Lecture #12 – Animal Immune Systems Blue macrophage ingesting green yeast cell
Key Concepts: Innate immunity provides broad-spectrum defense against many pathogens Acquired immunity is very specific, develops over time, and relies on B and T cells Antigen recognition properties of B and T cells B and T cell binding sites develop randomly! Integrated B and T cell function When the immune system goes wrong…
Pathogens have Antigens Some definitions…. Generates Pathology Pathogen = anything that causes disease Microbes (bacteria, protozoans), viruses, fungal spores, pollen, dust mites, etc Secretions (venoms, animal saliva) Non-self tissue cells (transplant rejections) Some cancer cells Antigens = cell surface proteins and other molecules that the body recognizes as non-self Generates Antibodies Pathogens have Antigens
Schematic of the human immune system The immune system is spread diffusely throughout the body – a system of organs, nodes and lymph vessels Schematic of the human immune system
Remember, the white blood cells are the defenders Diagram of the blood cells
Some WBC’s circulate though the lymph, the blood and the interstitial fluid Some are permanently housed in lymph nodes, thymus gland, spleen, appendix and a few other glands
Table showing the stages of defense Defense is step-wise 90% of pathogens are neutralized by innate immunity Multiple strategies to destroy pathogens Any remaining pathogens are normally attacked by the acquired immune system Two integrated stages Table showing the stages of defense
Innate Immunity – you are born with it Pathogens are ubiquitous Innate immunity includes both external and internal systems to eliminate pathogens Any and all pathogens are targeted This system does not recognize specific pathogens – it goes after any non-self cell or molecule Bacteria on the point of a straight pin
Innate Immunity – external defenses Skin – vital barrier Mucous membranes – trap, cilia evacuate Secretions – skin and mucous membranes secrete anti-microbial proteins; stomach secretes acids Sweeping cilia in trachea
Innate Immunity – internal defenses Sometimes pathogens get past the barriers and into the tissues Non-specific WBC’s attack Neutrophils Monocytes macrophages Dendritic cells Eosinophils Basophils
Innate Immunity – internal defenses Phagocytic WBC’s cells ingest and destroy microbes in the tissues Neutrophils – the most abundant, but short-lived Macrophages develop from monocytes – large and long-lived, also stimulate acquired Dendritic cells – mostly function to stimulate the acquired immune system
Model of a macrophage ingesting a fungal spore
Micrograph of macrophage ingesting bacteria
Innate Immunity – internal defenses Eosinophils destroy multi-cellular parasites by releasing toxic enzymes Also contribute to allergic responses Basophils contribute to inflammatory and allergic responses Among human parasitic diseases, schistosomiasis ranks second behind malaria in terms of socio-economic and public health importance in tropical and subtropical areas. Schistosoma mansoni
Additional Internal Defenses Antimicrobial proteins Lysosymes work in macrophages; also found in saliva, tears and mucous Complement proteins result in lysis; also help trigger inflammation and activate acquired immunity Interferons limit intra-cellular spread of viruses Defensins are secreted by macrophages, attack pathogens Natural killer cells attack virus-infected cells and cancer cells The inflammatory response
Diagram showing complement protein function Complement Protein Function: these proteins complement other immune system processes Diagram showing complement protein function
Additional Internal Defenses Antimicrobial proteins Lysosymes work in macrophages; also found in saliva, tears and mucous Complement proteins result in lysis; also help trigger inflammation and activate acquired immunity Interferons limit intra-cellular spread of viruses Defensins are secreted by macrophages, attack pathogens Natural killer cells attack virus-infected cells and cancer cells The inflammatory response
Diagram of interferon activity Interferons initiate production of proteins that inhibit viral reproduction Diagram of interferon activity
Additional Internal Defenses Antimicrobial proteins Lysosymes work in macrophages; also found in saliva, tears and mucous Complement proteins result in lysis; also help trigger inflammation and activate acquired immunity Interferons limit intra-cellular spread of viruses Defensins are secreted by macrophages, attack pathogens Natural killer cells attack virus-infected cells and cancer cells The inflammatory response
Additional Internal Defenses Antimicrobial proteins Lysosymes work in macrophages; also found in saliva, tears and mucous Complement proteins result in lysis; also help trigger inflammation and activate acquired immunity Interferons limit intra-cellular spread of viruses Defensins are secreted by macrophages, attack pathogens Natural killer cells attack virus-infected cells and cancer cells The inflammatory response
A natural killer cell (yellow) attacking a cancer cell (red).
