CHAPTER 24 The Immune System Modules 24.10 – 24.17

24.10 Antibodies are the weapons of humoral immunity An antibody molecule Figure 24.10A

An antibody molecule has antigen-binding sites specific to the antigenic determinants that elicited its secretion Antigen-binding sites Light chain Heavy chain Figure 24.10B

24.11 Antibodies mark antigens for elimination Antibodies may block harmful antigens on microbes clump bacteria or viruses together precipitate dissolved antigens activate complement proteins

Binding of antibodies to antigens inactivates antigens by Neutralization (blocks viral binding sites; coats bacterial toxins) Agglutination of microbes Precipitation of dissolved antigens Activation of complement Complement molecule Bacteria Virus Antigen molecules Bacterium Foreign cell Hole Enhances Leads to Phagocytosis Cell lysis Macrophage Figure 24.11

producing monoclonal antibodies 24.12 Connection: Monoclonal antibodies are powerful tools in the lab and clinic These molecules are produced by fusing B cells specific for a single antigenic determinant with easy-to-grow tumor cells Antigen injected into mouse Tumor cells grown in culture B cells (from spleen) Tumor cells Cells fused to generate hybrid cells Single hybrid cell grown in culture Antibody Hybrid cell culture, producing monoclonal antibodies Figure 24.12A

These cells are useful in medical diagnosis Example: home pregnancy tests They are also useful in the treatment of certain cancers Figure 24.12B

24.13 T cells mount the cell-mediated defense and aid humoral immunity Helper T cells and cytotoxic T cells are the main effectors of cell-mediated immunity Helper T cells also stimulate the humoral responses

Cell-mediated immunity An antigen-presenting cell (APC) first displays a foreign antigen and one of the body’s own self proteins to a helper T cell Microbe Macrophage (will become APC) 1 Antigen from microbe (nonself molecule) Self protein Self protein displaying antigen T cell receptor Binding site for self protein 3 2 Helper T cell 4 Binding site for antigen APC Figure 24.13A

The helper T cell’s receptors recognize the self-nonself complexes on the APC The interaction activates the helper T cells The helper T cell can then activate cytotoxic T cells with the same receptors Self protein displaying an antigen Cell-mediated immunity (attack on infected cells) Cytotoxic T cell T cell receptor Interleukin-2 stimulates cell division Interleukin-2 activates other T cells and B cells APC Helper T cell Humoral immunity (secretion of antibodies by plasma cells) B cell Interleukin-1 activates helper T cell Figure 24.13B

Cytotoxic T cells bind to infected body cells and destroy them 1 Cytotoxic T cell binds to infected cell 2 Perforin makes holes in infected cell’s membrane 3 Infected cell is destroyed Hole forming Foreign antigen INFECTED CELL Cytotoxic T cell Perforin molecule Figure 24.13C

24.14 Cytotoxic T cells may help prevent cancer Cytotoxic T cells may attack cancer cells The surface molecules of cancer cells are altered by the disease Figure 24.14

24.15 The immune system depends on our molecular fingerprints The immune system normally reacts only against nonself substances It generally rejects transplanted organs The cells of transplanted organs lack the recipient’s unique “fingerprint” of self proteins

DISORDERS OF THE IMMUNE SYSTEM 24.16 Connection: Malfunction or failure of the immune system causes disease Autoimmune diseases The system turns against the body’s own molecules Immunodeficiency diseases Immune components are lacking, and infections recur Physical and emotional stress may weaken the immune system

Allergies are abnormal sensitivities to allergens in the surroundings 24.17 Connection: Allergies are overreactions to certain environmental antigens Allergies are abnormal sensitivities to allergens in the surroundings B cell (plasma cell) Histamine Mast cell Antigenic determinant Antibodies attach to mast cell Allergen (pollen grain) B cells make antibodies Allergen binds to antibodies on mast cell Histamine is released, causing allergy symptoms SENSITIZATION: Initial exposure to allergen LATER EXPOSURE TO SAME ALLERGEN Figure 24.17

24.18 Connection: AIDS leaves the body defenseless The AIDS virus attacks helper T Cells This cripples both cell-mediated and humoral immunity So far, AIDS is incurable Drugs and vaccines offer hope for the future Practicing safer sex could save many lives