Adaptive Immunity and Immunization Introduction Immunity is the ability of an organism to recognize & defend itself against pathogens. Susceptibility is the vulnerability of the host to harm by the pathogen. Innate immunity: immunity produced by the host against pathogens regardless of their type. Adaptive (acquired) immunity: ability of host to build a defense against a specific pathogen. Adaptive immunity is either naturally acquired or artificially acquired but it is not hereditary. 1
Introduction Naturally acquired adaptive immunity is most commonly obtained by having a disease. e.g. a person is infected by a pathogen, the pathogen has antigens that activates lymphocytes of the immune system, antibodies are released specific to this pathogen & other immune responses are activated to protect against future invasion by similar pathogen. Immunity can be also naturally acquired by antibodies transferred to fetus across placenta or through breast milk. Artificially acquired adaptive immunity: obtained by receiving an antigen by injection or receiving immune serum (i.e. containing antibodies). Whether immunity is natural or artificial it is either active immunity or passive immunity Active immunity: Introduction It is created when the person s own immune system builds it. In this case, the immune system responds specifically against the antigen & remembers this specific antigen to respond to it any time in the future. Naturally acquired active immunity: is produced when a person is exposed to infectious agent. Artificially acquired active immunity: is produced when a person is vaccinated by inactive, weakened, or dead organism. 2
Introduction Passive immunity: It is created when ready made antibodies are introduced into the body, i.e. the host immune does not make the antibodies. Naturally acquired passive immunity: e.g. passed from mothers into offspring, or from mothers to infants who are breast fed especially in the first days after birth (colostrum: first fluid secreted by mammary glands after child birth, full of antibodies) Artificially acquired passive immunity: e.g. introducing antivenin (antibodies produced in another animal such as horses or rabbits) to a patient. The problem with passive immunity is that it lasts for short time (weeks to months) but then destroyed by the host & the host immune system will not make new one 3
Antigens Antigen (immunogen): a substance that the body identifies as foreign substance that elicits a specific immune response Most antigens are large proteins M. wt > 10,000 daltons, some are polysaccharides, few are glycoproteins or nucleoprotein Each antigen has several epitopes (antigenic determinants) Epitope is the area on the molecule that binds to antibodies The exact chemical structure of each antigen is determined genetically Antigens are found on surfaces of viruses, bacteria (flagella, pili, cell wall) other m.o. & human cells Antigens Hapten: small molecule that when binds to a large protein can elicit immune response. Hapten acts as epitope on the surface of protein. Hapten alone or the protein alone can not induce immune response. e.g. penicillin acts as a hapten, it binds to body proteins & elicit allergy rxn which is immune response. Immune system responds to a foreign substance (antigen) by producing antibodies (anti-antigens ) Antibodies or immunoglobulins are proteins produced in response to the presence of antigens, it binds to a specific epitope on the antigen 4
Lymphocytes Lymphocytes are part of the immune system, they develop from stem cells in bone marrow. They are divided into: B cells T cells Natural killer cells. Stem cells differentiate & mature into B-lymphocytes (cells) in the bone marrow (in birds this happens in Bursa of fabricus). B-cells are found in all lymphoid tissues (lymph nodes, tonsils, spleen, lymphoid tissue of digestive tract) & blood. They form 1/10 of lymphocytes circulating in the blood 5
Lymphocytes Other stem cells migrate to the thymus gland where they mature into T-lymphocytes (cells), They are found in all tissues where B cells are found. They form 3/4 lymphocytes circulating in the blood T cells differentiate further to 4 types of cells that migrate into lymphatic tissues & blood: Cytotoxic (killer) T cells Helper T cells Regulatory T cells Delayed hypersensitivity T-cells Lymphocytes Few lymphocytes (not identified as B cells or T cells) are found in tissues & circulating in blood, which include natural killer cells (NK cells). They naturally & non specifically kill cancer cells & cells infected by viruses. They release different cytotoxic molecules which can: Cause cell lysis by forming holes in the cells, or Enter the nucleus of the target cell, fragment its DNA leading to apoptosis (programmed cell death). 6