Additional Internal Defenses Antimicrobial proteins Lysosymes work in macrophages; also found in saliva, tears and mucous Complement proteins result in lysis; also help trigger inflammation and activate acquired immunity Interferons limit intra-cellular spread of viruses Defensins are secreted by macrophages, attack pathogens Natural killer cells attack virus-infected cells and cancer cells The inflammatory response
The Inflammatory Response Usually localized, in response to tissue injury Cascade of events May also be systemic – increased WBC release from bone marrow; fever Diagram of the inflammatory response
Critical Thinking Why do tissues swell near a cut???
Critical Thinking Why do tissues swell near a cut???
Invertebrates Also Have Innate Defense Systems Amoeboid cells ingest by phagocytosis in echinoderms Insect exoskeleton acts as a barrier similar to skin Hemocytes in insect hemolymph function similarly to vertebrate innate internal defenses Research indicates little immune system memory Little capacity for acquired immunity as seen in vertebrates
Same diagram of step-wise immune system function Defense is step-wise 90% of pathogens are neutralized by innate immunity – both external and internal Any remaining pathogens are normally attacked by the acquired immune system Same diagram of step-wise immune system function
Acquired Immunity Develops over time, in response to exposure to pathogens Highly specific – lymphocytes develop that match each incoming pathogen B cells and T cells Some circulate in tissues; some are permanently located in lymph nodes, the spleen and other lymph system structures Pathogen contact with lymphocytes, phagocytes, and other triggers initiates rapid immune responses
Remember – the lymph system is closely tied to the circulatory system Lymph vessels absorb excess fluids in capillary beds Pathogens in the blood are rapidly exposed to the phagocytes and lymphocytes in the lymph system Every heart beat pushes blood, and any pathogens it carries, past the immune system structures
The next 3 slides show the relationship between the capillary beds and the lymph vessels
Lymph fluid is returned to blood at shoulder ducts Diagram of lymphatic system
Remember – the lymph system is closely tied to the circulatory system Lymph vessels absorb excess fluids in capillary beds Pathogens in the blood are rapidly exposed to the phagocytes and lymphocytes in the lymph system Every heart beat pushes blood, and any pathogens it carries, past the immune system structures
Antigen Recognition by B and T Cells Remember, antigens are the non-self molecules that initiate the immune response Mostly cell surface proteins, other cell surface molecules, or toxins dissolved in fluid (venoms and other secretions) Most pathogens have several different kinds of antigens Because of this, there are usually several different lymphocytes that recognize and respond to the pathogen Antigens have specific binding sites
Membranes are complex, with many surface molecules Diagram showing structure of the cell membrane
Antigen Recognition by B and T Cells Remember, antigens are the non-self molecules that initiate the immune response Mostly cell surface proteins, other cell surface molecules, or toxins dissolved in fluid (venoms and other secretions) Most pathogens have several different kinds of antigens Because of this, there are usually several different lymphocytes that recognize and respond to the pathogen Antigens have specific binding sites
Epitopes are the specific binding sites found on all antigens Diagram showing epitope structure
Lymphocytes – B and T Cells Remember, lymphocytes are one of the categories of white blood cells Each B or T cell has ~100,000 antigen receptors – all of the exact same type Each B or T cell recognizes a single antigen The receptor molecules and recognition process are different for B cells vs. T cells Both types of receptors are protein-based Both have both constant and variable regions
Diagram showing development of all the blood cells and how lymphocytes have a separate origin from other white blood cells. A different cell lineage than the other WBC’s
Lymphocytes – B and T Cells Remember, lymphocytes are one of the categories of white blood cells Each B or T cell has ~100,000 antigen receptors – all of the exact same type Each B or T cell recognizes a single antigen The receptor molecules and recognition process are different for B cells vs. T cells Both types of receptors are protein-based Both have both constant and variable regions
Constant regions have stable amino acid sequences from cell to cell; Variable regions have different amino acid sequences from cell to cell Diagram showing the receptor molecules in B cells and T cells. This diagram is used several times in the next sequence of slides.
Antigen Recognition – B Cells B cell receptors are Y-shaped Each branch of the “Y” has 2 parts, called chains Inner, heavy chain makes the full “Y” Outer, light chain is located on the branches of the “Y” Both chains are proteins Chains are linked by chemical bonds The bottom of the “Y” is anchored in the B cell membrane
B Cell Receptor Structure
The protein structure of a B cell receptor
Antigen Recognition – B Cells The bottom regions of both chains have constant amino acid sequences The outer branches of both chains have variable amino acid sequences These variable ends are the antigen binding sites They bind directly to the epitopes B cells recognize unaltered antigens!
