1. Lesson 11 Adaptive Immunity “Specific Immunity” March 31,2015

2. Immunity Review Innate Immunity: defenses against any pathogen – Skin – Phagocytes – Inflammation – Fever Adaptive Immunity: induced resistance to a specific pathogen – Humoral (antibodies) – Cell-mediated

3. Extracellular antigens A B cell binds to the antigen for which it is specific. A T- dependent B cell requires cooperation with a T helper (T H ) cell. The B cell, often with stimulation by cytokines from a T H cell, differentiates into a plasma cell. Some B cells become memory cells. Plasma cells proliferate and produce antibodies against the antigen. Intracellular antigens are expressed on the surface of an APC, a cell infected by a virus, a bacterium, or a parasite. A T cell binds to MHC– antigen complexes on the surface of the infected cell, activating the T cell (with its cytokine receptors). Activation of macrophage (enhanced phagocytic activity). The CD8 + T cell becomes a cytotoxic T lymphocyte (CTL) able to induce apoptosis of the target cell. B cell Plasma cell T cell T H cell Cytotoxic T lymphocyte Cytokines Lysed target cell Cytokines activate macrophage. Cytokines from the T H cell transform B cells into antibody-producing plasma cells. Cytokines activate T helper (T H ) cell. Memory cell Some T and B cells differentiate into memory cells that respond rapidly to any secondary encounter with an antigen. Humoral (antibody-mediated) immune systemCellular (cell-mediated) immune system Control of freely circulating pathogensControl of intracellular pathogens Figure 17.20 The dual nature of the adaptive immune system.

4. Dual Nature of Adaptive Immunity Main components of Adaptive Immunity are T cells and B cells The fetal liver serves as the source of both T and B cells early in development and in bone marrow in adulthood – T cells develop in thymus in adults – B cells develop in the red bone marrow in adults

5. Stem cells develop in bone marrow or in fetal liver Stem cell (diverges into two cell lines) Red bone marrow of adults Differentiate to B cells in adult red bone marrow B cell Thymus Differentiate to T cells in thymus T cell Migrate to lymphoid tissue such as spleen, but especially lymph nodes Figure 17.8 Differentiation of T cells and B cells.

6. Dual Nature of Adaptive Immunity Humoral Immunity – Due to antibodies – B cells mature in the bone marrow Name derived from chickens: bursa of Fabricius – Bursa is an organ found in chickens that when removed results in the abrogation in antibody production

7. Dual Nature of Adaptive Immunity Cellular Immunity – Due to T cell activation – T cells mature in the thymus 1.Stimulate the production macrophages and other phagocytes 2.Activate B-cells into becoming plasma cells 3.Activate Cytotoxic T-Lymphocytes

8. The Nature of Antigens Antigen (Ag): a substance that causes the body to produce specific antibodies or sensitized T cells “foreign molecules” – Microbial cell wall components, proteins, capsules, flagella, toxins, viral coats – Non-microbial agents such as blood cell surface molecules can serve as antigens Organ rejection – Antibodies (Ab) interact with epitopes, or antigenic determinants which are specific regions on the antigen – Hapten: antigens that are too small to stimulate a immune response. Require carrier molecules

9. Binding sites Epitopes (antigenic determinants) on antigen Antibody A Antigens: components of cell wall Antibody B Bacterial cell Figure 17.1 Epitopes (antigenic determinants).

10. Hapten molecules Carrier molecule Hapten-carrier conjugate Figure 17.2 Haptens.

11. The Nature of Antibodies Globular proteins called Immunoglobulins Antigen-binding sites (ABS) are parts of the antibody that binds the epitope Valence refers to the number of antigen binding sites on an antibody – Two binding sites are bivalent (most common) – One binding site is monovalent

12. Figure 17.3ab The structure of a typical antibody molecule. Antigen- binding site Heavy chain Light chain Hinge region Antibody molecule Enlarged antigen-binding site bound to an epitope Fc (stem) region Epitope (antigenic determinant) Antigen Antigen- binding site

13. IgG Antibodies Monomer 80% of serum antibodies Fixes complement Found in blood, lymph, and intestine Cross placenta (passive immunity) Enhance phagocytosis; neutralize toxins and viruses; protect fetus and newborn Half-life = 23 days