Dual Nature of the Immune System There are two types of immune response: Humoral immunity (performed by antibodies) Cell-mediated immunity (performed by T cells) Humoral immunity: presence of antigen will stimulate B cells to initiate a process that leads to releasing antibodies Antibodies are very effective in defending the body against foreign substances outside cells e.g. bacteria, toxins, viruses before their entry to cells Cell-mediated immunity: occurs at cellular level when the antigen is inside host cell, or in cell membrane where it is inaccessible to antibodies Thus, it is effective against virus infected cells & also have a role against fungi, eukaryotic parasites, cancer & foreign tissue like transplanted organs 7
General Properties of the Immune System There are four general properties of immune system (humoral & cell mediated), which are: Recognition of self versus non-self Specificity Diversity Memory 1. Recognition of self versus non-self: The ability of immune system to tolerate host tissues while recognize & destroy foreign substances Immune system must differentiate between host substances (self) & foreign substances (non-self) in order to avoid damage of host tissues General Properties of the Immune System 1. Recognition of self versus non-self: (cont d) The clonal selection/deletion hypothesis (proposed 1950 by Sir Burnet/Australia): Lymphocytes are genetically programmed to recognize particular antigen to destroy it. If during lymphocytes development (in bone marrow for B cells & Thymus for T cells), these lymphocytes encounter their antigen as part of normal host substance (self), the lymphocyte is destroyed But if the development of lymphocyte was completed & then it encounters & recognizes an antigen, it divides repeatedly to produce a clone (a group of identical cells that make the same antibody) Clonal selection by removing lymphocytes that destroy host cells will create tolerance for self. Tolerance can be acquired by radiation (cancer therapy) & by the use of immunosuppressant (for transplantation), but this will suppress the ability of immune system to respond to foreign antigens leading to infections 8
General Properties of the Immune System 2. Specificity: The ability of immune system to react in a different & particular way to each foreign substance Immune system is fully mature at age of 2-3yrs, it recognizes huge no. of foreign substances as non-self T & B cells are specific for antigen i.e. they interact in a different way to each foreign substance & generally the response to one antigen has no effect on the other, BUT reactions of particular antibody with very similar antigen might occur this is called cross reactions e.g. Treponema Pallidum (syphilis) has hapten similar to human heart muscle cells, this allows antibodies against bacterial haptens to interact with muscle cells leading to damages in the heart 9
General Properties of the Immune System 3. Diversity: The ability of immune system to respond in a specific way to a great variety of different foreign antigens The immune system produces many different kinds of antibodies & T cells receptors, each reacts with a different epitope. If a bacterium has different epitopes, the immune system may make different antibody against each one Also immune system can make antibodies against newly synthesized antigens never before encountered by any immune system. B cells can produce antibodies to more than one billion (10 9 ) antigens Diversity of antibodies & T cell receptors does not necessitates exposure to antigen (e.g. animal raised in germ free environment has Ab to antigens never exposed to them). General Properties of the Immune System 4. Memory: The ability of immune system to recognize & respond quickly to foreign substances to which it has previously responded During the first rxn of the immune system to antigen, it produces Ab & also makes memory cells (B cells), these stays ready for years or decades, Therefore when the immune system encounters the same antigen in the future, the second response will be much more faster than the first one. 10