B Cell Receptor Structure
Antigen Recognition – T Cells T cell receptors are unbranched α chain and β chain are chemically linked Both are anchored in the membrane Both have basal constant regions and terminal variable regions A single antigen binding site is at the terminus
T cell receptor structure
T Cells DO NOT recognize intact antigens on intact pathogens T cells recognize antigen fragments that have been bound to a self-cell protein called a major histocompatibility molecule MHC major histocompatibility complex of genes codes for these molecules MHC molecules bind to antigen fragments inside a self-cell, and present the fragments at the surface of the cell T cells detect the presented antigen+MHC complex
MHC – self-cell proteins Diagram showing the production of MHC molecules, how they become attached to antigen fragments, and how the complex is presented at the cell surface. This diagram is used repeatedly in the next sequence of slides. Few MHC molecules per individual, but can form many unique shapes when bound to antigen fragments, and only then are they expressed at the cell surface
T Cells DO NOT recognize intact antigens on intact pathogens T cells recognize antigen fragments that have been bound to a self-cell protein called a major histocompatibility molecule MHC major histocompatibility complex of genes codes for these molecules MHC molecules bind to antigen fragments inside a self-cell, and present the fragments at the surface of the cell T cells detect the presented antigen+MHC complex
Development of MHC Variation MHC alleles are numerous Many more than just the 2 alleles common for most genes (ie: not just dominant vs. recessive) As a result, MHC molecules are the most polymorphic proteins known Because of the high degree of variation, it is very rare for any two individuals to have the exact same set of MHC molecules MHC molecules are unique to the “self” Help to distinguish “self” from “non-self” cells This is why transplants are rejected????
Development of MHC Variation MHC alleles are numerous Many more than just the 2 alleles common for most genes (ie: not just dominant vs. recessive) As a result, MHC molecules are the most polymorphic proteins known Because of the high degree of variation, it is very rare for any two individuals to have the exact same set of MHC molecules MHC molecules are unique to the “self” Help to distinguish “self” from “non-self” cells This is why transplants are rejected????
T Cells DO NOT recognize intact antigens on intact pathogens T cells recognize antigen fragments that have been bound to a self-cell protein called a major histocompatibility molecule MHC major histocompatibility complex of genes codes for these molecules MHC molecules bind to antigen fragments inside a self-cell, and present the fragments at the surface of the cell T cells detect the presented antigen+MHC complex
Two classes of MHC molecules: each found in a different type of antigen presenting cell
Class I MHC Found in most nucleated cells They bind antigen fragments if the cell has been infected, or is cancerous Class I MHC+antigen complexes are recognized by cytotoxic T cells Cytotoxic T cells then destroy the infected or cancerous cell
Antigen Presentation – Class I MHC molecules are presented on infected or cancerous cells
Class II MHC Found in dendritic cells, macrophages and B cells Present antigens from pathogens that have been engulfed by phagocytosis Class II MHC+antigen complexes are recognized by helper T cells Activated helper T cells begin a cascade of events that control the infection
Antigen Presentation – Class II MHC molecules are presented on phagocytic cells
In both cases, the T cell recognizes ONLY THE COMBINATION of antigen + self-protein Discuss similarities and differences
Review: B and T Cell Receptors B cell receptors bind directly to antigen on intact pathogen T cell receptors bind to MHC+antigen complex on self-cells
Review: B and T Cell Receptors Remember – both B and T cells have multiple receptors per cell (as many as 100,000), all identical
Lymphocyte (B & T cell) Development Lymphocytes are all produced from stem cells in the bone marrow Some mature in the bone marrow (B cells) The rest mature in the thymus gland (T cells)
Lymphocyte (B & T cell) Development Maturation = development of the B and T cell receptors Once the cells are fully differentiated, they migrate into the rest of the body Some stay permanently in the organs of the lymph system Some circulate constantly through blood, lymph and interstitial fluids
Lymphocyte (B & T cell) Development Step 1 – generation of diversity Step 2 – testing and removal Step 3 – clonal selection Steps 1 and 2 occur during the development of the B and T cells Step 3 occurs after exposure of the fully developed B and T cells to antigens
Lymphocyte (B & T cell) Development Step 1 – generation of diversity The genes that code for the antigen receptors are randomly rearranged by enzymes during lymphocyte maturation These are the genes that code for the variable regions of the light and heavy chains of B cells Ditto for the variable regions of the α and β chains of T cells These chains are then linked together to form the T cell receptor molecule
Example: gene re-alignment for the light chain of a B cell receptor. Diagram showing the development of diversity in the receptors of a B cell. This diagram is used repeatedly in the next sequence of slides.