14. IgM Antibodies Pentamer 5–10% of serum antibodies Fixes complement In blood, in lymph, and on B cells Agglutinate microbes; first Ab produced in response to infection Half-life = 5 days Disulfide Bond J-Chain

15. IgA Antibodies Dimer 10–15% of serum antibodies In secretions Mucosal protection Secretory Component protects Ab from degradation Most abundant antibody in the body Half-life = 6 days J-Chain Secretory Component

16. Monomer 0.2% of serum antibodies In blood, in lymph, and on B cells On B cells, initiate adaptive response Half-life = 3 days IgD Antibodies

17. Monomer 0.002% of serum antibodies On mast cells, on basophils, and in blood Allergic reactions; lysis of parasitic worms Attracts complement and phagocytic cells Half-life = 2 days IgE Antibodies

18. B-cells, when activated, become either plasma cells (Ab producers) or memory cells (responsible for enhanced secondary response) B-cells are capable of producing multiple Abs (IgM, IgG, IgA) with the same antigenic determinants (class switching) Major histocompatibility complex (MHC) expressed on mammalian cells are responsible for antigenic determinants – Molecules containing glycoproteins – Found on Antigen-presenting cells (APC) and lymphocytes – Presents antigen on the surface of the B cell Activation of B Cells

19. T-dependent antigens – Ag presented with (self) MHC to T H cell Distinguishes (self) from antigen to prevent antibody production against host cells – Lupus, Type I diabetes, and rheumatoid arthritis – T H cell produces cytokines that activate the B cell T-independent antigens – Antigens stimulate B cells directly to make Abs – Antigens are characterized by repeating subunits that bind multiple B cell receptors Polysaccharides Lipopolysaccharides

20. Figure 17.4 Activation of B cells to produce antibodies. Extracellular antigens Ag fragment MHC class II with Ag fragment B cell Immunoglobulin receptors coating B cell surface B cell Immunoglobulin receptors on B cell surface recognize and attach to antigen, which is then internalized and processed. Within the B cell a fragment of the antigen combines with MHC class II. MHC class II–antigen- fragment complex is displayed on B cell surface. Receptor on the T helper cell (T H ) recognizes complex of MHC class II and antigen fragment and is activated— producing cytokines, which activate the B cell. The T H cell has been previously activated by an antigen displayed on a dendritic cell (see Figure 17.10). B cell is activated by cytokines and begins clonal expansion. Some of the progeny become antibody- producing plasma cells. MHC class II with Ag fragment displayed on surface T H cell Cytokines Plasma cell Antibodies

21. Figure 17.6 T-independent antigens. Polysaccharide (T-independent antigen) Epitopes B cell receptors

22. Figure 17.5 Clonal selection and differentiation of B cells. Stem cell B cells Antigen Some B cells proliferate into long-lived memory cells, which at a later date can be stimulated to become antibody- producing plasma cells. See Figure 17.17. Memory cells B cell III complexes with its specific antigen and proliferates. Other B cells proliferate into antibody-producing plasma cells. Plasma cells Plasma cells secrete antibodies into circulation. Antigens in circulation now attached to circulating antibodies Cardiovascular system I II IIIIV Stem cells differentiate into mature B cells, each bearing surface immunoglobulins against a specific antigen.

23. Activation of B Cells Clonal Deletion eliminates harmful B cells – MHC molecules that contain self-antigens – Process in which the immune system does not attack an antigen is called immune tolerance Gestational immune tolerance protects a baby from the immune system of the parent – Produces cytokines to inhibit immune system detection (neurokinin B) – Do not contain receptors that bind cytotoxic Tcells

24. Antigen-Antibody Binding Affinity—is the strength of a bond b/w antigen and antibody – Antibodies recognize the shape of an antigen – Better the fit b/w the antigen’s shape and Ab the stronger the affinity – Abs are highly specific (discern minor differences in antigens)

25. Results of Antigen–Antibody Binding 1.Agglutination 2.Opsonization 3.Activation of complement 4.Antibody-dependent cell-mediated cytotoxicity 5.Neutralization

