Ch. 21 Notes
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Date Created: 02/20/15
BSC 216 Chapter 21 Notes The Lymphatic and Immune Systems 1 The lymphatic system a Three main functions of the lymphatic system i Fluid recovery 1 Fluid continually filters from the blood capillaries into the tissue spaces a Blood capillaries reabsorb 85 b Remaining 15 reabsorbed by lymphatic system ii Immunity 1 Excess filtered fluid picks up foreign cells and chemicals from the tissues a Passes through lymph nodes where immune cells stand guard against foreign matter b Activate a protective immune response iii Lipid absorption 1 Lacteals in small intestine absorb dietary lipids that are not absorbed by the blood capillaries b Bring fluids back home to the blood vascular system c Barracks for phagocytotic cells and lymphocytes immune d Components of the lymphatic system i Lymph fluid similar to blood plasma but LOW in protein content ii Lymphatic vessels iii Lymphoid tissues and organs e Fluid is forced out of blood at arterial end of capillary bed reabsorbed at venous end f Leaked fluid interstitial fluid has to be returned to blood to maintain proper blood volume g Lymphatic vessels collect interstitial fluid h Lymph flows in one direction towards the heart i Lymphatic capillaries associated with capillary beds and loose connective tissue of the body i Hidhlv permeable 1 Endothelial cells overlap forming minivalves 2 Increases in interstitial fluid volume causes minivalves to open fluid enters 3 Decreases in interstitial fluid volume close minivalves prevent backflow j Lymphatic capillaries l lymphatic collecting vessels internal valve prevent backflow l lymph nodes fluid detox destruction of bacteria viruses cancer cells l lymphatic trunks where collecting vessels unite l lymphatic ducts right lymphatic duct right upper arm right side of head and thorax and thoracic duct rest of body k Cisterna chyli Gateway to the thoracic duct Merger of two lumbar trunks intestinal trunk l Chyle fatty lymph absorbed from digestive organs 2 Lymph transport a Lymphatic system is not associated with a muscular pump b Flow of lymph relies upon I ii iii iv v vi Skeletal muscle milking Respiratory pumps changes in pressure of thoracic cavity breathing Valves to prevent backflow Close association with arteries arterial pulses promote lymph transport Smooth muscle in lymphatic trunks and thoracic duct Activity levels speed transport Very similar to venous return of blood to the heart 3 Lymphatic cells a Lymphocytes i i i iv Arise in red bone marrow Natural killer NK cells destroy bacteria and infected host cells Mature T lymphocytes direct attack on infected cells or invaders Mature B lymphocytes produce plasma cells that secrete antibodies against foreign substances antigens b Macrophages Phagocytize foreign substances Activate T lymphocytes c Dendritic cells Capture antigens transport to lymph nodes Activate T lymphocytes d Reticular cells Produce stroma fibrous network that supports other lymphoid cells 4 Lymphatic tissue a Critical for proper functioning of immune system Site for proliferation of lymphocytes Fibrous network provides good surveillance points for immune cells lymphocytes Diffuse lymphatic tissue in all organs consists of scattered reticular tissue iv Mucosaassociated lymphatic tissue MALT diffuse lymphatic tissues in walls of bronchi and mucosa of genital and urinary organs protects these open passageways from infection v Lymphoid follicles nodules concentrations of lymphoid tissue from larger lymphoid organs like lymph nodes germinal centers provide proliferation grounds for dendritic cells and B lymphocytes 1 Peyer patch cluster of lymphoid nodules in wall of small intestine a Clusters of lymphoid follicles also abound in the wall of the appendix tubelike offshoot of large intestine b Trap bacteria abundant in intestine c Facilitate creation of memory lymphocytes for long term immunity 5 Lymphatic organs a Lymphatic organs have welldefined anatomical sites i Have connective tissue capsule that separates the lymphatic tissue from neighboring tissues b Primary lymphatic organs i Red bone marrow and thvmus ii Site where T and B cells become immunocompetent able to recognize and respond to antigens c Secondary lymphatic organs i Lvmbh nodes tonsils and soleen ii Immunocompetent cells populate these tissues 6 Thymus a b Bilobed organ in inferior neck overlies heart i Particularly prominent during childhood i Abundant Tlymphocytes no Blymphocytes iii Main function Tlymphocyte maturation iv Bloodthymus barrier prevents direct contact with antigens v Stroma consists of epithelial cells not reticular