Immune Response Dr. Pendru Raghunath Reddy Assistant Professor Dept of MicrobiologyDr. VRK Women‟s Medical College
DefinitionThe specific reactivity induced in a host following anantigenic stimulus
Consequences of Immune response1. Beneficial to the host2. Indifferent3. Injurious to the hostThe state of specific nonreactivity (tolerance) induced by certaintypes of antigenic stimuli also included in it
Types of Immune responseThe immune response can be divided into two types1. The humoral (antibody mediated) immunity2. Cell-mediated immunity (CMI) These two are usually developed together, though at times one or other may be predominant or exclusive They usually act in conjunction but sometimes they may act in opposition
Role of humoral immunity1. Provides primary defense against most extracellular bacterial pathogens2. Helps in defense against viruses that infect through the respiratory or intestinal tracts3. Prevents recurrence of virus infections4. Participates in the pathogenesis of immediate (type 1, 2, and 3) hypersensitivity and certain autoimmune diseases
Role of cell – mediated immunity (CMI)1. Protect against fungi, viruses and facultative intracellular bacterial pathogens like Mycobacterium tuberculosis, M. leparae, Brucella and Salmonella and parasites like Leishmania and trypanosomes2. It provides immunological surveillance and immunity against cancer3. Participates in the rejection of homografts and garft-versus-host reaction4. Mediates the pathogenesis of delayed (type IV) hypersensitivity and certain autoimmune diseases
Humoral immunitySynthesis of antibodyAntibody production follows a characteristic pattern1. Lag phase2. Log phase3. A Plateau or Steady phase4. The phase of decline
Primary and secondary responsesThe kinetics and other characteristics of the humoral responsediffer considerably depending on whether the humoral responseresults from activation of naïve lymphocytes (primary response)or memory lymphocytes (secondary response)
Primary humoral response Characterised by a long lag phase and low titre of antibodies that do not persist for long The antibody formed is predominantly IgM A single injection of an antigen helps more in priming the immunocompetent cells to produce the particular antibody rather than in actual elaboration of high levels of antibody
Fate of antigens in tissuesThe manner in which an antigen is dealt within the bodydepends on many factors1. Physical and chemical nature of the antigen2. Dose and route of entry3. Speed of elimination4. Primary or secondary antigenic stimulus
Production of antibodies The majority of antigens will stimulate B cells only if they have the assistance of TH cells Antigens can be divided into two categories based on their apparent need for TH cells for the induction of antibody synthesis1. T – dependent antigens (TD – antigens) eg: Proteins and erythrocytes2. T – independent antigens (TI – antigens) eg: Polysaccharides and other structurally simple molecules with repeating epitopes
Polyclonal antibodiesSynthesised by several different clones of B cells against differentepitopes of the same antigenMonoclonal antibodiesProduced by a single clone of B cells and directed againat a singleantigenic determinant (epitope)
Principle• MYELOMA CELLS HAVE LOST the ability to synthesize hypoxanthine- guanine-phosphoribosyl transferase (HGPRT), an enzyme necessary for the salvage synthesis of nucleic acids• Which enables cells to synthesize purines by the salvage pathway here using an extracellular source of hypoxanthine as a precursor
• The selective culture medium is called HAT medium because it contains Hypoxanthine, Aminopterin, and Thymidine• Unfused myeloma cells cannot grow because they lack HGPRT• Unfused normal spleen cells cannot grow indefinitely because of their limited life span.
Problems with using mouse monoclonal antibodies• The therapeutic use of rodent monoclonal antibodies in humans is limited by their immunogenic, short circulating half-life, and inability to efficiently trigger human effectors mechanisms• This is due to differences between the mouse and humans.• Also severe allergic response in human when mouse mAb are introduced to a patients.• Also constant region of murine mAb are not effective in interacting with human effectors molecules.