26. Figure 17.7 The results of antigen–antibody binding. PROCTECTIVE MECHANISM OF BINDING ANTIBODIES TO ANTIGENS Activation of complement (see also Figure 16.9) Agglutination (see also Figure 18.5) Opsonization (see also Figure 16.9) Antibody-dependent cell-mediated cytotoxicity (see also Figure 17.16) Neutralization (see also Figure 18.9) Reduces number of infectious units to be dealt with Coating antigen with antibody enhances phagocytosis Phagocyte Bacteria Causes inflammation and cell lysis Bacterium Lysis Antibodies attached to target cell cause destruction by macrophages, eosinophils, and NK cells Epitopes Eosinophil Perforin and lytic enzymes Large target cell (parasite) Blocks adhesion of bacteria and viruses to mucosa Bacterium Virus Blocks attachment of toxin Toxin Complement

27. T Cells and Cellular Immunity T cells are derived from stem cells of bone marrow These stem cells migrate to the thymus where they mature into T cells – Thymic selection eliminates many immature T cells Similar to clonal deletion of B cells. Weed out cells that would otherwise attack “self” Mainly responsible for clearance of intracellular bacteria

28. Table 17.2 Principal Cells That Function in Cell-Mediated Immunity

29. T Cells and Cellular Immunity T cells require antigen-presenting cells (APCs) to become activated T cells respond to Ag by T-cell receptors (TCRs) – Similar to antibodies on the surface of B cells Pathogens entering the gastrointestinal or respiratory tracts pass through: – M (microfold) cells over – Peyer’s patches (secondary lymphoid organs), which contain APCs T cells are characterized by glycoproteins on surface – CD4—bind to MHC II molecules (found on lymphocytes) – CD8—bind to MHC I molecules (present on all nucleated cells)

30. Figure 17.9 M cells. Antigen M cell Microvilli on epithelial cell T H cell Pocket B cells Macrophage Epithelial cell (b) M cells facilitate contact between the antigens passing through the intestinal tract and cells of the body’s immune system. M cell on Peyer’s patch. Note the tips of the closely packed microvilli on the surrounding epithelial cells. (a )

31. T Cell Classification Certain glycoproteins on the surface of T cells help distinguish different classes of T cells – Clusters of Differentiation (CD) – Responsible for the adhesion of T cells to receptors – Cells containing CD4 are called CD4 + cells and cells containing CD8 are called CD8 + cells

32. T Helper Cells CD4 + or T H cells – TCRs recognize Ags and MHC II on APC – TLRs are a costimulatory signal on APC and T H – T H cells produce cytokines and differentiate into: T H 1cells T H 2 cells T H 17 cells T FH cells Memory cells

33. T Helper Cells – T H 1 produce IFN-  which activates cells related to cell-mediated immunity, macrophages, and Abs – T H 2 activate eosinophils and B cells to produce IgE – T H 17 stimulate the innate immune system – T FH stimulate B cells to produce plasma cells and are involved in class switching

34. Figure 17.11 Lineage of effector T helper cell classes and pathogens targeted. Antibodies B cell Recruits neutrophils; provides protection against extracellular bacteria and fungi Extracellular bacteria Fungi T H 1 cells T H 2 cells T H 17 cells IL-17 IL-4 IFN-  Cell-mediated immunity; control of intracellular pathogens, delayed hypersensitivity reactions (page 535); stimulates macrophages. Important in allergic responses, especially by production of IgE Stimulates activity of eosinophils to control extracellular parasites such as helminths (see ADCC, page 495). Neutrophil Mast cell Basophil Eosinophil Macrophage T H 1 cells T H 2 cells T H 17 cells Intracellular bacteria and protozoa T H cell Helminth

35. Figure 17.10 Activation of CD4 + T helper cells. APC (dendritic cell) Antigen Costimulatory molecule, (required to activate T cells that have not previously encountered antigen) Microorganism carrying antigens Antigen fragment (short peptides) T H cell receptor (TCR) Complex of MHC class II molecule and antigen fragment A receptor (TCR) on the surface of the CD4 + T helper cell (T H cell) binds to the MHC–antigen complex. If this includes a Toll-like receptor, the APC is stimulated to secrete a costimulatory molecule. These two signals activate the T H cell, which produces cytokines. The cytokines cause the T H cell (which recognizes a dendritic cell that is producing costimulatory molecules) to become activated. Cytokines T helper cell An APC encounters and ingests a microorganism. The antigen is enzymatically processed into short peptides, which combine with MHC class II molecules and are displayed on the surface of the APC.