fibers Epithelial cells secrete thymopoietin a hormone that enables T lymphocytes to recognize and attack foreign substances or pathogens c Thymic Hassall s corpuscles i Sites of Tlymphocyte destruction ii Produce cytokine TLSP which directs dendritic cell maturation 7 Lymph nodes weeps Lymph nodes rest stops for lymph where fluid filtration occurs Secondary organs of the lymphatic system Clustered around lymphatic vessels Embedded in connective tissue of body Lymph node functions Filtration of microorganisms and debris from loose connective tissue by macrophages prevents delivery to blood and other body parts Immune system activation when antigens are detected by resident lymphocytes attack is initiated f Surrounded by dense fibrous capsule g Trabeculae extend from capsule to partition of the node into compartments h Reticular connective tissue supports lymphoid cells i Cortex outer layer medulla inner layer iv Cortex contains densely packed follicles with germinal centers Germinal centers house proliferating B cells dendritic cells and active T cells surveillance Medullary cords thin inward extensions from cortex contain B and Tlymphocytes plasma cells Lymph sinuses lymph capillaries reticular fibers home to macrophages j Afferent lymphatic vessels lymph enters lymph node I subcapsular sinus l medullary sinus I hilum I efferent lymphatic vessels filtered lymph exits lymph node What is the advantage of having fewer efferent than afferent vessels 8 What happens when bacteria overwhelm lymphoid tissue a Bubonic Plague 1300 s Europe 1990 s India and USA aka Black Death b Buboes swollen lymph nodes caused by tremendous numbers of bacteria trapped in lymph node immune system can t keep up and inflammation 9 Spleen a Located in left side of abdominal cavity beneath diaphragm b Served by splenic artery and splenic vein enterexit through hilum c Functions Site of lymphocyte proliferation immune surveillance immune activation Resident macrophages cleanse blood 1 Removes aging or defective blood cells and platelets debris foreigners Recycles iron from hemoglobin Site of fetal erythrocyte production Stores platelets Capsule and trabecular similar to lymph nodes vii viii Splenic cords reticular tissue lots of macrophages Red pulp disposal of RBC s and pathogens ix White pulp lymphocytes reticular fibers immune function 10Tonsils a Tonsils ring of lymphatic tissue at entrance of throat i Palatine tonsils right and left sides of oral cavity largest of tonsHs ii Lingual tonsil collection of lymphoid follicles at base of tongue iii Pharyngeal tonsils posterior wall of nasopharynx b Follicles with germinal centers surrounded by lymphocytes c Tonsillar crypt invaginated epithelium that traps bacteria and particles d Why go out of your way to trap bacteria 11mmune system a Immunity resistance to disease i 3 lines of defense against disease ii 1St and 2nOI line nonspecific immunity innate mechanisms 1 1St line surface barriers 2 2nd line internal defenses a When surface protection is breached b Inflammation telltale sign of innate defense activation iii 3rd line specific immunity specificity memory 1 The special ops forces of your body 2 Attack specific foreign substances a Slower to mount an adaptive defense b Butmuch more potent 12 Innate defenses 1 surface barriers a Skin i Heavily keratinized epithelial membrane ii Keratin resistant to a host of chemicals even bacterial enzymes and toxins b Mucous membranes i Lining of body cavities open to exterior digestive respiratory urinary reproductive ii Often equipped with structure such as hairs and cilia that trap or mobilize invaders c Secretions by skin and mucous membranes i Acidity pH of 35 inhibits bacterial growth sweat sebum m mantle vaginal secretions ii Direct toxicity to bacteria sebum iii Stomach secretes hydrochloric acid and proteindigesting enzymes kills microorganisms iv Saliva oral cavity and teeth and lacrimal fluid tears contain lysozymes destroys bacteria v Mucous itself traps microorganisms in digestive and respiratory passages 13 Innate defenses 2 when the surface is breached a Cellular and chemical reinforcements called to duty i Cells and chemical defenses associated with innate immunity are nonspecific meaning that they recognize foreign substances but not soecific foreign substances b Phagocytes phagocytic cells with a voracious appetite for foreign matter c Five types of leukocytes involved in innate defenses i Neutrophils i Eosinophils iii Basophils iv Lymphocytes v Monocytes 14 lnnate defenses 2 leukocytes and macrophages a Neutrophils i Wander in connective tissue killing bacteria ii Simple method phagocytosis and digestion