Class 18 Adaptive Immunity PDF

11/13/24 Innate Immunity Barrier Mirrors what we do for food preservation. Can, Jar, or Plastic Low water (or high salt or other solutes) Acid pH Heat Friendly Microbes But what happens when Infectious Agents get through? 3...

11/13/24 Innate Immunity Barrier Mirrors what we do for food preservation. Can, Jar, or Plastic Low water (or high salt or other solutes) Acid pH Heat Friendly Microbes But what happens when Infectious Agents get through? 3 Innate Immunity: Protection force Phagocytic Cells Complement Defensins Cytokines Have evolved over hundreds of millions of years àBut Bacteria, Viruses, & Parasites have been evolving right along with those systems àAnd their lifespans are shorter, they evolve more quickly… àAlso. we need to recognize altered self (e.g., Cancer Cells), all while NOT ATTACKING self! 4 2  11/13/24 Develop an Array of PRR like receptors – but Rapidly Evolving! Recognize relatively small compound – or piece of a compound! Especially parts of different proteins! Start with a broad array of binding capabilities Goal is to recognize “anything” that is not self and attack “Not Self” is an essential control!!! 5 Generating diversity… Need to have a reasonable set of starting recognition. If tried to encode in DNA – all of population would have same set. If one mutant MO managed to avoid system, would infect entire population! If each organism randomly generates the starting recognition set Any one exception would only affect some of the population. Has been estimated that we can recognize over 100 million different “epitopes”. Not enough room in genome to encode. 6 3  11/13/24 Initial diversity Generated, not Stored. Not only randomly generate starting receptor set – we are then able to adjust starting recognition to create stronger, more specific generation. àThe ability to evolve in real time! àCombined with sufficient checks and balances to (usually) prevent our systems from attacking normal cells. Are two main branches of Immune System --- Cellular and Humoral Generate diversity and recognition with similar mechanisms Will cover Humoral Immunity in detail, Followed by Cellular Immunity overview. 7 Basic Unit of Immunoglobulin (Ig) (Also known as Antibody or Antitoxin) Y Shaped protein subunit (monomer) with different (overlapping) part names 2 identical “heavy chains” 2 identical “light chains” Can be easily split by proteases in the hinge region (see diagram) Antigen Binding fragment – Fab Constant Fragment – Fc The upper half of each Fab is known as the Variable Region The Antigen Binding site is at the top of both tips of the “Y” – and is the same on both tips. 8 4  11/13/24 There are Five Antibody (Ig) Classes, IgM, IgG, IgA, IgD, and IgE Each Class has a different ‘Constant’ region (but can have same binding sites!) Each Class has specific, distinct functions and properties. Can be multimeric (more than one basic unit connected together – all with same binding site!) One antibody, mono-, bi-, or penta-valent à all same binding site! One cell (B or T) à one set of receptors 9 IgM First class produced during primary response – and least specific! Principal class produced in response to some T-independent antigens 5% to 13% of circulating antibodies Pentamer Five monomeric subunits give 10 antigen-binding sites – amplifies possible weak binding! Aggregates very effectively Large size prevents crossing from bloodstream to tissues Primary role in bloodstream infections Most efficient class in triggering classical pathway of complement system 10 5  11/13/24 IgG 80% to 85% of total serum immunoglobulin Also exits bloodstream to enter tissues Provides longest-term protection: half-life is 21 days Generally first and most abundant circulating class produced during secondary response Protect via neutralization, aggregation, opsonization, complement activation, and antibody-dependent cellular cytotoxicity 11 IgG (cont) Transported across placenta to fetus’s bloodstream Women cautioned to avoid pathogens to which they lack immunity (e.g., Toxoplasma gondii) unless they already have protective antibodies! Degrade gradually over 6-month period By then, infant has had time to begin producing its own antibodies IgG found in colostrum (first breast milk); absorbed by newborn’s intestinal tract (also normal flora boost!) 