Immunology Flashcards
What are the 2 mechanisms of human defence?
Innate immunity
Adaptive immunity
Where do antibodies come from?
Synthesised and secreted by B lymphocytes
Before birth, they’re produced in the foetal liver
After birth- produced in the bone marrow from haematoitic stem cells
Outline the development of B cell sin the bone marrow
Bone marrow stroma provides the microenvironment for B lymphocytes to mature AND differentiate
Useful cells- exported to the periphery
Self- reactive B cells are disposed of
Ensures that each cell only has 1 specificity
Describe the T cell- independent responses
B cells (B-1) reside in the marginal zone of speed and are activated by polysaccharide and lipid antigens
Majority of B cells are follicular B cells (B-2) because they circulate through the follicles of lymphoid organs
Follicular B cells make the bulk of T-cell dependant class switches
They have a high-affinity antibody responses to protein antigens an give rise to long-lived plasma cells
Describe the 6 stages of B cell activation
- Membrane bound Ig recognises and binds to the microbe
- Receptor is non-covalently associated with 2 proteins: Ig alpha and beta
- Ig-alpha and Ig- beta contain conserved immunoreceptor tyrosine based activation motifs
- Signals initiated by binding to antigen receptor are transduced by recpetor-associated proteins
- Net result= Activation of transcription factors such as myc, NF-kb or AP-1
- Transcription factors switch on genes whose proteins are involved in cell proliferation and antibody synthesis
Describe the basic structure of an antibody
Glycoproteins
Range in size from 150kDa-900kDa
2x Heavy chains and 2x Light chains
N terminal= antigen binding site
C- terminal= Fc region
What are heavy chains?
Encoded by a single locus on human chromosome 14
Type of heavy chain present defines class of antibody
Distinct heavy chains differ in size and amino acid composition
What are light chains?
Encoded by 2 gene loci
Kappa locus is on human c-some 2
Lambda locus on human csome 22
Molecular weight of each light chain is approx 25kDa
Determine the specificity of antigen binding
2 identical light chains are either bases on either kappa or lambda
Describe the structure and function of immunoglobulins
Immunoglobulins are bifunctional proteins:
Thye must interact with a small number of specialised molecules- Fc receptors on cells- main classes include Fc alpha, Fc. , Fc gamma
They complement proteins
They’re intracellular cell signalling molecules whilst recognising an infinite array of antigenic determinants at the same time
Cell surface antigen receptor of B cells- Allows b cells to sense their antigenic environment
Secreted antibody- Neutralises toxins or microbes
Used for agglutination of viruses or immune complexes
Used to arm or recruit effector cells
Describe immunoglobulin G
Most abundant serum antibody and makes aprox 80% of total serum antibodies
Consists of 2 heavy chains and either 2 kappa/ lambda light changing
4 subclasses: 1-4
Longest half-life of all antibodies- aprox 23 days
What are the functions of IgG?
Major antibody of secondary immune system
IgG 3 &4 readily cross the placenta= important roles in the neonatal protection
IgG3 is the most effective activator of compliment
IgG4 does activate complement
IgG1/3 binds wiht a high affinity to Fc receptors on phagocytotic cells and thus mediates opsonisation
IgG aids immobilising bacterial threats and neutralises toxins and viruses
Describe immunoglobulin M
Largest Ig
Memebrane bound form is ALWAYS monomeric
Aprox 1000kDa
Found intravascularly
Aprox 10% of plasma Ig’s
Describe immunoglobulin A
Aprox 15% of plasma Ig
Major tissue Ig found lining mucosal surfaces: GI tract, Respiratory tract and Urogenital tract
The SC protect IgA from degradation by gastric acid and digestive proteolytic enzymes
Often found as a dimer
Key in neutralising antibody in the mucosal compartment
Exists in two subclasses and differs between 2 compartments where it is found- blood and mucosal secretions
Blood IgA is monomeric
Mucosal IgA is almost always dimeric
Describe immunoglobulin D
180kDA
Delta H chains= 70 kDa
Rarely detected in blood
Two classes: 1 and 2
Plays an important role in maturation and proliferation of B cells
Describe immunoglobulin E
200kDa
Epilson chains of approx 73 kDa
IgE constitutes 0.3% of serum Ig
Major immunoglobulins in mediating allergic and anti-parasitic responses
Cross-link to the FC epilsonR on mast cells
Triggers the release of histamine, tryptase and proteolytic enzymes by mast cells
How do antibodies interfere with HIV?