iii More complex methods produce a cloud of bactericidal chemicals iv Creates a killing zone when bacteria are detected in close proximity 1 Degranulation lysosomes are released into tissue fluid v Respiratory burst neutrophils rapidly absorb oxygen 1 Toxic chemicals are created 02 H202 HCIO vi Kill more bacteria with toxic chemicals than phagocytosis vii Unfortunately neutrophil is killed as well b Eosinophils i Found especially in the mucous membranes ii Stand guard against parasites allergens allergycausing agents and other pathogens iii Kill tapeworms and roundworms by producing superoxide hydrogen peroxide and toxic proteins iv Promote action of basoohils and mast cells v Phagocytize antioenantibodv complexes vi Limit action of histamine and other inflammatory chemicals c Basophils i Secrete chemicals that aid mobility and action of other leukocytes 1 Leukotrienes activate and attract neutrophils and eosinophils 2 Histamine a vasodilator which increases blood flow a Speeds delivery of leukocytes to the area 3 Heparin inhibits clot formation a Could impede leukocyte mobility ii Mast cells also secrete these substances 1 Type of connective tissue very similar to basophils d Lymphocytes i Three basic categories 1 Circulating blood contains a 80 T cells b 15 B cells c 5 Natural Killer NK cells 2 Many diverse functions a Immune surveillance and specific immunity e Monocytes i Emigrate from the blood into the connective tissue and transform into macrophages 1 Macrophage system al the body s avidly phagocytic cells except leukocytes a Wandering macrophages actively seeking pathogens i Widely distributed in loose connective tissue b Fixed macrophages phagocytize only pathogens that come to them i Microglia in central nervous system ii Alveolar macrophages in lungs iii Hepatic macrophages in liver 15 Innate defense 2 Fever a Fever abnormally high body temperatures b Leukocytes and macrophages release pyrogens in response to contact with invaders c Pyrogens reset the body s homeostatic temperature setpoint in an upward fashion i For example to 102 instead of 986 d Elevated temperature enhances actions of antimicrobial proteins and inhibits bacterial reproduction e Fever is an uncomfortable side effect of illness but does more good than harm f People with cold recover more quickly and are less infective to others when they allow a fever to run its course rather than taking antipyretics feverreducing medications such as aspirin or ibuprofen 16 Innate defenses 2 immune surveillance a A phenomenon in which NK cells continually patrol the body on the lookout for pathogens and diseased host cells i 1 NK cell releases perforins which polymerize and form a hole in the enemy cell membrane ii 2 Granzymes from NK cell enter perforin hole and degrade enemy cell enzymes iii 3 Enemy cell dies by apoptosis iv 4 Macrophage engulfs and digests dying cell 17 Innate defenses 2 antimicrobial proteins a Proteins that inhibit microbial reproduction and provide shortterm nonspecific resistance to pathogenic bacteria and viruses b Two families of antimicrobial proteins i lnterferons ii Complement system c Because viruses lack the cellular machinery to make energy ATP or synthesize proteins they hijack our cell machinery to reproduce 18 Innate defense 2 complement system a Complement system group of 30 proteins that make powerful contributions to both nonsoecific resistance and soecific immunitv i Synthesized mainly by the liver ii Circulate in the blood in inactive formActivated by presence of the pathogen iii Complements all branches of the immune system 19 Complement system a Activated complement brings about four methods of pathogen destruction i Inflammation ii Immune clearance iii Phagocytosis iv Cytolysis b Three routes of complement activation i Classical pathway ii Alternative pathway iii Lectin pathway c Classical pathway i Reduires antibodv molecule to get started ii Thus part of soecific immunitv iii Antibody binds to antigen on surface of the pathogenic organism 1 Forms antigenantibody AgAb complex iv Changes the antibody s shape 1 Exposing a pair of complementbinding sites 2 Binding of the first complement C1 sets off a reaction cascade called complement fixation a Results in a chain of complement protein attaching to the antibody d Alternative pathway i Nonspecific does not require antibody ii C3 breaks down in the blood to C3a and C3b 1 C3b binds directly to targets such as human tumor cells viruses bacteria and yeasts 2 Triggers cascade reaction with autocatalytic effect where more C3 is formed e Lectin pathway i Lectins plasma proteins that bind to carbohydrates 1 Bind to certain sugars of