1. Specific Immune Responses How the Immune System Handles Specific Antigens

2. Specific Immunity Involves T and/or B Lymphocytes. B cells produce antibodies – proteins that bind to antigens. Antibodies circulate freely in the blood – Immunoglobulins (Ig). T cells contain receptors for antigens.

4. Specific Immunity Each antibody & T cell recognizes a different antigen. Small part of an antigen – epitope. A given antibody can only bind to one part of a particular virus or a particular bacterium (think lock and key).

6. Humoral Immunity Involves antibodies. B cells produce antibodies. Targets bacterial cells, bacterial toxins, freely circulating viruses, allergens.

7. Cell-Mediated Immunity Involves T cells. They recognize our tissues that have gone awry or parasitic eukaryotes. Host cells infected with bacteria or viruses, cancerous cells, transplanted tissues, fungi, protists, worms.

8. Antigens Foreign substances that provoke an immune response. Usually protein or polysaccharide. Can be bacterial capsules, cell walls, flagella, secreted toxins, etc. Can be viral protein coat. Can be pollen, surface of cells in transplanted tissues/fluids, components of animal saliva, oils of plants, antibiotics, etc.

9. Antibodies Structure: 4 protein chains – 2 heavy and 2 light. Constant region & variable region. Variable regions bind to antigen. Each B cell produces antibodies with a unique variable region. There are 5 types of constant regions. Complement proteins & some WBCs can bind to constant regions of antibodies that are bound to antigen.

12. Antibody (Ig) Classes IgG: Most prevalent Ig in blood Can leave blood and enter tissues; maternal IgG can cross placenta Target circulating virus, bacteria, bacterial toxins; stimulate complement and phagocytosis Long-lived (can indicate past exposure)

13. Antibody (Ig) Classes IgM: Usually a complex of 5 antibodies held together by J polypeptide Tend to remain in blood; can be found on surface of B cells Responds to ABO blood antigens (transfusion) Stimulates the aggregation of antigen Part of early response; short-lived

14. IgM

15. Antibody (Ig) Classes IgA: Most common Ig in body as a whole Found in mucous membranes, mucus, tears, saliva, breast milk (esp. colostrum), other secretions Short-lived Most effective as a dimer Fights off infections in respiratory system, GU and GI tracts

16. IgA

17. Antibody (Ig) Classes IgD: Circulate, but unknown function in blood Also found on surfaces of B cells

18. Antibody (Ig) Classes IgE: Constant region binds to mast cells and basophils Stimulate release of histamine when bound to antigen Antigen binding also stimulates the recruitment of phagocytes

20. Outcomes Agglutination – clumping of antigen- antibody complexes aids phagocytes; IgM is especially effective. Opsonization – coating of antigen with antibody also eases phagocytosis. Neutralization – antibody may bind to key epitope, such as the part of the virus that binds to a host cell (interference).

21. Outcomes Trigger complement – complement proteins quickly recognize most antigen- antibody complexes; lysis and/or inflammation can result.

22. B Cell Selection Enormous number of B cells are needed to protect against an infinite number of different antigens. During infection many copies (clones) of a particular B cell is needed. The body cannot keep too many of any particular B cell on hand at any one time. Instead, proliferation of specific B cells occurs in response to exposure to its matching antigen.

24. B cell Selection B cells tend to remain in lymph nodes and spleen – they wait for antigen to come to them. IgM and IgD act as antigen receptors on B cell surfaces. Proliferation of the B cell is stimulated by IgM or IgD binding to antigen.

25. B cell Differentiation Clonal selection – a particular clone is selected to proliferate by the antigen. Clones that actively produce antibody are known as plasma cells. A small number of clones become memory cells. Memory cells are stored for future use.

27. Immunological Memory Memory cells can persist in the lymph nodes and spleen for decades. Only a small number is needed of each memory cell. Memory cells respond quickly and strongly to secondary infections. IgG production is especially high during secondary infections.

29. Cell-Mediated Immunity Mainly responds to intracellular antigens. Involves a wide array of chemical signals, known as cytokines. Cytokines can either stimulate an immune response or inhibit one.

30. Cytokines Interleukins (IL) carry signals from one WBC to another. Chemokines stimulate the movemnet of WBCs to a site of infection. Interferons stimulate anti-viral activity. Tumor Necrosis Factor (TNF). Colony Stimulating Factor (CSF).

32. T cell Proliferation T cells are selected in the same way as B cells. Some T cells are designated as memory cells.

33. T Cells Helper T cells (T H ), Cytotoxic T cells (T C ), Delayed Hypersensitivity T cells (T D ), Suppressor T cells (T S ). CD receptors can also define T cells: CD4 (helper, memory) and CD8 (cytotoxic, suppressor).

34. T Cells Each T cell is specific for a specific antigen/epitope. The epitope must be presented on the surface of an antigen-presenting cell (APC) – macrophages (or dendritic cells). Epitope may be on surface of infected cell.

35. Antigen-Presenting Cells Ingest antigen by phagocytosis. Digest proteins into small peptides. Display these peptides along with the major histocompatibility complex (MHC) on their surface. T Cells Receptors (TCR) – recognize MHC-antigen complexes.

37. T H Cells Secrete cytokines when stimulated by antigen. These cytokines stimulate other immune cells. Cytokines from T H 1 cells target macrophages, CD8 T cells, natural killer cells. Cytokines from T H 2 cells target B cells.

40. T H Cells T H Cell binding to APC causes APC to secrete IL-1. IL-1 stimulates T H Cell to secrete IL-2. IL-2 binds to receptors on that same T H Cell. T H Cell reproduces – specific because only those bound to antigen have IL-2 receptors (environmental control of gene expression). T H Cell clones produce cytokines to recruit more T cells and B cells.

42. T C Cells T C Cells also bind to MHC-antigen complex - on surface of target cells. Binding stimulates the T C Cells to secrete perforin, a protein which forms pores in the target cell’s membrane. These pores induce the lysis of the target cell.

44. B and T Cells Work Together B cells can work alone when the antigen is LPS or capsule polysaccharides. More often, though, T H cells bound to APC release IL-2 to recruit B cells. The recruited B cells develop into plasma cells and make antibodies against the antigen.