New Lecture:Immune System: Self/Non-self Recognition

• Living: Conservation of several processes. Determines mechanisms from organisms◦ Autopoieses or the dynamic process of self-generation

▪ Vertebrates conserve this by coherent internal relations among all self-production processes. This coherence is maintained through the nervous and immune systems

◦ Adaptation or the maintenance of a mutually satisfactory relationship between a living entity and its environment

The Immune System:• A dynamic (constantly changing) collection of organs, vessels, cells and molecules• Has access to virtually every part of the body• Distinguishes self from non-self in order to:

◦ protect against infection◦ recover from infection and tissue damage◦ Maintain an adequate relationship with the organisms environment

• Responds to molecular shapes◦ Unusual shapes (not normally found at or or in a particular place)◦ Familiar shapes in a unusual context (context is crucial, is this dangerous where it

currently is?, eg gut E.coli located elsewhere)

What can be recognised?• Antigen: often refers to the multiple epitopes that may exist on a biological substance (eg

protein) no a single structure• Microbes (or macromolecules from them)

◦ Viruses, fungi/yeasts, bacteria, parasites (multi and uni cellular)• Non-microbes:

◦ Dust particles: asthma isn't caused by an infectious agent – again context is crucial◦ Chemical polymers: even completely new molecules the body hasn't seen before◦ Cells form other people: very strong◦ Virtaully everythign above a certain molecular size and complexity (ie above 10-15

amino acids long for a protein – which is larger than many hormones)

Innate and Adaptive Immunity• Innate (non-specific):

◦ Resistance not improved by repeat infection

◦ Soluble factors: lysozymes, complement, acute phase proteins, cytokines, interferon

◦ Cells: Phagocytes, natural killer cells

• Adaptive (specific):◦ Resistance improved by repeat

infection (immunological memory)◦ Soluble factors: antibodies,

complement, cytokines◦ Cells: Lymphocytes (certain class(es)

per infection), phagocytes, antigen-presenting cells

Innate/Non-specific defenses:• First line barrier not improved by repeated exposure• Physical Barriers to infectious agents, microbicidal factors in body fluids (lysozyme and

complement), antiviral proteins (interferon), inflammatory response factors that lead to the recruitment and activation of phagocytic cells (neutrophils and macrophages)

• Generally very effective in preventing evasion and growth – act early and invoke adaptive• Two systems are interdependent

Protection of External Body Surfaces:• Importance shown in burn victims susceptibility to infection via damaged skin• Most infections enter the body via the epithelium of the masopharnyx, gut, lungs, or

genitourinary tract.

• Internal innate factors:◦ phagocytosis (cell eating)◦ the complement system

Antigens and Epitopes: means of recognition, ie receptors• Antibodies (soluble proteins) and the antigen-binding receptors on the surface of

(B)lymphocytes (cells bound) can only recognise and bind to structures of a certain size• The actual structures recognised are called antigenic determinants or epitopes

• The immunes system must be able to recognise an incredibly vast array or often never-before seen/unpredictable substances via its molecular shape – receptor interaction

Affinity and Cross-reactivity:• Receptors don't necessarily bind with a perfect fir to epitopes, for any particular antigen

some will fit well and others not so well• This strength of binding is called the affinity of the interaction and is achieved through non-

covalent forces• A perfect or high affinity is not necessarily required to produce an immune reaction

◦ This results in cross-reactivity, as some antibodies that have a high affinity for a particular antigen may also bind to other non-identical antigens of a similar shape to produce an immune response.