a microbial cell surface 2 Sets off another cascade of C3 production f Mechanisms of action of complement proteins i Inflammation 1 C3a stimulates mast cells and basophils to secrete histamine and other inflammatory chemicals 2 Activates and attracts neutrophils and macrophages 3 Speeds pathogen destruction in inflammation ii Immune clearance 1 C3b binds with antigenantibody complexes to red blood cells 2 These RBCs circulate through the liver and spleen 3 Macrophages of those organs strip off and destroy the Ag Ab complexes leaving the RBCs unharmed 4 Principal means of clearing foreign antigens from the bloodstream iii Phagocytosis 1 Neutrophils an macrophages cannot phagocytize naked bacteria viruses or other pathogens 2 C3b assists them by opsonization a Coats microbial cells and serves as binding sites for phagocyte attachment b Makes the foreign cell more appetizing iv Cytolysis 1 C3b splits other complement proteins 2 Bind to enemy cell 3 Attract more complement proteins membrane attack complex MAC forms a Forms a hole in the target cell b Electrolytes leak out water flows in rapidly cell ruptures 20The membrane attack complex a Complement proteins form ring in plasma membrane of target cell causing cytolysis 21 Innate defense 2 inflammation a Inflammation local defensive response to tissue injury of any kind including trauma and infection i Suffix it is denotes inflammation of specific organsarthritis pancreatitis dermatitis b General purposes of inflammation i Limits spread of pathogens then destroys them ii Removes debris from damaged tissue iii Initiates tissue repair c Four cardinal signs of inflammation i Redness ii Swelling iii Heat iv Pain d Three major purposes of inflammation i Mobilization of body defenses ii Containment and destruction of pathogens iii Tissue cleanup and repair 22 Mobilization of defenses a Most immediate requirement for dealing with tissue injury is to get the defensive leukocytes to the site quickly b Local hyperemia increasing blood flow beyond normal rate is a way to do this i Local vasodilation due to vasoactive chemicals 1 Histamine leukotrienes and other cytokines 2 Secreted by basophils mast cells cells damaged by trauma toxins or organisms triggering inflammation 3 Hyperemia washes toxins and metabolic waste from the site more rapidly c Cytokines class for chemicals that regulates inflammation and immunity i Secreted mainly by leukocytes ii Alter the physiology or behavior of receiving cell iii Act as short range neighboring cells paracrines or the same cell that secretes them autocrines iv Release of cytokines 1 Promote inflammation 2 Recruit white blood cells v Release of histamines 1 Dilate postcapillary venules 2 Increase vessel permeability 3 Local edema dilutes harmful substances d Vasoactive chemicals dilate local blood vessels i Endothelial cell separate increasing capillary permeability ii Fluid leukocytes and plasma proteins leave the blood stream 1 Complement antibodies and clotting factors iii Selectins celladhesion molecules made by endothelial cells that aid in the recruitment of leukocytes 1 Make membranes sticky and snag leukocytes e Neutrophil behavior i Margination 1 Selectins cause leukocytes to adhere to blood vessel walls ii Diapedesis emigration 1 Leukocytes squeeze between endothelial cells into tissue spaces 23 Containment and destruction of pathogens a Priority of inflammation is to prevent the pathogens from spreading throughout the body i Fibrinogen that filters into tissue fluid clots 1 Forms a sticky mesh that walls off microbes ii Heparin prevents clotting at site of injury 1 Pathogens are in a fluid pocket surrounded by clot 2 Attacked by antibodies phagocytes and other defenses b Neutrophils the chief enemy of bacteria accumulate at the injury site within an hour iAfter leaving the bloodstream they exhibit chemotaxis c Chemotaxis attraction to chemicals such as bradykinin and leukotrienes that guide them to the injury site d Neutroohils are duickest to resoond and kill bacteria bv i Phagocytosis ii Reparatory burst iii Secrete cytokines for recruitment of macrophages and additional neutrophils iv Macrophages and T cells secrete colonvstimulatind factor to stimulate leukopoesis 1 Neutrophilia 5000 cellsuL to 25000 cellsuL in bacterial infection 2 Eosinophilia elevated eosinophil count in allergy or parasitic infection 24Tissue cleanup and repair a Monocytes the primary agents of tissue clean up and repair i Arrive in 8 to 12 hours and become macrophages ii Engulf and destroy bacteria damaged host cells and dead and dying neutrophils b Edema contributes to tissue cleanup i Swelling compresses veins and reduces