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1.1.12. Understand how T and B cells are activated
1.1.13. Understand the mechanisms of T and B cell functions
See semester one study notes, this is covered well (appropriate parts are marked out)
B Cell Activation:
Free Antigen Helper T cells secrete cytokines and activate them
Once activated they will mature into either plasma cells or memory cells
T Cell Activation:
Need costimulation to activate T cells
Encounter antigen in the lymph nodes brought by an APC in an MHCII complex
Antigen
Phagocytosed B cell of that clone recognises
free antigen and binds
MHCII Complex
B cell presents antigen to CD4
on MHCII-antigen complex
Presented to CD4
B cell division and activation
(maturation)
CD4 cell proliferation
Memory B cells Plasma cells
(make specific antibody)
MHCII + Antigen
MHCII complex +
T cell
MHCII + T cell +
B cell
Plasma cell Memory cell
Helper T cell + Antigen
T cell proliferate
B cell + same antigen
B Cell display on MHCII present to helper T
Helper T stimulate B cell to divide
T Cell functions:
1. CD8 + Cytotoxic T cells recognise endogenous pathogens presented on an altered MHCI
molecule (i.e. virus) and lyse the offending cell2. CD4 + T helper cells interact with MHCII molecules and initiate and immune response to
exogenous pathogens
Endogenous pathway:
Infected cell has foreign protein
Proteosomes break it down
Protein peptides transported to endoplasmic reticulum
Assemply of peptide class I complexes
Surface expression of complexes
Cell lysed by CD8+
Exogenous Pathway:
Antigen in endocytotic vesicle
Lysosomes break it down
Transport of MHCII to endosome
MHCII + protein peptides
MHCII complex expressed on cell
MHCII complex presented to CD4+
B cell Functions:
Produce antibodies Recognise free antigens Memory
4.1.1. Understand the mechanisms behind the four types of hypersensitivities
Immediate hypersensitivity – Allergy:
Allergy – the state of having igE to specific antigens referred to as allergens Allergic reactions – clinical s/s caused by immediate hypersensitivity to specific allergens Atopy – the state of being at high risk of allergies
Types of Allergies:
1. Type 1 Mediated by IgE bound to the surface of mast cells Genetically linked Includes anaphylaxis, asthma and hayfever
2. Type 2 Antibody mediated cytotoxic – IgG Include haemolytic disease of the newborn, autoimmune diseases, Graves’ disease,
Myasthenia Gravis
3. Type 3 Immune complexes are deposited into the tissues Tissue complexes local inflammation Blood complexes can be deposited into the organs e.g. the kidneys Include serum sickness, systematic lupus erythematosus (SLE), RA
4. Type 4 Delayed type hypersensitivity – cell mediated Mediated by factors released by T cells and macrophages Include IDDM, RA, MS, Inflammatory bowel disease, contact dermatitis, granuloma
formation
Type 1 Allergies:
Induced by allergens – usually very small, soluble and proteolytic “Atopic individuals” (people who make too much IgE) Genetic link, therefore family Hx is important in Dx Caused by mast cell degranulation vasodilation, smooth muscle contraction Can cause local effects (hayfever) or systemic effects (anaphylaxis) Common allergens include proteins, pollens, drugs, insect products and foods
Mechanisms of T1 allergy:
B cell comes into contact with the allergen
With the help of a Th2 cell the B cell is activated
B cells produce IgE
The IgE binds to the FCer receptor on mast cells
Mast cell is re-exposed to the same antigen in the future
Activation of mast cell degranulation release of mediators:
Vasoactive amines, lipid mediators immediate hypersensitivity reaction (in minutes after exposure)
Cytokines late-phase reaction (2-4 hrs)
S/S of T1 allergy:
Oedema, mucus secretion, smooth mm contraction – if the allergen is contained in one compartment i.e. the airways, creating this local response
Anaphylaxis and systemic responses – if the allergen is not contained and is introduced into circulation
If exposure to the allergen persists, mast cells increase bone marrow production of eosinophils which migrate to the affected tissue causing a second wave of inflammation and damage with more lasting effects
The route of the allergen will determine the type of IgE reaction:
Intravenous high dose: systemic anaphylaxis Subcutaneous low dose: wheal and flare Inhalation low dose: allergic rhinitis (upper airway), asthma (lower airway) Ingestion urticaria (skin rash), smooth mm contraction
Why are people affected with T1 allergies:
Environmental and genetic factors are at play
The Mast Cell:
The main mediator of T1 allergies, has two responses -
1. Primary Produced before degranulation Stored in granules – histamine, proteases, eosinophil chemotactic factor, neutrophil
chemotactic factor, heparin, serotonin
2. Secondary Synthesised after target cell activation or released by the breakdown of membrane
lipids Platelet activating factor, leukotrienes, prostaglandins, bradykinin, various cytokines
Detection:
3. Skin test – scrape skin, drop a bit of allergen onto skin4. RIST/ELISA – IgE levels tested in the blood5. RAST – antigen specific IgE levels are tested to specific allergens
Treatment:
Remove offending source Hyposensitisation (giving small doses of allergen to desentisise the person Antihistamines (allergic rhinitis) Cromolym sodium Adrenaliin (particularly for anaphylaxis) Chorticosteroids Immune deviation (experiamental)
Type 2 – Antibody:
Antibody mediated destruction of RBCs Activates compliment (due to antibody-antigen complexes() Activates antibody dependent cytotoxic cells (ADCC) and neutrophils “transfusion reaction” giving someone the wrong blood type Causes haemolytic disease of the newborn and drug induced haemolytic anaemia Activating antibodies they may bind to a hormone receptor and mimick that
hormone or inhibit it from binding. E.g. mimics the thyroid antibody = Grave’s Disease, or antibody blocks Ach receptor = Myasthenia Gravis
See lecture note diagrams
T2 Disorders:
1. Haemolytic disease of the newborn – happens when an Rh- mother has an Rh + fetus. On first exposure to the antigen the mother produces antibodies, the problem occurs if the mother has a second child and her antigens attack the fetal blood
2. Graves disease – antibodies mimick the action of the thyroid overactive thyroid
Type 3 – Immune Complex
Antigen-antibody complexes deposit in tissue spaces (mainly in the kidey, blood vessels and joints)
This activates complement Frustrated phagocytosis – happens when the neutrophils can’t phagocytose a whole
antigen – as its lodged in the tissue, so they degranulate Damage is caused by the release of neutrophils enzymes Arthus reaction = localised effect Serum sickness = systemic effect See lecture slide!
Immune complexes:
Lattices of antibody and antigen, small complexes = soluble; large complexes = insoluble Activate the classical complement pathway If complexes form rapidly at infection site or overwhelm our capabilities of
phagocytosing them (RBCs attach to them and take them to spleen for phagocytosis) inflammation
Sensitivities can arise due to having large quantities of antigen too many immune complexes in one space (i.e. if breathed in lots complexes in lungs), triggering inflammation
In addition to this if large quantities of immune complexes form and enter circulation (i.e. if a large dose of antigen is injected e.g. penicillin), circulating immune complexes can trigger acute inflammation usually in the joints, skin and kidneys
Immune complexes should be quite large to be effectively picked up by phagocytic cells. If there is too much antibody or antigen present then they are no longer phagocytosed, and therefore they remain in circulation and get lodged in organs i.e. kidneys
Formation of immune complexes can often occur after the following:1. Autoimmune disease – SLE, RA, Goodpastures syndrome2. Drug reactions – penicillin, sulfonamides3. Infectious disease – post-streptococcal, glomerulonephritis, meningitis, hepatitis,
mononucleosis, malaria, trypanosomiasis
Type IV – Delayed
Some chronic infections with pathogens lead to inflammation dominated by T cells and macrophages. Macrophages secrete IL-12 and T cells secrete interferon-y. These reactions take several days to develop and IL-12 and IF-y stay until the infection is cleared. When this type of delayed response is a problem when it takes place in reaction to harmless environmental antigens
Delayed sensitivity can be induced by harmless environmental antigens, for example contact dermatitis were inflammation takes place at the site of contact with the allergen (nickel combines with normal host proteins and behaves as a hapten). The nickel-protien complex is taken to the lymph nodes by APCs Th1 cells are recruited next time of exposure T memory cells migrate and secrete interferon-y inflammatory response develops over several days
