1. COURSE: PHARMACEUTICAL MICROBIOLOGY II COURSE CODE: 203 COURSE TEACHER: IMON RAHMAN IMMUNOLOGY

2. IMMUNITY Protection of host cells from foreign organisms is called immunity. In response to foreign substances known as antigens, proteins called antibodies are produced that neutralize or destroy the antigens.

3. TYPES OF IMMUNTIY Immunity │ ___________________ │ Innate Acquired (nonspecific (specific immunity) immunity)

4. CLASSIFICATION Innate (nonspecific) immunity : Genetically predetermined resistance to certain diseases. Individual resistance is affected by gender, age, nutritional status and general health. Acquired immunity : Specific resistance to infection developed during the life of an individual.

5. Types of acquired immunity

6. Duality of the immune system: Humoral immunity (antibody mediated immunity) - It is in body fluids. - Involves antibodies produced by B-cells (B- lymphocytes) in response to a specific antigen. - These antibodies defends against bacteria, viruses and toxins found in extracellular fluids such as blood plasma, lymph and mucus secretions. Cell mediated immunity : - depends on T-cells and does not involve antibody production - Cellular immunity is primarily a response to intracellular bacteria and viruses, multicellular parasites, transplanted tissues and cancer cells.

7. Antigens Proteins or large polysaccharides Components of invading microbes such as capsules, cell walls, flagella, fimbriae, toxins of bacteria, coats of viruses or surface of other types of microbes. Molecular weight 10,000 or higher Epitopes – Generally antibodies recognize and interact with specific regions on antigens called antigenic determinants or epitopes. (refer to figure 17.3, pg- 515, Tortora).

8. Haptens : Foreign substance that has a low molecular weight and is often not antigenic unless attached to a carrier molecule usually a serum protein. Hapten + carrier molecule ------ › complete antigen A good example of hapten is Penicillin. (refer to Fig 17.4, page – 515, Tortora).

9. Antibodies Proteins (immunoglobulins) that are made in response to antigen and can recognize and bind to that antigen They neutralize or destroy the antigens Each antibody has two identical sites known as antigen-binding sites that bind to antigenic determinants on the antigen. Number of antigen-binding sites on an antibody is called valence. Example : Most human antibodies have tow binding sites so they are bivalent.

10. Structure of an antibody

11. Bivalent antibody has the simplest molecular structure and so is called a monomer. Monomer has 4 protein chains : - two identical light chains (L) - two identical heavy chains (H) Light and heavy refer to the relative molecular weights. The chains are joined by disulphide links and other bonds to form a Y-shaped molecule.

12. The two sections at the end of the Y’s arms are called variable (V) regions. The amino acid sequences and the three dimensional structure of these two regions are identical. These are the two antigen binding sites found on each antibody monomer. The stem of the antibody monomer and the lower parts of the Y’s arms are called constant (C) regions. These are the 5 major types of C regions that accounts for 5 major classes of immunoglobulins.

13. Summary of Immunoglobulin Classes

14. Differentiation of B-cells and T-cells Both B-cells and T-cells originate from the stem cells in adult red bone marrow or in the fetal liver. Some cells pass through the thymus and emerge as mature T-cells. Others probably remain in the bone marrow and become B-cells and then migrate to lymphoid organs such as lymph nodes or spleen. B cells recognizes antigens by means of antigen receptors found on their cell surfaces. When B-cells are exposed to antigens, they become activated and divides and differentiates into a clone of many effector cells called plasma cells that produce antibodies against the specific antigen.

15. Apoptosis Lymphocytes that are not needed undergo apoptosis or programmed cell death and are destroyed by phagocytes. If apoptosis does not occur then it would lead to proliferation of lymphocytes resulting in a disease known as leukemia.

16. Clonal Selection Each B cell can produce antibodies against only one specific antigen. When the appropriate antigen binds to the antigen receptors on the B-cell, the B-cell proliferates into a large clone of cells with the same immunological specificity, a phenomenon known as clonal selection.

17. Clonal selection and differentiation of B-cells

18. Activation of antibody-producing cells by clonal selection According to the clonal selection theory, a B cell becomes activated when an antigen reacts with antigen receptors on its surface. The activated B cells produces a clone of plasma cells and memory cells. Plasma cells secrete antibodies. Long- lived Memory cells recognize pathogen from previous encounters. T and B cells that react with self antigens are destroyed during fetal development; this is called clonal deletion.

19. Antigen-Antibody Binding + Results An antigen binds to the antigen-binding site of an antibody to form an antigen-antibody complex. This tags foreign cells and molecules for destruction by phagocytes and complement. Results of antigen-antibody binding : - Agglutination - Opsonization - Neutralization - Antibody-dependant cell-mediated toxicity - Action of complement causing cell lysis and inflammation

20. Results of antigen-antibody binding

21. IMMUNOLOGICAL MEMORY The amount of antibody in serum is called the antibody titer. The response of the body to the first contact with an antigen is called the primary response. It is characterized by the appearance of IgM followed by IgG. Subsequent contact with the same antigen results in a very high antibody titer and is called the secondary, anamnestic or memory response. The antibodies are primarily IgG.