◦ Eg antibodies of Cowpox (vaccinia) can also bind to those of Smallpox (variola)

Quantities of the Specific/Adaptive Immune System:• Defence• Specificity: Response is directed principally against the agent that stimulates it• Diversity:• Adaptivity: Response to unexpected stimuli to provide rapid, appropriate responses• Self/non-self antigen discriminations and tolerance• Memory: Of previous responses and foreign agents – more vigorous responses subsequently

Innate Immunity(cellular and humoral factors)

Inflammation

Adaptive Immunity(cellular and humoral factors)

Distinguishing Self from Non-self• One of the most important characteristics of the immune system• The immune system in inherently able to response to both foreign and self antigens but

“learns” not to responsd to itself early in development• This was show in 1945 – normally when tissues are transplanted from one individual to

another are recognised and destroyed. However non-identical (dizygous) cattle twins that had exchanged blood cells in utero due to fused placentas were found to accept skin grafts from one another

• The continuous prescience of non-self antigens in utere before the immune system has matured leads to permanent unresponsiveness to those particular antigens. This is called acquired immunological tolerance (induced antigen-specific unresponsiveness)

• This process is not based on genetics, as then you would have responses against your mothers shapes from your fathers genes are responses against your fathers shapes from your mother genes.

Undesirable Consequences of Immunity:• Innocuous agents eg dust can cause hypersensitivity reactions (allergies)• Grafts of organs etc can be rejected• Autoimmune reactions in which self-antigens are targets (loss of tolerance)

New Lecture:Structure of the Immune System

Phagocytosis:• Blood phagocytes include granulocytes (mostly short-lived neutrophils) and blood

monocytes (longer lived, related to antigen-presenting cells). Both can migrate into tissues in response to a suitable stimulus

• Neutrophils are polymorphonuclear (nucleus of different shapes) leukocytes which are made in the bone marrow and are the main phagocytic cells. Along with bacterial fragments / whole bacteria they make up pus

Local Infection, Inflammation, The Complement System and Recruitment of Phagocytes:

Neutrophils and the Innate Immunity:• Phagocytosis is promoted by:

◦ Receptors for common bacterial cell wall components (weak interaction)◦ Receptors for C3b complement component (complement mediated opsonisation – high

affinity)◦ Receptors for the Fc region of antibodies (very high affinity)

Phagocytes of the Reticuloendothelial System:• all these cells are related to each other, all derived from bone marrow stem cells• they are much longer lived than neutrophils, and are strategically placed where we may find

a new antigen1. Brain microglial cells2. Skin Langerhans Cells3. Alveolar Macrophages4. Liver Kupffer Cells5. Kidney Mesanglial Cells6. Lymph Node Macrophages + dendritic cells7. Synovial A Cells8. Splenic macrophages

Immune System Organisation• Primary Lymphoid Organs: export their

cells to secondary organs◦ Thymus (produces T lymphocytes)◦ Bone Marrow (B lymphocytes)◦ Thoracic Duct◦ Lymphatic Vessels◦ Fetal Liver (stem cells + B

lymphocytes)

• Secondary Lymphoid Organs: contain mature lymphocytes – filter foreign bodies out of fluids are are usually the site of immune responses◦ Adenoids◦ Tonsils◦ Spleen◦ Appendix◦ Peyers Patches◦ Lymph nodes

Connections:• blood vessels (arteries, capillaries, veins)• lymphatic vessels – no circulatory pump other than muscles, form a tree structures• Thoracic duct breaks out into smaller and smaller vessels plus lymph nodes

The Role of the Lymphatic System:• Consist of leaky vessels with single celled valves and muscle

pumps to ensure one-way flow• Allows immune tissue to be collected from around cells and

taken to the lymph nodes for filtering• Parasites can block then leading to immense swelling

Blood Cells and Blood Cell Development: bone marrow of long bones predominatly• Lymphocytes come from pleuripotent haemopoietic stem cells which give rise to all blood

cells including RBCs, WBCs + phagocytes and platelets under the influence of cell surface molecules, cytokines and hormones

• Some stem cells mature in the bone marrow are are then exported through the blood stream to secondary lymphoid organs (B lymphocytes)

• Other progenies migrate from the bone marrow via the blood to the thymus (T-lymphocytes). These are also exported to other lymphoid tissues

• Basophil: mucosal surface protection• Eosinophil: anti-parasite immunity• Neutrophil: phagocyte of the innate immune system• Lymphocyte: adaptive immunity responses• Monocyte: long lived neutrophil, phagocytosis and antigen presenting

Blood Cells and Blood Development:

