Lecture #12 – Animal Immune Systems

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

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

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

Diagram of interferon activity
Interferons initiate production of proteins that inhibit viral reproduction Diagram of interferon activity

A natural killer cell (yellow) attacking a cancer cell (red).

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???

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

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????

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)

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

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???

Differentiation and testing result in an enormous variety of B and T cells – each capable of recognizing a single antigen ~ 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???

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

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

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???

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

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

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???

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…..

Rheumatoid Arthritis

Diabetes

Multiple Sclerosis

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…

Lecture #12 – Animal Immune Systems

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Lecture #12 – Animal Immune Systems

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