22. MONOCLONAL ANTIBODIES + THEIR USES + PRODUCTION Hybridomas are produced in the laboratory by fusing a cancerous cell with an antibody-secreting plasma cell. A hybridoma cell culture produces large quantities of the plasma cell’s antibody called monoclonal antibodies (Mab). Mab are useful for 3 reasons: they are uniform, highly specific and can be produced readily in large quantities. - For these reasons they can be used in serologic identification tests. They can be used to prevent tissue rejections and to make immunotoxins to treat cancer. Immunotoxins can be made by combining a monoclonal antibody and a toxin. The toxin will then kill a specific antigen. Production (Refer to pg 524, Fig. 17.11 Tortora).

23. CYTOKINES Cells of the immune system communicate with each other by means of chemicals called cytokines. Interleukins (IL) are cytokines that serve as communicators between leukocytes. Interferons are cytokines that protect cells against viruses. Chemokines are cytokines that cause leukocytes to move to the site of infection. Tumour necrosis factor (TNF) are cytokines that are important in inflammatory reactions. Colony stimulating factor (CSF) are cytokines stimulate the formation of various blood cells. Table 17.2, Pg- 526 Tortora.

25. CELLULAR COMPONENTS OF IMMUNITY T cells are responsible for cell – mediated immunity. After differentiation in the thymus gland, T cells migrate to lymphoid tissue. T cells differentiate into effector T cells when they are stimulated by an antigen. Some effector T cells become memory cells.

26. TYPES OF T CELLS According to their function: Helper T (Th) cells Cytotoxic T (Tc) cells Delayed hypersensitivity T (Td) cells Suppresor T (Ts) cells According to cell-surface receptors (CD): CD 4 cells (mainly Th cells & long lived memory cells) CD 8 cells (include both cytotoxic and Ts cells).

27. Antigen presenting cells Individual T cells interact specifically with only a single antigen that must be displayed on a cell surface. These cells are known as antigen-presenting cells (APCs). The primary APCs are macrophages or dendritic cells. Dendritic cells have no function. APC ingests and processes the antigen. It then displays fragments of the antigen, each of about 10 amino acids on the cell surface.

28. Antigen presenting cells (contd) A T–cell will recognize an antigenic fragment on APC only if it is in close association with certain cell surface self molecules. These self molecules are components of the major histocompatibility complex (MHC). The particular MHC proteins a person carries are unique to that individual. Thus they serve as signals by which the immune system distinguishes self from nonself.

29. Types of Helper T (Th) cells Th1 cells produce cytokines that mostly activate cells related to cell-mediated immunity. Among these are macrophages, CD8 T cells and natural killer cells. Th2 cells produce cytokines that cause B cells to begin producing eosionophils that are protective against certain parasites, IgM and IgE, an important factor in allergic reactions.

30. Central Role of Helper T cells

31. Cytotoxic T (Tc) cells Cytotoxic T (Tc) cells destroy target cells on contact. Cytotoxic T (Tc) or CD8 cells release perforin to lyse cells carrying the target antigen and MHC.

32. Cell – mediated cytotoxicity

33. Delayed Hypersensitivity T (Td) cells are associated with certain types of allergic reactions and transplant rejection. Suppresor T (Ts) appear to regulate the immune response. Nonspecific Cellular components : Activated Macrophages Natural Killer cells Table 17.3 Pg 530 Tortora

34. Nonspecific cellular components Activated macrophages : Macrophages that are stimulated by ingesting an antigen or by cytokines become activated to have enhanced phagocytic activity. Natural killer (NK) cells lyse virus-infected and tumour cells. They are not T cells and are not antigenically specific.

36. Interrelationship of Cell-mediated and Humoral Immunity The production of antibodies against certain antigens requires the assistance of helper T cells and so this type of antigen is therefore known as a T-dependent antigen. T-dependent antigens are mainly proteins such as those found on bacteria and viruses, foreign red blood cells and combinations of haptens and carrier molecules. Antigens that can stimulate B cells directly without the help of T cells, usually composed of polysaccharides or lipopolysaccharides, characterized by repeating subunits that can crosslink several antigen receptors on the same B cell are called T-independent antigens. Example : bacterial capsules.

37. T- Independent Antigen

38. How helper T cells may activate B cells to make antibodies against T-dependent antigens ?

39. Antibody-Dependant Cell- Mediated Cytotoxicity (ADCC) With the help of antibodies produced by the humoral immune system, the cell-mediated immune system can stimulate NK cells, macrophages and other leukocytes to kill targeted cells. In this way, an organism, such as a protozoan or helminth, that is too large for ingestion and destruction by phagocytosis can be attacked by immune system cells that remain external to it. This process is refereed to as antibody-dependant cell-mediated cytotoxicity (ADCC).

40. Antibody-Dependant Cell-Mediated Cytotoxicity (ADCC). In this process The target cell (parasite) must first be coated with antibodies, leaving their Fc (Stem) regions pointing outward. Immune system cells such as NK cells, macrophages, neutrophils and eosinophils have receptors that will bind to these protruding Fc regions and thus to the target cell. The target cell is then lysed by substances such as cytokines, perforin and other lytic enzymes secreted by the attacking cells that destroy the parasite. By this process the immune systems successfully attack many relatively large organisms such as the various life-cycle stages of parasitic helminths.

41. Antibody-Dependant Cell- Mediated Cytotoxicity (ADCC)

42. Duality of the Immune System

43. Pathogens can come into contact with antibodies only when they circulate in the blood or spaces outside cells. However, viruses and some bacterial pathogens and parasites reproduce only inside living cells where circulating antibodies cannot reach them. Elimination of these intracellular pathogens requires cell mediated immune responses that depend on T cells, especially cytotoxic T cells.