Haptens:
Become antigenic after they combine with host proteins
T cell mediated (Th1) (as opposed to antibody mediated in T1-3) takes 48hours for symptoms
Delayed onset due to T cell activation via APCs and migration to site of infection Cytokine release – macrophage accumulation and release of enzymes tissue
destruction Intracellular parasites and bacteria activate bacteria, leading to:1. Possible granuloma formation2. Possible tubercle formation3. Tuberculin response4. Contact sensitivity
RA
Cytokine Delayed hyperactivity
MS
Syndrome Antigen ConsequenceDelayed Hypersensitivity Proteins
Insect venom Myobacterial proteins
Local skin skin swelling Erythema Induration
Cellular infiltrate Dermatitis Granuloma
Contact sensitivity Haptens Small molecules
Local epidermal reaction
Cellular infiltrate Contact dermatitis
Coeliac disease Gliadin Vilous atrophy Malabsorption
T cells stimulate macrophages by secreting interferon-y Both cells secrete TNF, which has a wide range of targets Macrophages stimulate T cells by secreting IL-12
Contact sensitivity:
Antigen is picked up in the skin by APC takes to lymph presents to T cell rash
T cell mediated diseases:
See lecture slides, diseases mentioned are –
Type 1 diabetees Ra MS
Type 4 RA (also type 3) - see lecture slides
6.1.11. Understand the concept of immunological tolerance
T Cell Tolerance and the thymus – central tolerance :
T cells develop in the thymus until early adolescence when they are proliferated outside of the thymus in the bone marrow and are matured in the thymus
When T cells enter the thymus for maturation the thymus uses two criteria to select T cells which are allowed to pass through the thymus and enter the periphery
1. T cells must recognise either self MHC 1 (using CD8) or self MHC 2 (using CD4). Cells which recognise the different MHCs are selected for being self restricted and are selected through positive selection
2. Normal self peptides are constantly entering the antigen presenting pathways, these self peptides must not be recognised by T cells. T cells which are selected as they do not recognise self peptides are described as being self tolerant and are selected through negative selection
Both of these steps take place one after the other in the thymus cortex (primary selection) and the thymus medulla (negative selection)
Over 90% of thymocytes die during positive and negative selection
Positive selection explained:
Pre-T cells migrate from the liver to the thymus (cortex) In the thymus they are called thymocytes and they express both CD4 and CD8 receptors In the thymus if a thymocytes receptor recognises self MHC on the thymic epithelium a
signal is transmitted allowing that cell to survive if not that cell dies (apoptosis) T cells which recognise MHC1 lose CD4 and those that recognise MHC 2 lose CD8
Negative selection explained:
Thymocytes move to the thymic medulla and encounter dendritic cells which actively sample and display a wide range of normal self-peptides from the environment and from the cytoplasm
In the thymic medulla if a T cell receptor recognises self-peptides and MHC it transmits a signal that forces the cell to die so cells capable of recognizing self-peptides p.lus MHC are deleted
The results of positive and negative selection is CD8 cells (which recognise intracellular infection) and CD4 cells (which can recognise antigens from phagosomes)
T cell Tolerance – central tolerance :
Tollerance = the state in which clones of T cells do not recognise self antigen
In the thymus tolerance is bought about by negative selection this process is not always 100% and sometimes T cells may escape into the periphery were they can cause damage
In order to avoid this we have peripheral tolerance
Peripheral tolerance = T cells that react to antigen and MHC in the absence of a danger signal (i.e. cytokines) provided by the innate immune system do not become activated, instead they become anergic
Anergic = permanently dormant and unable to respond to antigen again
Autoimmune Diseases:
For autoimmune disease to develop tolerance to self antigen must break down at 2 levels: in the thymus and in the periphery
Breakdown of thymic tolerance central tolerance – genetic factors:
Using diabetes as an example
1. Individual may inherit an MHC allele that does not strongly bind to the self antigen. For example if someone inherits an allele that does not present islet cell antigens well auto reactive T cells cannot be deleted
2. For negative selection to be effective the thymus must be able to display a wide range of the body’s normal peptide antigens. If an individual inherits gene polymorphisms that reduce the thymic expression of insulin there is a risk that T cells that recognise insulin will not be deleted
If one or both of these genetic factors are present self-reactive T cells may escape to the periphery
Although not everyone with these genetic factors will develop the disease as peripheral tolerance must break down before disease can develop
Breakdown of peripheral tolerance :
Using diabetes as an example
Even after binding to antigen plus MHC T cells will not react unless they recieive a danger signal. Self reactive T cells may reach the pancreatic islet and recognise antigen, however unless they receive the danger signal they will become anergic
A likely factor which could lead to a danger signal is infection which could lead to the innate immune system secreting cytokines. These danger signals would increase lymphocyte migration into the organ and then stimulate self reactive T cell responses
SO: genes and the environment work together to break down tolerance
So autoimmune diseases are a result of a genetic problem coupled with infection/inflammation
Other causes of autoimmune diseases:
Molecular mimic – some bacteria particles can look like self tissue we produce a CD8 for it AID
New epithelium – you own antigens may be altered for example by a drug AID
6.1.13. Apply the concepts of immunology to the practice of immunisation
Principles of vaccination:
Vaccines induce a primary adaptive immune response in the host, thereby establishing immunological memory. When the host is re-exposited a secondary response develops this is more rapid and includes high levels of high affinity antibody = active immunity
NB: passive immunity = transfer of effecter components e.g. immunoglobulin, T cells from one individual to another
Effect of antibodies produced by vaccines:
1. Prevent infection taking place in the first place – i.e. prevent a virus from binding to cells2. Induce antibodies that opsonize pathogens or promote antibody dependent cellular
cytotoxicity3. Induce antibodies that may not prevent an infection from taking place but prevent the
infection from causing harm
Effectiveness of antibody at preventing many infections explains why most current vaccines aim to produce memory B and Th2 cells, which together produce high-titre antibody on exposure to the pathogen or its toxin
NB: Titre = the degree of dilution of a substance such as an antibody, reflecting the strength of the solution.
Vaccines elicit adaptive immune responses in just the same way as pathogens:
Initial exposure (priming) antigen is conveyed to the lymph node by dendritic cells Dendritic cells must be stimulated by an innate immune system danger signal
How vaccines induce innate immune system danger signals:
Some vaccines stimulate a danger signal by introducing a live organism that reproduces in vivo initiating an innate response these vaccines elicit the best type of response
Others contain whole dead organisms which produce a weaker response Vaccines which only contain peptide components of pathogens (subunit vaccines) do
not produce a danger signal and will not induce a good immune response Another strategy is to add an adjuvant. An adjuvant is a chemical that directly causes
low grad inflammation thus activating the innate immune system and providing a danger signal
Initial vaccination usually induces only weak responses and poor immunological memory. Vaccine boosters improve antibody titres by somatic hypermutation and sustain the duration of immunological memory
Vaccine Components:
Organism Suspending fluid Preservatives to maintain shape and increase shelf life Stabilizers Antibiotics Adjuvant Induce an inflammatory response
o Aluminum hydroxideo Aluminum phosphate
Herd immunity describes a form of immunity that occurs when the vaccination of a significant portion of a population (or herd) provides a measure of protection for individuals who have not developed immunity.