Circulatory Paths:• Most lymphocytes reside in lymphoid organs but B and T lymphocytes can circulate in

blood and lymph (around 10% at anyone time)• They can leave the blood stream, through High Endothelial Venules (HEV) and enter

various tissues, including all lymph nodes• The then percolate through larger and larger lymph vessels, through nodes, to the Thoracic

Duct and back to the blood• Not all classes recirculate as such – mediated by cell-surface and other signals• This ensures that the appropriate lymphocytes will eventually come into contact with an

antigen (and each other) and serves to disperse the resulting activated lymphocytes through the body's lymphoid tissues

Lymph Nodes (tissue immune organs):• Lymphocytes enter lymph nodes from afferent lymphatic vessels or in blood which

circulates through small vessels within the node called post-capillary venules• Only B and T cells are able to adhere specifically to specialised endothelial cells in the

venules walls, then enter the node through them• Lymphocytes within the node may leave via efferent lymphatic vessels which empty into the

thoracic duct.

Spleen (blood immune organ):• Antigens entering the blood are usually filtered out as blood passes through the spleen

Peyers Patches (digestive system immune organs):• Antigens that enter via the upper respiratory and GI systems are filterd through local lymph

nodes and several specialised lymphoid organs in the gut or Gut-Associated Lymphoid Tissue (GALT):◦ Tonsils, Adenoids, Peyers patches, appendix

• The blood and lymphatic vessels connect them all to give a comprehensive network that can rapidly bring antigens into contact with effector mechanisms

New Lecture:Antibodies in Immunity

How foreign antigens meet the immune system1. Antigen uptake by Langerhan's cells int eh skin (or other reticuloendothelial / Antigen-

Presenting cells)2. Langerhan's cells leave the skin and enter the lymphatic system3. They then enter a lymph node to become dendritic cells, presenting their antigen on the

surface4. T cells then cluster around them and those with receptors specific for that antigen become

stimulated (proliferate, differentiate and produce antibodies)

• The response of the immune system depends on the nature of the substance and route of entry (non-infections almost certainly treated differently than infectious)

• Intracellular infectious organisms (viruses) will be treated differently that extracellular (bacteria / parasites)

• Foreign material entering through the skin or respiratory or GI tract epithelium will meet phagocytic cells there that function of ingest foreign matter◦ some of these localised (non-specific) phagocytes will also be able to present antigens to

antigen-sensitive lymphocytes of the immune system (usually in secondary lymphoid organs)

◦ Here they are presented to lymphocytes by APC's using specialised structures which hold fragments and present them to specific receptors on lymphocytes

Lymphocytes and Immune Responses:• Effector subpopulations of lymphocytes: responsible for recognising, responding to and

disposing of antigenic substances ◦ Antibody production (B-lymphocytes)◦ Antigen-specific cytotoxicity (CD8 T-lymphocytes)◦ ADDC – Antibody dependent, cell mediated cytotoxicity (K cells)◦ Natural Killer cell activity (NK cells – don't use antibodies, but can also function as K

cells)• Regulator subpopulations: primarily involved in control of the effector cells

◦ Cytokine production (CD4 T-lymphocytes – control all the above processes) ◦ Helper T cells◦ Regulatory T cells

Shape Recognition:1. Recognition of common components of many microbes, eg bacterial wall cell sugars (non-

specific immunity)2. Recognition of some unique/uncommon characteristic of a particular foreign substance, eg

viral capsid protein (specific immunity)3. Common components in an uncommon context – eg myocardium shapes or streptococci

bacterium – cross reactivity

• B-lymphocytes can recognise a wide range of antigenic substances• T-lymphocytes can only recognise short peptide antigens (8-20 amino acids)

Antibodies:• Antibodies or immunoglobins (Ig) constitute a major group of blood proteins (~1%)• All immunoglobins have the same basic structure, although the blood of partticular

individuals has a unique set• They are made up of 4 polypeptide chains held together by disulfide bonds and non-covalent

interactions• There are two heavy chains (made up of 4 domains) and 2 light chains (each made up of 2

domains)• Each heavy chain and each light chain has a variable domain at its end, with hypervariable

regions which act as the fingers that bind to epitopes at the antigen binding site• The hinge region is made up of many proline residues• The CH2 (constant heavy chain domain 2) acts as the complement binding region• The CH2 and CH3 domains act as the Fc region which can bind to other immune cells

receptors (neutrophils and macrophages etc)