Humoral Immunity Each kind of B cells carries its specific antibody on its membrane & can bind to a specific antigen. When it binds to antigen, it becomes activated or sensitized & divides many times, some of the daughter cells are memory cells & most are plasma cells. Plasma cells are large lymphocytes that synthesize & release many antibodies like those on their membrane (each plasma cell can produce up to 2000 Ab/sec). Humoral Immunity When the Ag binds to the Ab on B cell, they are taken into the cell where it processes the Ag (breaking the Ag into short fragments) which binds to Major Histocompatibility Complex II (MHCII) molecule on the surface of the B cell. This is called presenting the antigen. Dendritic cells & macrophages also present antigens in this way. At this stage T cells have two important roles: T cells recognize the presented antigen (Ag + MHCII) & become activated, they produce interleukin 2 (IL2) which help B cell to multiply & mature into plasma cells (produce Ab). When T helper cell contacts Ag-presenting B cell, it will stimulate it to proliferate & produce more memory cells. 11
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Antibodies (Immunoglobulins) The basic unit of immunoglobulins (Ig) is a Y-shaped protein composed of 4 polypeptide chains: 2 identical Light (L) chains, 2 identical Heavy (H) chains. The chains are held together by disulfide bonds Immunoglobulins have constant regions and variable regions: Constant regions: determines the class of the Ig, similar in all Ig of same class Variable regions: at the end of L & H chain and have specific shape & charge that enable the Ig to bind to specific Ag, i.e. they are binding sites of Ab to Ag & are identical to the corresponding receptors on B cells First antibodies produced by B cells are embedded into their membranes to form receptors. 13
Antibodies (Immunoglobulins) Immunoglobulins can be cleaved (by papain enzyme) into three fragments: Two antibody-binding fragments (Fab fragment) that bind to antigen epitopes One crystallizable fragment (Fc fragment) which is formed by parts of heavy chain in the tail of Y and has sites that bind with complement, combine with phagocytes in opsonization & participate in allergic rxns. Classes of Immunoglobulins Five classes of Ig have been identified in humans which are: IgG, IgA, IgM, IgE and IgD Each class has its particular constant region IgG: Main class of antibodies in blood, forms ~80% of total blood Ig Produced in large amounts during secondary response. The only Ig that crosses the placenta which provides protection to the fetus & also secreted in breast milk. It binds to antigen on the pathogen & the Fc fragment binds to phagocytes, thus bringing phagocytes to engulf the pathogen. Ig can also activate complement system. 14
Classes of Immunoglobulins IgA: Occurs in small amounts in blood (~10% of total blood Ig), but in large amounts in secretions like tears, milk (mainly colostrum), saliva, mucus & attached to lining of digestive, respiratory & genitourinary tracts. It is secreted into blood & pass through tissues to either be released into the secretions or attached to the linings. In blood it is present mainly in the form of monomers, but small amounts of dimers, trimers & tetramers may be found Secretory IgA is in form of dimers which consist of 2 monomers held together by a joining chain (J chain) & has an attached secretory piece which protects it from digestion & helps solubilizing it in secretions It binds to antigens on microbes before they invade tissues & activates the complement system Classes of Immunoglobulins IgM: It forms about 5% of total blood Ig Secreted from plasma cells in form of pentamers (5 units joined by their tails, so having 10 Ag binding sites) The 1 st Ab produced in life (formed in fetus) & the 1 st Ig secreted into blood during early primary response, thus high levels in blood indicates recent infection It activates the complement system & also causes microbes to clump together 15
Classes of Immunoglobulins IgE: Found mainly in body fluids & skin, but rare in blood (only ~0.005% of blood Ig) Has high affinity to receptors on the membranes of basophils in blood & mast cells in tissues, therefore, It plays a major role in allergies It binds to basophils & mast cells from tissue binding site & antigen binding site binds to antigens (drugs, pollens, food) & causes the cells to release histamine responsible for allergy symptoms IgE is elevated in patients with allergy or having worm parasites Classes of Immunoglobulins IgD: Found mainly on B cell membranes & rarely secreted Unknown function, may help initiate immune response or allergic reactions IgD level increase in autoimmune disease 16
Primary & Secondary Immune Responses Primary response: Occurs when Ag is first recognized by and bound to a suitable clone of B cells After Ag binding, B cells can divide to form plasma cells that synthesize Ab s which increase in concentration over a period of 1 to 10 weeks IgM is produced first and then a shift occurs to the production of IgG; i.e. IgM decreases as IgG increases. At some point, the production of both IgM & IgG ceases and their concentrations become low During the primary response, B memory cells are also produced and persist in lymphoid tissues, they don t participate in the initial response but they survive without dividing for months to many years retaining the ability to recognize the Ag. 17