Chimeric monoclonal antibodies• Monoclonal antibodies are genetically engineered using a molecular approach• Chimeric Abs are obtained by genetically fusing the mouse variable domains to human constant domains• Variable regions are isolated using polymerase chain reaction (PCR)
Humanized monoclonal antibodies The complementarity determining regions (CDRs), which are the responsible for antigen binding within the variable regions, have been transferred to human frameworks creating „„CDR-grafted‟‟ or „„humanized‟‟ antibodies
Uses of monoclonal antibodies1. Diagnostic use Many commercial diagnostic systems use monoclonal antibodies for identification of bacterial, viral and other antigens2. Used in cancer therapy3. Used in the identification of tumor and other surface antigens4. Used in identification of functional populations of different types of T cells5. Purification of desired proteins
Age The embryo is immunologically immature Production of antibodies starts after the development and differentiation of lymphoid organs Immunocompetence is not complete at birth Full competence is acquired only by the age of 5 – 7 years for IgG and 10 – 15 years for IgA
Genetic factors The immune response is under genetic control The immune response in different individuals to same antigen varies due to genetic factors Persons capable of responding to a particular antigen are called “responder” and those who do not respond are termed “nonresponder”
Multiple antigensWhen two or more antigens are administered simultaneously,the effects may vary1. No effect (eg: Mixture of typhoid and cholera vaccines)2. Synergistic effect (eg: DPT vaccine)3. Antagonistic effect (eg: Mixture of diphtheria and tetanus toxoids with one in excess)
Adjuvants Any substance that enhances the immunogenicity of an antigen is called adjuvant Adjuvants are often used in research and clinical settings to boost the immune response when an antigen has low immunogenicity or when only small amounts of an antigen are available
Precisely how adjuvants augment the immuneresponse is notentirely clearAction of adjuvants1. Sustained release of antigen (Depot)2. Enhance costimulatory signals3. Stimulate lymphocytes non-specifically4. Activate macrophages and stimulate CMI5. Increase local inflammation
Types of adjuvants1. Depot Aluminium hydoxide or phosphate, aluminium potassium sulfate (alum) and Freund‟s incomplete adjuvant (water in oil suspension) When an antigen is mixed with alum, the salt precipitates the antigen. Injection of this alum precipitate results in slower release of antigen from the injection site Freund‟s incomplete adjuvant contains antigen in aqueous solution, mineral oil and emulsifying agent such as manninde monooleate, which disperses the oil into small droplets surrounding the antigen
2. BacterialExamples: Freund‟s complete adjuvant and bacterial lipopolysaccharides Freund‟s complete adjuvant: Freund‟s incomplete adjuvant + heat-killed Mycobacteria Muramyl dipeptide, a component of the mycobacterial cell wall activates dendritic cells and macrophages3. ChemicalBentonite, calcium alginate and silica particles
Cell-Mediated Immunity• It is a specific acquired immune response mediated by specific cells of the immune system – Primarily T lymphocytes (T cells), but also macrophages, NK cells and K cells – This type of immunity can be transferred from one organism to another by intact lymphoid cells, but not by antisera• T cells are the main agents of cellular immunity
– Involves specialized set of lymphocytes called T cells that recognize foreign antigens on the surface of cells, organisms, or tissues: • Helper T cells • Cytotoxic T cells– T cells regulate proliferation and activity of other cells of the immune system: B cells, macrophages, neutrophils, etc.– Defense against: • Bacteria and viruses that are inside host cells and are inaccessible to antibodies. • Fungi, protozoa, and helminthes • Cancer cells • Transplanted tissue
CD4+ T helper Subsets• CD4+ Thelper cells can be divided into subsets based on their cytokine production.• Th1 cells produce IL-2, IFN-γ, TNF-β cytokines which activate cell mediated immunity• Th2 cells produce IL-4, IL-6, IL-10 that activate humoral immunity• These subsets were originally identified using mouse T- cell clones.
Cytotoxic T cells• CTLs recognize cells that have been infected – Virus – Transformed to tumor• CTL activation is divided into 2 phases – Activation and differentiation of naïve CTL – Effector recognizes Class I MHC/peptide and destroys target
Naïve CTLs cannot Kill; referred to as CTL-Ps (precursors) 3 signals needed for activation 1. Ag specific signal through TCR/MHC I+Ag 2. Co-stimulatory signal CD28(CTL)/B7 (APC) 3. IL-2 signaling inducing proliferation IL-2 is provided by TH1 or CTL-P itself IL-2R is expressed only after activation
How CTLs kill• 4 Phases In CTL Killing – Conjugate formation • LFA-1 (CTL) binds ICAMs (Target) – LFA-1 changes to high avidity if Ag Is Recognized – Activated LFA-1 persists for 5-10 mins – Membrane attack • Requires Ca2+ and energy – Granules release Perforins (65 kDa) and Granzymes (serine proteases) at the junctional space – Perforins polymerize forming cylindrical pores (5-20 nm), Ca2+ is needed – Granzymes enter target cell – Granzyme B can enter thru mannose-6-phosphate receptor in a vesicle – DNA fragmentation – CTL dissociation – Target cell destruction • Apoptotic death within a few hours
One cytotoxic T cell can kill multiple targets• A cytotoxic T cell causes its target to undergo apoptosis (cell suicide) by the focussed secretion of vesicles carrying cytotoxins.• The T cell binds to its target, delivers its cytotoxins, and moves on before it has a chance to be hurt itself (one T cell can kill another, so a T cell is not immune to the cytotoxins).
Cytokines These are biologically active substances secreted by lymphocytes, leucocytes, monocytes and other cells• Interleukins - Cytokines secreted by leukocytes that have the ability to act as signal molecules between different population of leukocytesIL-1~IL-29• Lymphokines - produced by lymphocytes• Monokines - produced exclusively by monocytes• Interferons - involved in antiviral responses• Colony Stimulating Factors - support the growth of cells in semisolid medias• Chemokines - promote chemotaxis.