A. Virus- target cell interaction blocked
B. Post attachment stages blocked
C. Virus- cell fusion inhibited
D. Virus uncoating prevented, viral assembly and maturation interrupted and virus budding inhibited
E. Infectious virions aggregated
What are the stages of complement action?
- Pattern recognition acts as trigger
- Protease cascade amplified
- Inflammation, phagocytosis or membrane attack
What is RAG1 and RAG-2?
2 DN shuffling enzymes that are encoded by recombination activating genes aka V(D)J
B and T cells express enzymes here during development and maturation
They initiate the molecular processes that lead to lymphocyte receptor formation by V(D)J segment recombination
Nonsense mutation in RA1/2 cause the most profound immunodeficiency, sever recombined immunodeficiency (SCID)
Describe central tolerance
The processes of eliminating and developing T/B lymphocytes that are auto reactive
Bone marrow stromal cells and blood plasma provide ncomplete encyclopaedia of self antigens to train B cells
Any B cell that binds strongly to these self antigens are either deleted/ recycled
What is peripheral tolerance?
When auto reactive B cell are released into peripheral lymphoid organs and they’re silenced by several mechanisms
A major mechanism being- B anergy in which auto reactive B cells are maintained in an antigen unresponsive state
Describe the principles of cellular co-operation
Antibody production is improved when T cells are present
“Purified” lymphocytes were shown to produce immune responses in vitro to antigens
Contaminated with macrophages, when truly purified with little response, therefore macrophages are important- seen to have another role in antigen presentation
What is MHC?
MAJOR HISTOCOMBATABIITY COMPLEX= Special structre cells used to present antigen to lymphocytes
B cells recognise antigen without MHC- surface bound antibody referred to as B cell Receptor
These can act as antigen presenting cells (APCS)
T cells can only “see” antigen presented on MHC
What are T cells?
Lymphoid cels of adaptive immune system
Formed from lymphoid stem cells, produced in the bone marrow
Mature in the thymus where they produce a T cell receptor (TCR)
2 main subsets of T cells:
1. Helper T cells- Produce proteins to act as messengers for the rest of the immune system, in the form of cytokines
2. Killer T cells- cause infected cells to apotose
Once triggered hey can kill a cell in an antigen specific manner
A 3rd subset has been characterised recently: Regulatory T cells= Keep other T cells under control
What are APC’s?
Antigen presenting cells
Professional APCs deliver antigen to CD4+ T cells
Dendritic cells, macrophages and B cells
All APCs require class II MHC molecules in addition to other co-stimulatory molecules on their surface
What is the function of dendritic cells?
Main function in periphery= act as ‘gatekeepers’
They capture foreign material and transport them to secondary lympoid tissues
Immature DCs halve pattern recognition receptors (PRRs)
During migration to the lymphoid tissue they mature
Describe MHC class I
Made up of an alpha chain and a light chain of Beta2 microglobulin (encoded on c-some 15)
There are 3 domains on the alpha chain, with a peptide binding cleft between alpha-1 and alpha-2
Peptide binding region is closed at both ends
MHC class I peptide cleft binds peptides of 8-10 amino acids length
Individuals can possess up to 6 different HLA class I molecules on their cells, with the possibility of 2A, 2B and 2C antigens
Therefore a restricted number of clas I molecules can present and bind a range of different foreign peptides
Describe MHC class II
Made up of an alpha and beta chain- both encoded by MHC
Peptide binding cleft between alpha 1 and beta 1 region
Can accommodate larger peptides of between 15-30amino acids
Fundamental difference between the clefts of class 1 and II molecules;
Peptide binding left of class II molecules is open at both ends
Cleft in class I is closed
Class II molecules can present significantly larger peptides
An individual possesses a restricted number of different class II MHC molecules
MHC class II molecules interact with a co-recpetor molecule, the CD4 expressed on helper T cell populations
CD4 interacts wiht the beta 2 domain of the constant part of the molecules
T helper cells are clas II MHC restricted
What are co-receptor molecules?