12 6  11/13/24 IgA Most IgA is a dimer: secretory IgA (sIgA) Important in mucosal immunity Gastrointestinal, genitourinary, and respiratory tracts Secretions including saliva, tears, breast milk Protects breast-fed infants against intestinal pathogens Protection primarily via neutralizing toxins, viruses and interfering with attachment of microbes to host cells Produced by plasma cells of mucosa-associated lymphoid tissue (MALT); secretory component (polypeptide) added to attach antibody to mucus IgA is most abundant immunoglobulin class produced; Although in serum, monomeric form is only 10% to 13% of antibodies 13 Other Igs IgD Less than 1% of serum immunoglobulins Involved with development and maturation of antibody response Function in serum not clearly defined IgE Barely detectable in serum; most is tightly bound via Fc region to basophils and mast cells Allows these cells to detect, respond to antigens Antigen binds to two adjacent IgE molecules carried by mast cell, cell releases histamine and other inflammatory mediators Important in eliminating parasitic worms Basophils and mast cells also release chemicals when IgE binds to normally harmless foods, dusts, pollens, yielding allergic reactions of coughing, sneezing, swelling Some allergic (hypersensitivity) reactions can be life-threatening 14 7  11/13/24 Story Line -- Creating initial diversity Haemopoietic stem cell has 40 variable region segments 25 diversits region gene segments 6 joining regions Flexibility as to how cut and paste together through DNA deletion as well as transcription/splicing. Heavy Chain and Light Chain variable regions assembled differently 15 Clonal Selection Naïve B cells display a wide range of “possible affinities” for different antigens When one recognizes an antigen, it is primed for Activation 16 8  11/13/24 B Cell Activation Simple version: Antigen binding triggers proliferation of reactive B cell. But need to double check not reacting (or cross-reacting) with “self” 17 B Cell Activation – expanded... Before going all Rambo, B cell needs to have confirmation from a special T Helper lymphocyte that this is a threat and not self! 18 9  11/13/24 Generation of Plasma and Memory Cells – Simple version – selected cells expand, Some converting to Plasma Cells – secreting lots of antibody Others Form Memory cells (more in a moment) 19 Expansion – Primary Response When the few naive B cells that recognize a particular antigen are activated, they multiply to generate a population of clones Some form antibody-secreting plasma cells; produce IgM Others form a region the secondary lymphoid organ called a germinal center (GC) A region where various cell types, including YH cells direct activated B cells to optimize the response This results in changes to the proliferating B cells, including affinity maturation and class switching. 20 10  11/13/24 Affinity Maturation Form of natural selection among proliferating B cells Spontaneous mutations occur in multiplying B cells resulting in slight changes in B-cell receptor B cells that bind antigen longest are most likely to proliferate Plasma cells that descend from these cells secrete antibodies that bind the antigen more effectively 21 Class Switching B cells are originally programmed to produce plasma cells that secrete IgM As cells multiply some are induced to differentiate into plasma cells that secrete other antibody classes B cells in lymph nodes usually switch to IgG B cells in mucosal environments switch to IgA Activated B cells continue multiplying and generating increasing numbers of plasma cells as long as antigen is present; titer of antibody molecules increases. à Memory cells come from these B cells with enhanced affinity! 22 11  11/13/24 Primary Response Resolution Plasma cells undergo apoptosis after several days, but are replaced as long as antigen is present Antibody produced by those plasma cells are even more effective because of affinity maturation and class switching Some of the B-cell clones become memory B cells, which are long- lived even in the absence of antigen Specialized cells in the germinal center retains some of the antigens, maintaining memory of the response Once the antigen is cleared, the antibody response decreases As fewer molecules of antigen remain to stimulate the lymphocytes, activated lymphocytes undergo apoptosis 23 Secondary Response Mediated by memory cells Significantly faster, more effective than primary Pathogens usually eliminated before causing harm Vaccination exploits this natural phenomenon 24 12  11/13/24 Secondary Response Resolution Memory B cells that respond to a specific antigen are present in greater numbers Receptors already fine-tuned through affinity maturation Antibodies coded by these cells bind antigen effectively When activated, some quickly become plasma cells, producing IgG or IgA due to class switching Proliferating cells again undergo affinity maturation, generating even more effective antibodies Future exposures elicit an even stronger response 25 Roles for Antibodies Neutralization: prevents toxins, viruses from binding Opsonization: enhancement of phagocytosis Complement system activation: classical pathway Immobilization and prevention of adherence: binding to bacterial flagella or pili interferes Cross-linking: two arms of antigen bind separate antigens Antibody-dependent cellular cytotoxicity (ADCC): targets cell for destruction by natural killer (NK) cells 26 13  11/13/24 Antibody roles – illustrated. 