venous drainage ii Forces open valves of lymphatic capillaries promoting lymphatic drainage iii Lymphatics collect and remove bacteria dead cells proteins and tissue debris better than blood capillaries c Pus accumulation of dead neutrophils bacteria other cellular debris and tissue fluid form a pool of yellowish fluid i Abscess accumulation of pus in a tissue cavity d Plateletderived growth factor secreted by blood platelets and endothelial cells in injured area i Stimulates fibroblasts to multiply ii Synthesizes collagen e Hyperemia delivers oxygen amino acids and other necessities for protein synthesis f Increased heat increases metabolic rate speeds mitosis and tissue repair g Fibrin clot forms a scaffold for tissue reconstruction h Pain makes us limit the use of a body part so it has a chance to rest and heal 25 Specific adaptive immune system a Specific defense system i Must come into contact with foreign substance antigen at least once prior to protecting body 1 Rememberthe innate defenses can do serious damage to foreign substances on their first meeting ii Two characteristics distinguish specific adaptive immunity from nonspecific innate resistance 1 Specificity immunity directed against a particular pathogen ex E coli as opposed to any bacteria 2 Memory when reexposed to the same pathogen the body reacts so quickly that there is no noticeable illness you are immune iii Two major branches 1 Cellular cellmediated immunity T cells a Lymphocytes directlv attack and destroy foreign cells or diseased host cells b Means of ridding the body of pathogens that reside inside human cells that harbor pathogens 2 Humoral antibodymediated immunity B cells a Mediated by antibodies that do not directlv destrov a m b Antibodies mark infected cells for destruction other immunity cells will then kill the cells marked for death c Can only work against the extracellular stage of infectious microorganisms b What s the difference between active and passive acquired immunity N BiillJlr i If i acqul red l Artificially acquired f39 l l l the 39 pmiim amine quot ames infection Antibodies Fasting Injection content from dead or minimum with mother tn attenuated serum pathogen fema via pathogens gamma placenta glahullln far to infant in fur millsquot 26Antigens a Any nonself substances that can provoke an immune response b Epitopes antigenic determinants certain regions of an antigen molecule that stimulates immune responses c Haptens too small to be antigenic in themselves i Must combine with a host macromolecule ii Create a unique complex that the body recognizes as nonself iii Cosmetics detergents industrial chemicals poison ivy and animal dander iv Penicillin binds to host proteins in allergic individuals d Antigenic determinants small portions of antigen are immunogenic 27 Cells of adaptive immunity a T lymphocytes cellmediated immunity b B lymphocytes humoral immunity c Antigen presenting cells APC i B and T lymphocytes must be educated either in the thymus T or bone marrow B 1 lmmunocompetence a Appearance of cellsurface antigen receptors b Ability to recognize one specific antigen c Ability to bind to that antigen 2 Self tolerance a Learn about self antigens SA b Make sure to stay unresponsive to SA don t attack one s own cells ii B and T cells must pass an exam before leaving school 28 Educating the lymphocytes a While in the thymus developing T cells are presented with self antigens like a final exam b Two ways to fail the exam i Inability to recognize self antigens 1 Would be incapable of recognizing a foreign attack on the body ii Reaching to the self antigen 1 T cells would attack one s own tissues c B cells go through a similar exam in the bone marrow 29Antigen presenting cells a T cells cannot recognize their antigens on their own b Antigenpresenting cells APC s are required to help i Dendritic cells macrophages reticular cells and B cells function as APC s c Function of APC s depends on maior histocompatibilitv MHC complex proteins i Act as cell identification tags that label every cell of your body as belonging to you ii Structurally unique for each individual except for identical twins 30 Selfantigens a Not recognized as foreign b Major histocompatibility complex MHC cell surface proteins that identify as self c Class I MHC found on all body cells i Can bind to pieces of material found within cells protein breakdown and present them at the cell surface these are self an gens d Class II MHC found only on cells that patrol body and pick up antigenic material i Bind proteins that come from outside the cell and the patrolling cell presents the material at its cell surface these are