Chapter 21: Immune System

1. Chapter 21: Immune System

2. INTRODUCTION The immune system protects against assaults on the body External assaults :include microorganisms: protozoans, bacteria, and viruses Internal assaults: abnormal cells reproduce and form tumors that may become cancerous and spread

3. ORGANIZATION OF THE IMMUNE SYSTEM The immune system is continually at work patrolling and protecting the body Identification of cells and other particles Markers or antigens are unique molecules recognized by the immune system Self-markers: molecules on the surface of cells that are unique to an individual, thus identifying the cell as “self” to the immune system Non–self-markers: molecules on the surface of foreign or abnormal cells or particles that identify the particle as “non-self” to the immune system Self-tolerance: the ability of the immune system to attack abnormal or foreign cells but spare normal cells

4. ORGANIZATION OF THE IMMUNE SYSTEM (cont.) Two major categories of immune mechanisms: innate immunity and adaptive immunity Innate immunity provides a general, nonspecific defense against anything that is not “self” epithelial barrier cells, phagocytes (neutrophils, macrophages, dendritic cells), and natural killer cells Adaptive immunity acts as a specific defense against specific threatening agents Primary types of cells for adaptive immunity: lymphocytes called T cells and B cells

6. INNATE IMMUNITY Mechanical and chemical barriers: first line of defense Species resistance: genetic characteristics of an organism or species that defend against pathogens Internal environment of the body is protected by a barrier composed of the skin and mucous membranes Skin and mucous membranes provide additional immune mechanisms: sebum, mucus, enzymes, and hydrochloric acid in the stomach

7. INNATE IMMUNITY (cont.) Inflammation and fever: second line of defense Inflammatory response: tissue damage elicits responses to counteract injury and promote normalcy Inflammation mediators: include histamine, kinins, prostaglandins, and related compounds Characteristic signs of inflammation: heat, redness, pain, and swelling Systemic inflammation: occurs from a bodywide inflammatory response

8. INNATE IMMUNITY (cont.) Fever: abnormally high temperature triggered by inflammation mediators Triggered in systemic inflammatory response syndrome and events such as viral infections, tumors, allergies Fever is believed to increase immune function and inhibit pathogens Phagocytosis: ingestion and destruction of microorganisms or other small particles by phagocytes Antigen-presenting cells: phagocytes that ingest foreign particles, isolate protein segments (peptides), and display them as antigens on their surface to trigger an immune response when recognized by a specific (adaptive) immune cell

9. INNATE IMMUNITY (cont.) Neutrophil: most numerous phagocyte; usually first to arrive at site of injury; migrates out of bloodstream during diapedesis; forms pus Phagocytes Macrophage: large phagocytic cells Dendritic cell: type of phagocytic antigen-presenting cell with long branches or extensions

10. INNATE IMMUNITY (cont.) Natural killer (NK) cells: lymphocytes that kill tumor cells and cells infected by viruses Method of recognizing abnormal or non-self cells: target cell is killed if killer-inhibiting receptor on NK cell does not bind to a proper major histocompatibility complex surface protein Method of killing cells: lysing cells by damaging plasma membranes Interferon: protein synthesized and released into circulation by certain cells if invaded by viruses to signal other nearby cells to enter a protective antiviral state

11. INNATE IMMUNITY (cont.) Complement: group of enzymes that produce a cascade of reactions resulting in a variety of immune responses Lyse cells when activated by either adaptive or innate mechanisms Opsonization: mark cells for destruction by phagocytes Variety of other immune responses Toll-like receptors: pattern recognition receptors in the membranes of host cells; when triggered, stimulate many different kinds of innate immune responses

13. OVERVIEW OF ADAPTIVE IMMUNITY Adaptive immunity is part of the third line of defense consisting of lymphocytes—two different classes of a type of white blood cell Two classes of lymphocytes B lymphocytes (B cells) and T lymphocytes (T cells) Lymphocytes flow through the bloodstream, become distributed in tissues, and return to the bloodstream in a continuous recirculation

14. OVERVIEW OF ADAPTIVE IMMUNITY Light Chain Heavy Chain

16. OVERVIEW OF ADAPTIVE IMMUNITY (cont.) B-cell mechanisms: antibody-mediated immunity (humoral immunity); produce antibodies that attack pathogens T cells attack pathogens more directly; classified as cell-mediated immunity (cellular immunity) Activation of lymphocytes requires two stimuli: a specific antigen and activating chemicals

18. B CELLS AND ANTIBODY-MEDIATED IMMUNITY (cont.) Antibody molecules produce antibody-mediated immunity (humoral immunity) within plasma Antibodies resist disease first by recognizing foreign or abnormal substances Epitopes bind to an antibody molecule’s antigen-binding sites, which forms an antigen-antibody complex that may have several effects

19. B CELLS AND ANTIBODY-MEDIATED IMMUNITY (cont.)

20. B CELLS AND ANTIBODY-MEDIATED IMMUNITY (cont.) Primary and secondary responses Primary response: initial encounter with a specific antigen triggers the formation and release of specific antibodies that reaches its peak in a few days Secondary response: a later encounter with the same antigen triggers a much quicker response; B memory cells rapidly divide, producing more plasma cells and thus more antibodies

22. T CELLS AND CELL-MEDIATED IMMUNITY T cells: lymphocytes that go through the thymus gland before migrating to the lymph nodes and spleen Pre–T cells develop into thymocytes while in the thymus Thymocytes stream into the blood and are carried to the T cell–dependent zones in the spleen and the lymph nodes

23. T CELLS AND CELL-MEDIATED IMMUNITY Activation of T cells T cells display antigen receptors on their surface membranes that are similar to antibodies A T cell is activated when an antigen (presented by an antigen-presenting complex) binds to its receptors (at an immunological synapse), causing the T cell to divide repeatedly to form a clone of identical T cells Cells of the clone differentiate into effector T cells and memory T cells Effector T cells go to the site where the antigen entered, bind to antigens, and begin their attack

25. TYPES OF ADAPTIVE IMMUNITY Innate immunity (inborn or inherited immunity): genetic mechanisms put innate immune mechanisms in place during development in the womb Adaptive or acquired immunity: resistance developed after birth; two types: Natural immunity results from nondeliberate exposure to antigens Artificial immunity results from deliberate exposure to antigens, called immunization

26. SUMMARY OF ADAPTIVE IMMUNITY Adaptive immunity is specific immunity targeting specific antigens Adaptive immunity involves two classes of lymphocyte: B cells and T cells B cells: antibody-mediated (humoral) immunity T cells: cell-mediated (cellular) immunity Adaptive immunity occurs in a series of stages Recognition of antigen Activation of lymphocytes Effector phase (immune attack) Decline of antigen causes lymphocyte death (homeostatic balance) Memory cells remain for later response if needed B cells and T cells work together in a coordinated system of adaptive immunity

28. THE BIG PICTURE: IMMUNE SYSTEM AND THE WHOLE BODY Immune system regulated to some degree by nervous and endocrine systems Agents of the immune system include blood cells, skin cells, mucosal cells, brain cells, liver cells, and other types of cells and their secretions