Antibodies at Birth:• Before birth the babies antibody levels rise from around 2 months onward as some of the

maternal antibodies can cross the placenta (baby isn't making any of their own)• After birth there is a decrease in overall antibodies as the maternal ones are docmposing and

the baby is only gaining a few from the mother milk. In the first few months after birth we begin to develop our own, so total level start to rise

Primary and Secondary Antibody Responses:• The initial response to an antigen not previously encountered is called the primary response

◦ There is usually a lag of a few day before antigen-specific antibodies appear in the blood◦ These antibodies lead to the neutralisation and removal of the antigen◦ In the absence of antigen, the antibodies of the primary response eventually decay

• Is a second exposure occurs to the same antigen, the secondary response is much more rapid and vigorous.◦ This is an indication that the immune system as remember the primary encounter and

adapted its resources to respond more effectively

B Cells and the Generation of Antibody Responses• Carried out mainly by B-lymphocytes (or B cells)• Primary responses come from the activation of virgin, antigen-sensitive B cells while

secondary responses come fro the activation of long0lived memory cells1. Antigens select and bind to the surface immunoglobin receptors (sIg) on antigen-sensitive B

cells in secondary lymphoid organs, that have receptors with sufficiently high affinity to bind enough antigen to overcome their activation threshold

2. Further activation signals are required from helper T-lymphocytes that allow determination of dangerous and non-dangerous antigens

3. Activation of antigen sensitive B-cells results in proliferation and maturation to form (cloned) plasma cells which secrete antibodies of the same antigen binding specificity as was present on the sIg of the precursor B cell.

4. A population of memory B cells is also made that can be re-stimulated subsequently to produce secondary responses

How Antibodies Work within the Body:

1. Direct Neutralisation• bind to the antigen (eg viral surface protein) and cover it up, preventing the antigen from

completing its lifecycle (ie preventing viral entry into a host cell)• Eg preventing bacterial metabolite production

2. Opsonisation• Phagocytic cells (eg granulocytes in the blood and macrophages in the tissues), through

cell surface receptors for Fc and C3b can bind with high affinity to antibody0coated foreign material or material which C3b (complement system protein) had adhered to

• Tis results in an increases efficiency of phagocytosis called opsonisation

3. Antibody Dependant Cell-Mediated Cytotoxicity ADCC• small population of lymphocytes in the blood which do not have classical B or T

lymphocyte characteristics – there are referred to as K cells• These cells have Fc receptors and C3b receptors on their surface and are thus able to

recognise and bind to antibody-coated or C3b-coated foreign antigenic material• However K cells are not phagocytic – they kill cellular material by delivering short

range cytotoxicity

4. Complement Activation (complement can act as both innate and specific in recognising foreign components)• Complement = group of 20 enzymes in the blood which act as an enzyme cascade

(amplify their effects)• Antigen-antibody complexes can trigger the Classical Pathway of the complement

cascade which is important in destroying or removing foreign material◦ Chemotaxis: attraction of phagocytic cells to the site of complement activation◦ Opsonisation: enhancement of the phagocytic process◦ Lysis: destruction of cell membranes (eg bacterial) to which the antibodies that

activated the complement cascade are attached.▪ This achieved by the insertion of complement proteins as a Membrane Attack

Complex (MAC) into the cell membrane

New LectureT-Cells in Immunity

Comparison of humoral and Cell-mediated Immunity:Humoral Immunity:

• Products of B-lymphocytes• Involves antibodies• Effective against antigens extracellular

antigens◦ viruses◦ toxins◦ extracellular bacteria

Cell-Mediated Immunity• Product of T-lymphocytes• Does not involve antibodies• Effective agains intracellular antigens

◦ virus infected cells◦ tumor cells◦ transplanted organs (different cells)