27 Immune Tolerance -- Central Tolerance As lymphocytes mature (T cells in thymus and B cells in bone marrow), immature T and B cells that recognize “self” are eliminated. Peripheral tolerance Prevents mature T and B cells that were not eliminated during central tolerance from reacting against self or other harmless molecules. 28 14  11/13/24 Cell Mediated Immunity -- Immune response cannot begin until a lymphocyte becomes activated Dendritic cells (DCs) help activate the naive T cells DCs collect various antigens, including material from invading microbes Travel to regions where naive T cells gather Present pieces of the antigen Producing surface proteins, called co-stimulatory molecules if the antigen being presented is microbial or otherwise represents “danger” If a T cell TCR binds an antigen presented by a dendritic cell that also has co-stimulatory molecules, T-cell activation may result 29 T-Cell Activation – two+ paths Cytotoxic T cells (also CTL) Check cell surfaces displaying the antigen it recognizes. If found, induces the infected cells to undergo apoptosis Also produces memory cytotoxic T cells. Helper T Cells Produce a variety of cytokines that activate macrophages and B cells and support other T cells. Also produces memory helper cells. Regulatory T cells – once called suppressor T cells. Reduce immune responses – preventing overreaction! 30 15  11/13/24 T cell Activation Recognize that there are many subclassifications of T cells. (E.g., CD8, CD4, numerous Greek letters) For this section, I will only ask about Helper and Cytotoxic T-cells Other names will come up later, in context (e.g., HIV) 31 What is an antigen? A molecule that reacts specifically with antibody, a B-cell receptor, or a T-cell receptor Antigens differ in their effectiveness in stimulating an immune response An antigen that elicits immune response is immunogenic Proteins generally induce a strong response. lipids and nucleic acids do not (usually) Small molecules are usually not immunogenic T-dependent antigens—B cells that recognize them cannot be activated without TH cell help T-independent antigens—activate B cells without TH cell help Lipopolysaccharide (LPS) and carbohydrates with repeating subunits 32 16  11/13/24 Lymphatic System A collection of tissues and organs Brings populations of lymphocytes into contact with antigens Important because each lymphocyte recognizes only a specific epitope Appropriate lymphocyte must encounter appropriate antigen before immune response can develop 33 Lymphatic Vessels (or Lymphatics) Carry lymph Some fluid (not rbcs) from blood flowing through capillaries is pushed out to join tissue fluid; some re-enters capillaries, but some enters lymphatic vessels Lymph bathes tissues and contains antigens from tissues Lymph flows through lymph nodes before emptying back into circulatory system Inflammatory response causes more fluid to enter tissues; increases antigen-containing fluids entering lymphatic system 34 17  11/13/24 Primary Lymphoid Organs (include Bone Marrow and Thymus) Organs where lymphocytes develop Hematopoietic stem cells reside in bone marrow; give rise to all blood cells including lymphocytes B cells mature in bone marrow T cells migrate to thymus and mature there As lymphocytes develop, they acquire the ability to recognize distinct epitopes Central tolerance eliminates self-reactive lymphocytes Once mature, lymphocytes move via lymphatics to secondary lymphoid organs 35 Secondary Lymphoid Organs (Lymph nodes, spleen, tonsils, etc) Lymph nodes, spleen, tonsils Situated throughout body Peyer’s patches allow sampling of intestinal contents via specialized M cells, dendritic cells Part of mucosa-associated lymphoid tissue (MALT) Mucosal immunity prevents microbial invasion via mucous membranes 36 18  11/13/24 Natural Killer (NK) Cells Induce apoptosis in antibody-bound self cells Antibody-dependent cellular cytotoxicity (ADCC) Detects infected cells – triggers them to suicide Also recognizes cells missing Major Histocompatibility (MHC or ‘self’) markers Some Viruses use to deter cytotoxic T cells (which require antigen + MHC to respond) 37 “Flu-Like” Symptoms (NCI dictionary) Fever and chills Headache Muscle or body aches Cough Sore throat Runny nose Fatigue Nasea Vomiting diarrhea 38 19  11/13/24 Summary.... 39 If time – Piggyback on Antibodies Testing for antibodies in a person’s serum that react with known pathogens à evidence that has been exposed, possibly immune àAlso Blood Typing Using known antibodies to identify MOs or antigens Fluorescent antibody (often with microscopy) Direct and Indirect test (specificity vs sensitivity) Enzyme-linked immunosorbent assay (ELISA) Antibody 1 on plate. Add Antigen – capture and concentrate Antibody 2 – detect COVID Antigen test; Pregnancy test (human chorionic gonadotropin or HCG) 40 20