Primary & Secondary Immune Responses Primary response: The primary response of B cells can occur by two mechanisms: T-independent mechanism: B cells are directly activated (sensitized) by Ag binding so they divide to form plasma cells which mainly produce IgM Ab s. This step does not need T helper cells & no memory cells are produced T-dependent mechanism: the Ag is presented by B cell, which makes contact with activated T helper cells. T H secrete lymphokines (cytokines) that further activate B cells to proliferate & produce memory cells & plasma cells where the later cells switch from IgM to IgG production Primary & Secondary Immune Responses Secondary response: When another exposure to the antigen happens, it is recognized by memory cells and a secondary response occurs Memory cells are present in larger no. than the original clone of B cells, that s why the secondary response is much faster than primary one Some memory cells divide producing more memory cells, other memory cells rapidly divides producing plasma cells that produce & release large quantities of Ab IgM is produced before IgG but in smaller quantities over shorter period of time, while IgG is produced sooner & in much greater quantities than in primary response 18
Primary & Secondary Immune Responses Antigen-Antibody Reactions Inactivation or destruction of infectious agents starts by the formation of Ag-Ab complex. This is then accomplished by different mechanisms which include: Agglutination: clumping or sticking together of microbes as a result of their interaction with Ab. IgG produces weak while IgM produces strong agglutination rxn with some bacteria which is visible to the naked eye. Opsonization: Ab s (e.g. IgG) interact with microbes & coat them so that they can be phagocytized e.g. by macrophages 19
Antigen-Antibody Reactions Neutralization: binding of Ab with small molecules like bacterial toxin can directly inactivate them (prevent them from exerting their toxic effects). IgG is the main neutralizer for bacterial toxins. Viruses can also be inactivated by neutralization. IgM, IgA & IgG are effective neutralizers against viruses. Complement system activation: IgG & IgM are powerful activators of complement system, that causes lyses to the cell (i.e. by MAC formation). Cell lysis: IgM can directly lyse microbial cells without the aid of complement system Cell Mediated Immunity This involves the direct action of different types of T cells on foreign antigens e.g. bacteria, viruses, eukaryotic parasites, tumor cells, transplanted organs & some allergic rxns. It involves also production of chemical mediators called cytokines (lymphokines & interleukins) 20
Cell Mediated Immunity How it works? T cells can not be activated directly by Ag s, the Ag should be presented on the surface of cells along with major histocompatibility complex MHC protein. The cells that present the Ag s include dendritic cells, B cells & macrophages, these cells are called antigen presenting cells (APC) There are 2 classes of MHC protein; MHCI present in all nucleated cells i.e. all body tissues & MHCII present on APC T H cells are activated by antigen presented by MHCII, while T C cells are activated by antigen presented by MHCI T cells with proper receptor to the presented Ag will bind to the presented Ag (T cell-ag-mhc-cell). This binding will cause T cells to differentiate to different types of T cells including memory cells (to recognize this Ag if encountered in future to respond to it more rapidly). 21
Cell Mediated Immunity Role of each cell type in immunity T H cells: Play a role in humoral immunity; they activate B cells Have a role in cell mediated immunity by releasing lymphokines to activate T D cells & T C cells. T H causes differentiation of undifferentiated lymphocytes to natural killer cells. T D cells: Secrete lymphokines that are chemtactic to macrophages, Activate macrophages & prevent macrophages from leaving the site of infection. T D cells secrete certain lymphokines that cause the macrophage to increase the production of toxic hydrogen peroxide & enzymes that attack the bacteria. Also play a role in allergic rxns. Cell Mediated Immunity Role of each cell type in immunity (cont d) T C cells: Kill infected host cells, mainly virally infected cells. T C cells can be activated by antigen presented by MHCI T C, NK & eosinophils synthesize a lethal protein (perforin) that is released when they bind to target cell. Perforin makes pores in the target cell membrane causing release of cellular content, it affects only infected cells. Thus, these cells by destroying infected host cells prevent the spread of infection NK cells: Act mainly on tumor cells, transplanted tissues & cells infected with viruses or intracellular bacteria. In contrast to T C they don t need the Ag to be presented by macrophages, thus, they act directly on malignant cells without the help of macrophages. 22