Types of Lymphocytes produced by the Thymus: exported to secondary lymphoid organs• Both cell types express T cell receptors (TCR) which recognise antigenic peptide fragments

presented to them by Major Histocompatability Complex (MHC) structures (each T cell only recognises one type of antigen – but there is a large amount of them)

• All T cells also express CD3 on their cell surfaces• CD8 exists on the surface of cytotoxic T cells and interacts with class I MHC determinants• CD4 on the surface of helper T cells interacts with calls II MHC determinants

T Cell Responses:1. Cytotoxic Responses: ability to kill cells that carry the antigen on their surface2. Helper Responses: produce helper cytokines (immunological hormones) that assist other

cells to respond3. Regulator Responses: produce inhibitory cytokines (immunological hormones)

• these cells have a marker called CD45RB• T cells can only recognise and bind to antigenic epitopes which are presented to them on the

surfaec of specialised structures called MHC determinants

The Major Histocompatability Complex (MHC):• MHC genes code for the cell surface structures that present T-lymphocytes with antigens

(also what is recognised in graft rejection)• These structures are called Human Leukocyte Antigens, HLA• These genes can be divided into class I and class II loci

Class I HLA:• surface structures found on virtually all

nucleated cells (not RBC's)• Co-dominantly expressed (ie both mum

and dads are expressed)• Polymorphic genes (many alleles in the

population, very small change of having the same as someone else's)

• Present antigenic peptides to CD8 (cytotoxic) T-cells

Class II HLA:• Surface structured found on specialised

antigen presenting cells and B-cells• Co-dominantly expressed• Polymorphic genes• Present antigenic peptides to CD4 (helper)

T-cells

Antigen Processing and Presentation:• The way antigens are processed depends on whether they are ingested (phagocytosis or

pinocytosis) or whether they originate from within the cell

1. Antigenic fragments that get presented on class I HLA's are usually derived from an infections process (eg viral infection) or due to normal breakdown of metabolites

◦ eg a cells is infected with a virus, during its lifecycle, some of the proteins are broken down and fragments are presented on class 1 MHC structures, they can then be recognised by cytotoxic CD8 T-lymphocytes

Activation:• Class I MHC present antigen fragments to cytotoxic T cell precursors by binding strongly,

causing activation◦ Typically this involves presentation by virus infected cells

• With the help of cytokines from helper T-lymphocytes, these activated cells will divide and differentiate into populations of effector cells which can recognise specific targets and kill them

2. Antigenic material can be taken up of APC's (macrophages or B-lymphocytes), broken down into small (peptide) fragments are presented on class II MHC structures. These will be presented to CD4 helper T-lymphocytes.

◦ Antigenic determinants of this variety usually result from processing of phagocytosed and degraded antigens

Activation:• Helper T cell precursors recognise antigens presenting by class II MHC structures on

antigen-presenting cells• When activated they divide and differentiate in a similar manner toe cytotoxic T cells and B

cells to form a population of cells which secrete helper factors called cytokines.

General Model of Lymphocyte Activation:1. Antigen binding to specific receptors on the surface of antigen-sensitive B or T lymphocytes

delivers the first activation signal2. The cell then expresses receptors for a growth hormone (cytokine)3. Following the receipt of a second signal, or cytokine secreted by an inducer (accessory) cell,

the lymphocyte is fully activated an is able to response by division and differentiation to its immune effector state.

Cytokines and Their Functions: immunological hormones• Low weight glycoproteins from a variety of cells• Regular the quality, amplitude and duration of immune and inflammatory reactions• Can act with both paracrine (most common), autocrine and endocrine actions• Often have multiple, overlapping actions depending on concentration, presence of other

cytokines, cell type / history of cell responding• May also be produced by cells from other blood cell lineages• Also have an effect on non-immune systems (eg nervous) and influence behavior (eg

sickness)• Often called interleukins (between leukocytes)• Responsible fro many symptoms of sickness – not exercise, social activity, eating,

temperature rises, hyperalgesia, wish to sleep more, desire nurturing – evolved responses to limit spread