Characteristics of cytokines1. Cytokines are peptide mediators or intracellular messengers which regulate immunological, inflammatory and reparative host responses2. They are highly potent hormone-like substances, active even at femtomolar (10-15 M) concentrations3. They differ from endocrine hormones in being produced not by specialised glands but by widely distributed cells4. Cytokines can affect the same cell responsible for their production (an autocrine function), near by cells (a paracrine function) or can be distributed by the circulatory system to their target cells (an endocrine function)5. Their production is influenced by nonspecific stimuli. Some cytokines also can induce the production
One kind of cytokines can be produced by differentcells. One kind of cells can secrete different cytokines IL-2 IL-4，6 TH1 IL-3,GM-CSF,TNF- TH2 IFN-γ,TNF-β IL-5
Cytokines initiate their actions by binding to specificmembrane receptors on target cells
self autocrineCytokines take effectin three ways paracrine Nearby Blood circulation endocrine Distance
Properties of cytokines The effects of cytokines are often pleiotropism, redundant, synergy, antagonism, and form a cytokine network• Pleiotropism refers to the ability of one cytokine having multiple effects on diverse cell types.
• Redundancy refers to the property of multiple cytokines having the same or overlapping functional effects
• Synergy refers to the property of two or more cytokines having greater than additive effects.
• Antagonism refers to the ability of one cytokine inhibiting the action of another
Interleukin-1• Interleukin I divided into Alpha and Beta• IL1 is secreted by Macrophages, Monocytes other nucleated cells• Stimulated by antigens, toxins, injury, inflammation• Inhibited by cyclosporins,corticosteiods,prostaglandins
Functions of Interleukin-1• IL1 stimulates T cells for the production of IL-2 and other lymhokines• Helps B cell proliferation• Synthesizes antibodies• Helps neutrophils in chemotaxis• Promotes phagocytosis• Promotes metabolic physiological and inflammatory responses by action on bone marrow
IL-1 initiates Fever• IL-1 is crucial in promoting fever and called as Pyrogen.• With the help of Tumor necrosis factor causes hematological changes in Septicemias, Shock and bacterial meningitis 61
IL-2• A T cell growth factor (TCGF) produced by activated T cells• IL-2 important actions: – It can increase immunoglobulin synthesis and J-chain transcription – Proliferation in B cells (with IL-4) – potently augment the cytolytic activity of natural killer (NK) cells – Stimulates cytotoxic T cells – Converts certain null cells (LGL) into lymphokine-activated killer (LAK) cells – Due to its effects on T-cells and B-cells IL-2 is a central regulator of immune response – Passes Blood Brain Barrier
IL-4• IL-4, like IL-2, is produced principally by activated CD4+ T cells• It is also produced by natural killer cells, and by mast cells and basophils
IL-4 Actions• IL-4 is the major B-cell growth factor (BCGF-1)• Vital for immunoglobulin class switch IgG to IgE and inhibits the synthesis of IgM and other IgG subtypes• IL-4 induces expression of class II major histocompatibility complex (MHC) molecules on B cells• Enhances the activity of cytotoxic T cells• IL-4 can inhibit responses of cells to IL-2
Tumor necrosis factor (TNF) TNFs were originally thought of as selective antitumour agents, but are now known to have a multiplicity of actions Occurs as two types; TNF- and TNF- TNF- is produced mainly by LPS activated monocytes and macrophages Resembles IL-1 in having a very wide spectrum of biological activities (endotoxic shock) Has immunomodulatory influences on other cytokines TNF- (lymphotoxin, LT) is produced mainly by activated Th0 and Th1 TNF- actions are similar to those of TNF-
InterferonsA group of glycoproteins that produced by human oranimal cells following the infection of virus andexposure to various inducing agents
Comparison of IFN- , IFN- , IFN-_____________________________________ Types Produced cells Main functions____________________________________IFN- leukocytes anti-virus,immune regulationIFN- fibroblasts anti virus, anti-tumorIFN- Th1,NK cells immunoregulatory functions_____________________________________
Immunological Tolerance Defined as the absence of specific immune response resulting from a previous exposure to the inducing antigen This nonreactivity is specific to the particular antigen, immune reactivity to other antigens being unaffected Tolerance to self is initially induced during embryonic life Tolerance occurs in both T and B cells Multiple mechanisms of tolerance exist
Immune tolerance is of two types 1. Natural tolerance 2. Acquired tolerance Natural tolerance Non-responsiveness to self antigens Any antigen that comes in contact with the immunological system during its embryonic life would be recognised as self antigen and would not provoke an immune response in the mature animal
Burnet‟s clonal selection model:DEVELOPMENT MATURITY Clonal Deletion Anti-self Self AgLymphocyte Activation DifferentiationAnti-non-self Foreign Ag + second signalLymphocyte
x Medawar’s experiment demonstratingneonatal tolerance induction (Nobel Prize)
The induction of tolerance depends on the species and immunocompetence of the host, physical nature, dose and route of administration of antigen Tolerance to humoral and cellular immunity is usually induced simultaneously When unresponsiveness is established for one branch of the immune response, it is called “split tolerance” Tolerance can be overcome spontaneously or by an injection of cross reacting immunogens
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