Mediators/ Actors of Innate Immunity 1) PRRs, TLRs, PAMPs, DAMPs 2) Complement System PAMPs: Pathogen Associated Molecular Patterns PRRs: Pathogen Recognition Receptors TLRs: Toll Like Receptors (on innate immune cells) PAMPs Intracellular Signalling Pathway Activation of Nuclear Factor kappa B Induction of gene transcription Cytokines PAMPs/TLRs: Innate Immunity Signalling TLR Signalling Pathway Immunity 34, May 27, 2011 PAMPs/DAMPs and PPRs TRENDS in Immunology, 2007; Vol.28 No.10 DAMPs: Damage Associated Molecular Patterns Trends in Immunology April 2011, Vol. 32, No. 4 Complement Pathway Complement Pathway: Outcome ANTIGEN (Ag) ANTIGEN (Antibody generator) Definition: Any foreign material that provokes a specific immune response upon entry of organism into the body is termed as antigen. Bacteria Virus Allergen Immunogenicity is the ability to induce humoral or cell mediated immune response. Antigenicity is ability of Ag to combine specifically with the end products of humoral or cell mediated immune response. All molecules having immunogenicity also have antigenicity but the reverse is not possible. Sites of entry of antigen (Ag) The antigen can enter through: • Skin • Mucus membrane • Wound • Cuts/ abrasions • Arthropods biting e.g Dengue virus Characteristics of an Antigen •Foreignness (Non-Self) The antigen is not normally present in the body and is foreign to the body •Molecular Size Large proteins act as potent immunogens, molecules < 10,000 Da are weakly immunogenic and very small one are also non-immunogenic. FOREIGNNESS The degree of immunogenicity depends upon the degree of foreigness. A molecule must be recognized as non self by biological system. Ability to recognize as self/non self arise during lymphocytic development. The greater the difference between two species the greater will be the immunogenicity. MOLECULAR SIZE The best immunogen tend to have a size approaching 100000(lac) daltons. Substance having molecular size less than 5000-10000 daltons are poor immunogens. CHEMICAL COMPOSITION Synthetic homopolymers tend to lack immunogenicity regardless of their size. Copolymers having two or more different amino acids are immunogenic. The addition of aromatic amino acid tend to enhance immunogenicity of the synthetic polymers. Homopolymers: Less immunogenic Heteropolymers: More immunogenic CONTRIBUTION OF BIOLOGICAL SYSTEM TO IMMUNOGENICITY Dose and route of immunogen Adjuvants DOSAGE AND ROUTE An insufficient dose will not stimulate a immune response either because it fails to activate enough lymphocytes or because it induces a non responsive state. An excessively high dose also can fail to induce a immune response because it causes lymphocytes to enter into a non responsive state. ADJUVANTS Adjuvants are the substances that when mixed with antigen and injected with them, enhance the immunogenicity of that antigen. Adjuvants are often used to boost the immune response when an antigen has low immunogenicity or small amounts of an antigens is available. Functions of Adjuvant Enhance immunogenicity and size of Ag. Prolong antigen persistence. Enhance co stimulatory signals. Induce granuloma formation. Stimulate lymphocytic proliferation non specifically. EPITOPES (antigenic determinants) The smallest unit of a complex antigen that is capable of inducing an immune response is called as epitope or Ag-determinant. An epitope is of roughly 5 amino acids or a sugar size. Virus EPITOPES (antigenic determinants) Epitopes are discrete sites on an antigen which are recognized by lymphocytes rather than the whole antigen. These are immunologically active regions of antigen that bind to antigen specific membrane receptors on lymphocytes or secreted antibodies. HAPTENS Haptens are small organic molecules that are antigenic but not immunogenic. The conjugate formed by coupling a hapten to a large carrier protein is immunogenic and elicits production of antihapten antibodies when injected into an animal. Example is drug allergy (penicillin allergy) Types of Antigen 1: Autoantigen These are the molecules on self tissues for which tolerance is inadequate. During lymphocyte differentiation some body molecules are not recognized and they act as antigen later on in life e.g. cells in retina of eye and in thyroid gland. 2: Alloantigen Alloantigens are cell surface markers of one individual that act as an antigen for other individuals of the same species. These are named after the Greek word “Allos” meaning “other” e.g. different blood group mismatching and organ transplant allergy. 3: Heterophilic Antigens The molecules from unrelated species that bear similar antigenic determinants leading to same lymphocytic response e.g. carbohydrate residues on surface of bacteria and RBCs, group A streptococci and human heart tissue, cardiolipin a phospholipid present in wide assortment of living tissues. 4: Super Antigens These are the bacterial toxins that are potent stimuli of T-Cells. Their presence in an infection activates T-Cell 100 times more than an ordinary antigen. In this process more cytokines are released leading to the cell death e.g. TSS (Toxic Shock Syndrome), Staphylococcus enterotoxins, Streptococcus pyrogenic exotoxins, these toxins are active in very low concentration (10-9 mol/L). 