Cell Mediated Immunity Role of each cell type in immunity (cont d) AIDS virus invades T H cells & disturbs their function & kills them. This will impair humoral & cell mediated immunity including the destruction of malignant cells. That s why AIDS patients are susceptible to opportunistic infections & malignancies. Cell Mediated Immunity Superantigens: Antigens that exhibit highly potent unspecific T cell activation. They are not degraded by APC, rather they bind intact to MHC & T cell receptors. This binding causes 100-500 times more activation to T cells than in normal Ag binding. T helper cells then release enormous amounts of IL2 that circulates in blood & enters various tissues causing various symptoms 23
Cell Mediated Immunity Superantigens: (cont d) e.g. Toxic shock syndrome toxin-1 (TSST-1): Induced by a toxin produced from S. aureus Can cause fever, hypotension, rash, desquamation, kidney failure, liver failure, abnormalities in blood and finally death. Some believe that superantigens play a role in autoimmune diseases like multiple sclerosis & rheumatoid arthritis. The explanation is that not all T cells that recognize self are deleted by clonal deletion. when these multiply excessively, host tissues are attacked. Factors Affecting Immune Responses Age: Below age of 2 the immune system is not fully developed. Elderly people are more exposed to infections & malignancies because their immune system especially cell mediated is declining. Race: e.g. black people are more prone to TB than white people Season: T cells have annual cycle, their lowest levels are in June. Diet & nutrition: adequate vitamins & proteins are essential for intact skin & mucus membrane. Also important for production of lymphocytes & Ab s. 24
Factors Affecting Immune Responses Moderate exercise: Increase Ab s production during exercise & 1 hr afterwards. NK cells activity increases. But excessive exercise depresses immunity, e.g. Marathoners are more vulnerable to infections especially upper respiratory tract. Pregnancy: reduces cell mediated immunity. Sleeping disorders: reduce NK cells, good night rest return it to normal Factors Affecting Immune Responses Traumatic injuries: Provides access to microbes Lower resistance to infections because tissue repair competes with immune system in synthesizing the required proteins. Environmental factors: Pollution (including tobacco smoking) damage respiratory membranes & depress activity of phagocytes. Radiation damages cells of immune system. Genetic defects: absence of B cells, T cells or both Diseases: e.g. AIDS destroys T cells Drugs: immunosuppressant drugs 25
Immunization Immunization is either active or passive Active immunization: the process of inducing active response by the host immune system similar to that occurring during an infection. Passive immunization: the administration of ready-made antibodies into the body in order to provide immediate immunity to a nonimmune person who is exposed to a pathogenic agent Active Immunization Active immunization is induced by administering a vaccine or toxoid Vaccine: a substance that contains an antigen to which the immune system responds. Types of vaccines: Killed whole-cell m.o.: the m.o. is killed using chemicals, heat or radiation (e.g. cholera, rabies and intramuscular polio vaccine) Attenuated live m.o.: either the same m.o. but with reduced virulence or a another closely related m.o. that is less risky (e.g. BCG for TB, oral polio vaccine and MMR vaccine) Advantage: live vaccines produce higher & longer lasting immunity, Disadvantage: possibility of the organism to retain its virulence (e.g. too risky for an AIDS vaccine) 26