The coding gene has 40 variable (V) segments and 5 joining (J) segments
Recombinase enzymes randomly snip and join! During differentiation of each B cell, one V segment is snipped out and attached to one J segment. Recombinase enzymes randomly snip and join!
40 V regions x 5 J regions = 200 possible combinations of V and J in the functional gene. Each cell ends up with only one of these possible combinations for the light chain. See Freeman for explanation of how diversity develops – light chain and heavy chain form independently, heavy chain has more V regions
The V+J segment is attached via an intron to the C segment that codes for the constant region of the light chain.
This “new” gene is processed and translated into the protein that makes up the light chain
The DNA coding for the heavy chain goes through the same kind of random rearrangement process, but there are more V regions See Freeman for explanation of how diversity develops – light chain and heavy chain form independently, heavy chain has more V regions
The light and heavy chains form independently and are then linked Additional variation occurs during the linkage Thus the enormous number of possible receptors Many millions of different receptors are produced in B cells!!! See Freeman for explanation of how diversity develops – light chain and heavy chain form independently, heavy chain has more V regions
Lymphocyte (B & T cell) Development Step 1 – generation of diversity The genes that code for the antigen receptors are randomly rearranged by enzymes during lymphocyte maturation These are the genes that code for the variable regions of the light and heavy chains of B cells Ditto for the variable regions of the α and β chains of T cells These chains are then linked together to form the T cell receptor molecule
Lymphocyte (B & T cell) Development Step 2 – testing and removal The rearrangement process is entirely random Each new receptor is “tested” against self-cells – both during development and during migration into lymph system organs Receptors that bind to self-cells or self-MHC molecules are eliminated or deactivated
Critical Thinking Why would testing be so important???
Critical Thinking Why would testing be so important???
Differentiation and testing result in an enormous variety of B and T cells – each capable of recognizing a single antigen ~ 1010 - 1014 different B cells!!!! Similar numbers of different T cells Usually no duplication – you start out with a single cell of each type Clonal selection (the next step) builds a population of duplicate lymphocytes
Lymphocyte (B & T cell) Development Step 3 – clonal selection Each B and T cell has receptors that are specific to a single antigen Incoming pathogens typically display several antigens Virtually always, there is a B or T cell receptor to match at least one of the pathogen’s antigens
Critical Thinking How are incoming pathogens exposed to these myriad B and T cells???
Critical Thinking How are incoming pathogens exposed to these myriad B and T cells???
Lymphocyte (B & T cell) Development Step 3 – clonal selection When a lymphocyte receptor encounters a matching antigen, the lymphocyte is activated Activation = stimulation of the lymphocyte to begin mitotic cloning Diagram showing clonal expansion of selected B cell
Lymphocyte (B & T cell) Development Step 3 – clonal selection Duplicate lymphocytes are rapidly produced Two clonal populations form Effector cells are short-lived and carry out the immune system response (varies based on type of lymphocyte – more later) Memory cells are long-lived and “remember” the epitope Memory cells allow for rapid response to that same pathogen the next time it enters the body Memory cells confer active immunity
Clones divide into two populations: effector and memory Diagram showing clonal expansion of selected B cell
Lymphocyte (B & T cell) Development Step 3 – clonal selection Duplicate lymphocytes are rapidly produced Two clonal populations form Effector cells are short-lived and carry out the immune system response (varies based on type of lymphocyte – more later) Memory cells are long-lived and “remember” the epitope Memory cells allow for rapid response to that same pathogen the next time it enters the body Memory cells confer active immunity
Step 3 – clonal selection Memory cells accumulate over repeated exposure to the same pathogen EX is for B cells; T cells also accumulate Graph showing accumulation of memory cells after repeated exposures.
Critical thinking If the immune system response is so rapidly initiated, why do we ever get sick???
Critical thinking If the immune system response is so rapidly initiated, why do we ever get sick???
Integrated B and T Cell Function Diagram showing how B cell and T cell functions are integrated
Simultaneous
Diagram of helper T cell binding to antigen presenting cell. Helper T Cell Function Nearly all pathogens activate helper T cells Dendritic phagocytes 1o activate naïve helper T cells Important in primary immune response Macrophages 1o activate memory helper T cells Important in secondary immune response Diagram of helper T cell binding to antigen presenting cell.
Helper T Cell Function Clones of active and memory T cells develop after exposure Active helper T cells secrete proteins that stimulate cytotoxic T cells and B cells
Diagram showing activated helper T cell functions. Active helper T cells stimulate the rest of the immune system: both cytotoxic T cells and B cells Diagram showing activated helper T cell functions.