foreign antigens 31 The action antigenpresenting cells 1 Phagocytosis of antigen 2 Lysosome fuses with phagosome 3 Antigen and enzyme mix in phagolysosome 4 Antigen is degraded 5 Antigen residue is voided by exocytosis 6 Processed antigen fragments epitopes displayed on macrophage surface 32 Cellular immunity a Cellular cellmediated immunity i A form of specific defense in which the T lymphocytes directly attack and destroy diseased or foreign cells ii The immune system remembers the antigen and prevents them from causing disease in the future iii 4 classes of T cells 1 Cytotoxic T Tc cells killer T cells nweoce a Effectors of cellular immunity carry out attack on enemy cells 2 HelperT TH cells a Help promote TC cell and B cell action and nonspecific resistance 3 Regulatory T TR cells Tregs a lnhibit multiplication and cytokine secretion by other T cells important in preventing autoimmune disease 4 MemoryT TM cells a Descend from the cytotoxic T cells b Responsible for memory in cellular immunity b Both cellular and humoral immunity occur in three stages i Recognition ii Attack iii Memory c Thought of as the three R s of immunity i Recognize ii React iii Remember d Antibodies not good enough to defend against pathogens inside the cellsso in come the T cells 33Attack Helper T cells a Helper and cvtotoxic T cells play different roles in the attack phase b Helper T TH cell necessary for most immune responses c Play central role in coordinating both cellular and humoral immunity d When helper T cell recognizes the MHC II complex i Secrete interleukins that exert three effects 1 Attract neutrophils and NK cells 2 Attract macrophages stimulate their phagocytic activity and inhibit them from leaving the area 3 Stimulate T and B cell mitosis and maturation 34 Attack Cytotoxic T cells a Cytotoxic T Tc cells are the only T cells that directly attack other cells b When To cell recognizes a complex of antigen and MHCl protein on a diseased or foreign cell it docks on that cell i Perforin and granzymes kill cells in the same manner as NK cells ii lnterferons inhibit viral replication iii Tumor necrosis factor TNF aids in macrophage activation and kills cancer cells 35 Memory a Immune memory follows primary response b Following clonal selection some TC and TH cells become memory cells TM i Longlived ii More numerous than naive T cells iii Fewer steps to be activated so they respond more rapidly c T cell recall response i Upon reexposure to the same pathogen later in life memory cells launch a quick attack so that no noticeable illness occurs ii The person is immune to the disease 36Humoral immunity a Humoral immunity is a more indirect method of defense than cellular immune system i B lvmphocvtes of humoral immunity produce antibodies that can bind to antigens and tag them for destruction by other means 1 No direct attack of enemy cell 2 Cells are marked for death ii Works in 3 stages just like cellular immunity 1 Recognition 2 Attack 3 Memory b B cells have thousands of surface receptors antibodies for one antiden i Recognition 1 Antigen recognition antigen binds only to B cells with complementary receptors 2 Antigen presentation B cell internalizes antigen and displays processed epitope Helper T cell bind to B cell and secrets interleukin 3 Clonal selection some cells of the clone become memory B cells Most differentiate into plasma cells ii Attack plasma cells synthesize and secrete antibody Antibody employs various means to render antigen harmless 37 Antibody functions a Cannot destroy antigens but function to inactive them and tag for death 4 main ways i Begins with formation of antigenantibody complex 38 Humoral immune response a Primed memory cells mount almost immediate response to antigen upon 2nOI encounter b Immunological memory i Adaptive defense l humoral immunity 39Antibodies immunoglobulins a lgG monomer i Most abundant in plasma ii Only one that passes from mom to fetus iii Bind and activate complement iv Main antibody of 1 2 responses lgD monomer i Always bound to B lymphocytes ii B lymphocyte antigen receptor iii Binding to lgD activates B lymphocytes lgE monomer i Stem bound to mast cells ii Antigen bind receptor end histamine release iii Tightly linked to allergic reactions iv Found in small quantities in plasma lgA dimer i Found in mucus saliva sweat milk ii Prevents pathogen attachment to epithelial surfaces iii Found in small quantities in plasma lgM pentamer i First to be released by plasma cells to the blood during 1 response indicates infection ii Agglutinates clumps antigens very well iii Binds and activates comlement
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