36. T Cytotoxic Cells CD8 + or T C cells Target cells are self-cells carrying endogenous antigens Activated into cytotoxic T lymphocytes (CTLs) – CTLs recognize Ag + MHC I – Induce apoptosis in target cell CTL releases perforin and granzymes

37. Figure 17.12 Killing of virus-infected target cell by cytotoxic T lymphocyte. Processed antigen MHC class I Virus-infected cell (example of endogenous antigen) Virus-infected cellCytotoxic T lymphocyte (CTL) T cell receptors Infected target cell is lysed A normal cell will not trigger a response by a cytotoxic T lymphocyte (CTL), but a virus-infected cell (shown here) or a cancer cell produces abnormal endogenous antigens. The abnormal antigen is presented on the cell surface in association with MHC class I molecules. CD8 + T cells with receptors for the antigen are transformed into CTLs. The CTL induces destruction of the virus-infected cell by apoptosis. Processed antigen presented with MHC class I CTL

38. Figure 17.13 Apoptosis.

39. T Regulatory Cells T reg cells  CD4 and CD25 on surface Suppress T cells against self

40. Antigen-Presenting Cells Digest antigen Ag fragments on APC surface with MHC – B cells – Dendritic Cells – Activated Macrophages

41. Figure 17.14 A dendritic cell.

42. Figure 17.15 Activated macrophages. Activated macrophages Resting (inactive) macrophage

43. Granular leukocytes destroy cells that don’t express MHC I Kill virus-infected and tumor cells Attacks parasites Natural Killer (NK) Cells

44. ADCC Antibody-dependent cell-mediated cytotoxicity

45. Figure 17.16a Antibody-dependent cell-mediated cytotoxicity (ADCC). Organisms, such as many parasites, that are too large for ingestion by phagocytic cells must be attacked externally. Cytotoxic cytokines Lytic enzymes Perforin enzymes KEY Macrophage Fc region Large parasite Eosinophil Extracellular damage Epitope Antibody (a)

46. Figure 17.16b Antibody-dependent cell-mediated cytotoxicity (ADCC). Eosinophils adhering to the larval stage of a parasitic fluke. Fluke Eosinophils

47. Cytokines Chemical messengers – Allows for communication between cells Overproduction leads to cytokine storm

48. Cytokines CytokineRepresentative Activity Interleukin-1 (IL-1)Stimulates T H cells in presence of antigens; attracts phagocytes Interleukin-2 (IL-2)Proliferation of antigen-stimulated CD4 + T helper cells, proliferation and differentiation of B cells; activation of CD8 + T cells and NK cells Interleukin-12 (IL-12)Inhibits humoral immunity; activates T H 1 cellular immunity

49. Cytokines CytokineRepresentative Activity ChemokinesInduce the migration of leukocytes TNF-αPromotes inflammation Hematopoietic cytokines Influence differentiation of blood stem cells IFN-  and IFN-  Response to viral infection; interfere with protein synthesis IFN-  Stimulates macrophage activity

50. Immunological Memory Antibody titer is the amount of Ab in serum Primary response occurs after initial contact with Ag Secondary (memory) response occurs after second exposure

51. Figure 17.17 The primary and secondary immune responses to an antigen. IgG IgM Initial exposure to antigen Second exposure to antigen Time (days) Antibody titer in serum

52. Types of Adaptive Immunity Naturally acquired active immunity – Resulting from infection Naturally acquired passive immunity – Transplacental or via colostrum Artificially acquired active immunity – Injection of Ag (vaccination) Artificially acquired passive immunity – Injection of Ab

53. Terminology of Adaptive Immunity Serology: the study of reactions between antibodies and antigens Antiserum: the generic term for serum because it contains Ab Globulins: serum proteins Immunoglobulins: antibodies Gamma (  ) globulin: serum fraction containing Ab