Cytotoxic T Cell Function Activated cytotoxic T cells release proteins that perforate target cells & initiate apoptosis The activated T cell releases, and moves on to target additional infected or cancer cells Diagram showing cytotoxic T cell function Class I MHC molecule
B Cell Function Remember, B cells recognize and bind to specific intact pathogens B cells also engulf some pathogens by phagocytosis Antigens are presented on the B cell surface These antigens are recognized by helper T cells Helper T cells activate the B cell Only its one specific antigen can be presented by each type of B cell
Some B cells are activated directly by exposure to the antigen
B Cell Function Remember, B cells recognize and bind to specific intact pathogens B cells also engulf some pathogens by phagocytosis Antigens are presented on the B cell surface These antigens are recognized by helper T cells Helper T cells activate the B cell Only its one specific antigen can be presented by each type of B cell
Diagram showing an activated helper T activating a B cell Most B cells are activated by proteins secreted from active helper T cells Diagram showing an activated helper T activating a B cell
B Cell Function Remember, B cells recognize and bind to specific intact pathogens B cells also engulf some pathogens by phagocytosis Antigens are presented on the B cell surface These antigens are recognized by helper T cells Helper T cells activate the B cell Only its one specific antigen can be presented by each type of B cell
Diagram showing secretion of antibodies from activated B cell B Cell Function Activated B cells form 2 clones – plasma cells and memory cells Plasma cells release antibodies Diagram showing secretion of antibodies from activated B cell
Table of antibodies and their functions Each activated B cells produces thousands of clones Each clonal B cell releases nearly a billion antibodies 2000 antibodies per second Each B cell has a 4 – 5 day life span Table of antibodies and their functions
Antibodies Five classes of antibodies are secreted Each recognizes and attacks specific pathogens Read through this table for understanding; don’t memorize
Antibodies Only one antibody per type of B cell But remember, most pathogens have multiple antigens with multiple epitopes Many B cells are activated
Antibody Mediated Pathogen Disposal Diagram showing how antibodies work
Integrated B and T Cell Function Responses to pathogens are coordinated and simultaneous, NOT mutually exclusive All components of the immune system are activated Positive feedback increases function
Active vs. Passive Immunity Active immunity is generated when the acquired immune system is activated Memory cells are generated Exposure to pathogen OR vaccination with inactivated pathogen that still retains antigens Confers long-term protection (often, lifetime) Passive immunity is generated when antibodies alone are transferred Does not generate memory cells Antibodies cross placenta; are injected Short-term protection
Critical Thinking What would be the advantage of passive immunity???
Critical Thinking What would be the advantage of passive immunity???
Immune System Failure Allergic responses Autoimmune diseases Hypersensitive response to allergenic antigens Antibody tails bind to mast cells Exposure causes massive histamine release Autoimmune diseases Immune system fails to distinguish self-cells Immunodeficiency diseases Immune system fails Can be genetic, developmental, or acquired AIDS; also some cancers, chemotherapy, stress
Allergic Responses Most generated by IgE antibodies Antibody tail binds to mast cells IgE accumulates on mast cell surface Eventually, allergen binds between 2 IgE This triggers massive release of histamine Histamine dilates blood vessels…..
Immune System Failure Allergic responses Autoimmune diseases Hypersensitive response to allergenic antigens Antibody tails bind to mast cells Exposure causes massive histamine release Autoimmune diseases Immune system fails to distinguish self-cells Immunodeficiency diseases Immune system fails Can be genetic, developmental, or acquired AIDS; also some cancers, chemotherapy, stress
Rheumatoid Arthritis
Diabetes
Multiple Sclerosis
Lupus
Immune System Failure Allergic responses Autoimmune diseases Hypersensitive response to allergenic antigens Antibody tails bind to mast cells Exposure causes massive histamine release Autoimmune diseases Immune system fails to distinguish self-cells Immunodeficiency diseases Immune system fails Can be genetic, developmental, or acquired AIDS; also some cancers, chemotherapy, stress
T Cell HIV
2007 – 40 million people are infected by HIV; 15 million children have been orphaned by AIDS Graph showing relationship between HIV concentration, antibody concentration and T cell concentration over time. HIV attacks helper T cells especially
REVIEW – Key Concepts: Innate immunity provides broad-spectrum defense against many pathogens Acquired immunity is very specific, develops over time, and relies on B and T cells Antigen recognition properties of B and T cells B and T cell binding sites develop randomly! Integrated B and T cell function When the immune system goes wrong…