Active Immunization Types of vaccines: (cont d) Subunit vaccines: purified immunogenic portion (i.e. antigen) of the m.o. (e.g. Hepatitis B (surface proteins), Anthrax (cellular proteins)). Recombinant vector vaccines: produced by inserting the gene for specific Ag into the genome of nonpathogenic m.o. (cloning), (e.g. rabies vaccine by inserting its glycoprotein gene into vaccinia virus) Toxoid: an inactivated toxin (by chemical or heat treatment) that is no more harmful but has antigenic properties. (e.g. tetanus & diphtheria toxoids) Active Immunization When vaccine or toxoid is administered, the immune system recognizes it as foreign substance & produces Ab s, cytotoxic T cells & memory cells; similar rxn to disease but because the organism is attenuated or killed the disease itself does not occur. Sometimes, organisms in attenuated vaccines multiply but without producing disease. Longevity of vaccines: vaccines made with live organisms provide longer lasting immunity than dead organisms or toxoids. e.g. oral poilo & measles vaccines provide lifelong immunity while killed typhoid vaccine provides 3-5 yrs immunity Booster shots: immunity is not always lifelong. The first dose of vaccine stimulates primary immune response, next doses (booster shots) stimulate secondary immune response & greatly increase Ab production 27
Active Immunization Route of administration: oral vaccines are more effective for GIT infection than IM ones, whereas nasal aerosols are better given for respiratory infections In general, active immunization is not useful to prevent a disease after the person is exposed to it (because immune response takes time to develop) except in situations were the incubation period of the disease is long e.g. rabies virus which has long incubation period. Active Immunization Hazards of vaccines: Active immunization may cause fever, malaise & soreness at the site of injection. So patients with active infections should not receive vaccines as they worsen the case & also because the patient s immune system is overburdened by the disease & may not respond adequately to the vaccine. Pregnancy: pregnant women must not receive live vaccines as they may cross placenta & infect the fetus whose immune system is not yet developed. Immunocompromised patients should not receive live vaccines as the attenuated virus may still have some virulence enough to cause a disease in these patients. 28
Active Immunization - Jordan Each country has a list of vaccines for routine immunization of infants and children. This is done by coordination with WHO The national Jordanian program of vaccination includes the following vaccines: BCG: Bacillus Calmette Guérin vaccine (for TB) DTaP: diphtheria, tetanus and acellular pertusis HBV: hepatitis B virus Hib: Haemophilus influenzae type b OPV/IPV: oral polio vaccine/intramuscular polio vaccine MMR: measles, mumps and rubella Active Immunization - Jordan Age (months) Vaccine <1 2 3 4 9 18 BCG DTaP HBV Hib Polio IPV IPV+OPV OPV OPV OPV Measles MMR 29
Passive Immunization The introduction of ready made antibodies into the body. In this approach, immunity is produced quickly but it is temporary; i.e. lasts as long as there is a high titer of Ab s. It is established by administering gamma globulin, hyperimmune serum or antitoxin. Gamma globulin (antiserum): pooled gamma globulins from many individuals which typically contains Ab s for several diseases such as measles, mumps & Hepatitis B Hyperimmune serum: gamma globulins that are prepared to have high titer of a specific antibody. e.g. gamma globulins taken from patients recovering from mumps or from a person recently vaccinated with mumps, or collecting A sb from serum of animal (e.g.horse) who was exposed to the m.o. Antitoxin: are antibodies against specific toxins like those against diphtheria, tetanus and reptile venom (antivenin) Passive Immunization Passive immunization gives immediate immunity to a non-immune person who is exposed to disease or at least lessens the severity of disease. The sooner these Ab s are given, the more effective they are in counteracting the antigen. Example applications: Person exposed to hepatitis B (contaminated needle) administration of Hepatitis B immune globulins Snake bite antivenin (react with venom that did not reach the tissues) Bite from rabid dog or cat rabies immune globulins A mother who is Rh ve carrying a Rh +ve fetus within 72 hrs of child birth or miscarriage, anti Rh antibodies should be given to the mother to prevent the mother s immune system from producing anti Rh antibodies, because if produced, they will attack &destroy the RBCs of next Rh+ve fetus 30
Passive Immunization Hazards of passive immunization: Allergy: since some antitoxins are prepared in other animals or eggs, some proteins with these antitoxins are allergic, that s why antitoxins prepared in humans are safer. Also immune globulin & hyperimmune sera (due to large size of their molecules) should be given IM, if given IV will cause allergic rxns. New immune globulin can be given IV, it contains smaller molecules. Interference with host s immunity: administered Ab s may interfere with body s active immunity by binding to Ag thus preventing the host immune system from being stimulated. 31