5: Allergens These are the antigens which provoke the allergic reactions in the body e.g. pollen, penicillin drugs, milk or fish proteins. Viral Epitopes: Molecular Patterns F-protein: Fusion protein in some viruses act as antigen Fate of the Antigen in the Body Antigen entry (Intravenous): becomes localized to liver, spleen, bone marrow, kidney and lungs. If by other routes: Ag localizes in lymphatic fluid and concentrated by the lymph nodes. Lymph nodes and the spleen are considered as very important in filtering and concentrating the antigen and circulate them to interact with the lymphocytes. Fate of Ag in the Body Th1 cytokines: IFN-γ, TNF-α Th2 cytokines: IL-4, IL-5, IL-10, IL-13, IL-33 Abs Production Chemoattraction & engulfment of Ag Structure of Antibody Structure of Antibody Structure of Antibody Classes of Antibody IgG (Characteristics) • 160 KDa MW, • Gamma (γ) epitopes on its heavy chain • Highest concentration in serum • Plays a major role in antibody mediated defense mechanisms • Can opsonize and agglutinate the antigen • Major isotype in secondary immune response • Can activate complement system if sufficient molecules are present • Monomer in nature • Class of Igs which can cross placenta • Also present in clostrum • Average life span in serum 23 days IgM (Characteristics) • 5-10% of the total serum • As monomer on B-cells and pentamer when secreted by plasma cells • 180 KDa molecular weight • Heavy chain have (μ) epitopes and have 10 antigen binding sites • J-chain join IgM molecule • Major isotype in primary immune response • Activates complement system, opsonization, neutralization, agglutination of Ag are major functions. • First antibody synthesized in the neonate • Monomeric IgM can be detected in chicken eggs and day old chicks • It is thought to be derived from oviduct secretions in hen • Average life span 5 days IgA (Characteristics) • 150 KDa molecular weight • These are monomeric in blood as IgA • These are also found as dimmer • Synthesized by plasma cells • These are major isotypes in secretions (13 %) • In intestinal tract it is bind to a secretory component (S IgA which protects it from proteolytic enzymes). • These are present in respiratory tract, UGT (uro-genital system), mammary glands, eyes and nasal secretions and prevent adherence of Ags to mucosal surfaces • IgA cannot activate complement mediated Ag lysis • It can agglutinate and neutralize the viruses (Ag) • Production/ day of this Ig class is more than other Ig classes • In Chicken (IgA) • Chicken IgA in monomer form has 170 KDa and in dimmer form it has the weight of 340 KDa • Chicken IgA has 4 heavy chain constant domains where as mammalian IgA has 3. • Average life span is 6 days IgE (Characteristics) • The molecular weight is 200 KDa • These have a low concentration in serum up to 1% • It mediates type-I hypersensitivity • It is largely responsible for immunity to invading parasites • FC receptors are present on mast cells and basophiles that lead to degranulation of these cells resulting in increased release of histamine causing allergic or hypersensitivity reactions like asthma. • It has a role in parasitic infections • IgE is named because E is from E-Ag of ragweed pollen that induces this class of Ig • Average life span of these Igs is 2.5 days IgD (Characteristics) • These are not found in serum • It is primarily act as B-cell Ag receptor (BCR) found on the surface of B-cells in association with IgM • These are monomers • These don’t activate complement system • These also play a role in immune suppression • The average life span of these Igs is 3 days • Its amount is 0.002% with respect to other Ig classes which is less Summary • IgG can cross placenta • IgG can activate and fix complement • Fc receptor binds to phagocytes in (IgG, IgA), B- lymphocytes (IgM, IgD) • On mast cells and basophills (IgE) receptors present. • IgM, IgG and IgD synthesized in spleen, lymph nodes • IgA and IgE important in intestinal and respiratory tract or mucosal surface infections IgY (Vet. Immunology by C. Tizzard) •It is the principal Ig in the chicken serum. It is somewhat similar to IgG and IgE. •It has 3 sub classes which are; IgY1, IgY2 and IgY3. •It consists of two heavy and two light chains. •Heavy chains are called as upsilon (υ), consists of one variable and four constant domains. •Its truncated isoform is also present that consists of two constant domains having molecular weight of 120 KDa. •These don’t have Fc region so cannot activate complement system. These are produced from splicing of H-chains mRNA. •Ducks and geese both have full sized and truncated IgY but the chicken has only full sized IgY not truncated. •This also lack hinge region, so these are somewhat inflexible and •can cause precipitation or agglutination only. IgY is related with anaphylaxis in chicken which has following signs; • Increased salivation • Defecation • Ruffled feathers • Dyspnea • Convulsions • Cyanosis • Collapse and finally death The major organs involved are lungs that lead to death. The chemical mediators involved are, histamine, serotonin, kinins and leukotrines.
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