotoxins *Many toxins have receptor-binding and toxic functions on separate polypeptide chains *Exotoxin disease prevented by antibodies that block toxin binding *Neutralizing anitbodies *Antibodies raised by vaccination against toxins using *Toxoids (modified toxins)

ANTIBODIES PROTECT AGAINST VIRAL AND BACTERIAL INFECTION *Initial step in microbial pathogenesis is attachment to host tissues mediated by adhesins *Microbial adhesins *Molecules *Gp120 of HIV *Hemagglutinin (HA) of Influenza viruses *Organelles *Fimbriae of Escherichia coli *Neutralizing antibodies against adhesins prevent attachment

DESTRUCTION OF ANTIBODY COATED PATHOGENS *Phagocytes are agents of destruction *Macrophages and neutrophils *Mechanism *Antibodies bind to pathogens *Phagocytes have Fc-gamma receptors on surface *Fc-gamma receptor of phagocytes binds to Fc region of antibody *Low affinity binding *Antibody coating of pathogens enhances phagocytosis *Encapsulated pathogens

DESTRUCTION OF ANTIBODY COATED PARASITES *Adult parasites are the largest of microorganisms *Diphyllobothrium latum ( meters) *Ascaris lumbricoides (20 – 35 cm) *Mechanism of destruction *IgE coating of parasites *Binding of mast cells, basophils and activated eosinophils *Release of granules contents onto surface *Mechanism of elimination *Inflammatory mediators promote physical removal *Constriction of smooth muscle *Increased blood vessel permeability

CASE STUDY *45 year old white female *Presents to family physician with complaint *Mild fatigue for one month *Passed “Ribbon-like worm” with bowel movement *Patient history *No exotic travel *No pets *Eats out 3 to 4 times a week (patron of sushi / sashimi)

CASE STUDY *Laboratory testing *CBC with differential normal *Basic metabolic panel normal *Physical examination *Extraction from rectum of ribbon-like worm *Approximately 80 cm *Specimen (worm) sent to laboratory

CASE STUDY – QUESTIONS *What is the diagnosis *What is the etiological agent *How is the etiological agent transmitted *What is the recommended treatment *United States *Japan

CASE STUDY – ANSWERS TO QUESTIONS *Diagnosis *Diphyllobothriasis *Etiological agent *Diphyllobothrium latum *Transmission *Ingestion of raw fish containing plerocercoid larvae

CASE STUDY – ANSWERS TO QUESTIONS *Treatment in the United States *Praziquantel *Niclosamide *Treatment in Japan *Amidotrizoic (diatrizoic) acid (Gastrografin) *Administered either orally or by injection into duodenum by endoscopy

CASE STUDY *87 year old female presents to family physician with *Erythematous lesion on left thigh *Three weeks duration *Lesion biopsy submitted for pathology exam *Pathology report *Subcutaneous Dirofilaria infestation with granulomatous and eosinophilic dermatitis

MAST CELLS, EOSINOPHILS, BASOPHILS AND IgE ANTIBODY *Mast cells, basophils and activated eosinophils have IgE receptor *Fc-epsilon-RI *High affinity for Fc region of IgE *Mast cells *Fc-epsilon-RI receptors and cytoplasmic granules are constitutive *Cytoplasmic granules contain “inflammatory mediators” *Histamine *Antigen binding initiates degranulation of mast cells

Fc RECEPTORS AND NATURAL KILLER CELLS *NK cells *Express Fc-gamma-RIII receptor for IgG1 and IgG3 *Fc-gamma-RIII important in ADCC *ADCC *Mediated primarily by NK cells *Illustrates that antibody can direct specific attack by effector cells that have no specificity for antigen *Mechanism *Virus infected cells express viral proteins on surface *Antibodies bind viral proteins *NK cells bind to antibodies, release granules, kill cells