T cells bind to MHC antigen via their TR and a co-recprtor
T- cells are subdivided into 2 major populations depending upon which co-receptor molecule is expressed on the cell surface, CD4/ CD8
Which T cells express the CD4 receptor?
T helper cells
Which T cells express the CD8 receptor?
Cytoxic T cells
Describe TAP transporters and how they work
The endogenous peptides gain access to the ER lumen and MHC class I molecules through:
2 proteins in the ER membrane are responsible for transporting peptide into the ER
Memebers of the family of proteins known as the ATP-binding cassette proteins (ABC)
Transporters asscotiated with he Antigen Processing-1 and 2 (TAP-1 and 2) is encoded by genes within the MHC
After, two TAP proteins form a heterodimer, link the cytoplasm of the cell with the lumen of ER
How are peptides generated in cytosol?
Proteins within cells re continually being degraded and replaced wiht newly synthesis proteins
Degradation is carried up by a multi catalytic protease complex named proteasome
Large cylindrical omelet of 28 sub-units that are arranged into 4 stacked rings
Proteins to be degraded are introduced into the core and are broken don into smaller peptide fragments
2 sub-units (LMP1 and 2) of the proteosome are endowed within the MHC, close to the TAP-1 and 2 genes
Outline the process of MHC class I presentation
Newly synthesised MHC class I alpha chains rapidly bind to an 88kD Calnexin (CNX)
A chaperone protein retains MHC class I molecule in a partially folded state wihtin the ER, until it binds to the Beta 2 micro globulin
Binding of Beta2M to MHC class I heavy chain causes dissociation from CNX
CNX is replaced by another chaperones Calreticulin and ERp57
MHC class I interact with TAP via tapasin to form the PLC
TAP molecules actively transport endogenous peptides into the lumen of the ER
Tapain helps select the best fitting peptide
Binding of a suitable peptide to partially folded MHC class I molecules alows it to be released from the PLC
MHC-1 peptide complex transits the Golgi apparatus to cel surface
Describe Class II MHC presentation
Newly synthesised mMHC class II molecules are held within the ER by and invariant chain (li)
Li chain directs class II out off the ER to acidified endosome
Within the acidified endosome the invariant chain is digested in the low pH environment
The peptides are free to bind to MHC class II binding site
Class III MHC + peptide is expressed on the cell membrane
The competed MHC Class II peptide complex is then directed towards the cell surface
List the different tissues in which lymphocytes can be found
Slpeen
Lymph nodes
Bone marrow
Blood
Skin
Intestines
Liver
Lungs
What is tolerance?
Under normal circumstances the immune system it does not react with the host’s own potentially antigenic substances – so called self antigens
This unresponsiveness to self is called immunological tolerance
Failure in tolerance results in autoimmune diseases such as
Rheumatoid arthritis (RA)
Describe alpha-beta TCRs
<95% of T cells express these receptors
Each T cell has approx 30,000 surface receptors
Only one antigen specificity per cell
Develops in the thymus in a similar manner to B cell receptors but without somatic mutation
α-chain: similar rearrangements to light chain Ig
β-chain: similar rearrangements to heavy chain Ig
Cannot signal on it’s own
Describe the CD3 T cell receptor complex
Series of proteins which allow signalling
Associated with TCR in membrane
Essential role in function and mutual association required for TCR and CD3 expression in the membrane
Also associated with CD4/8 molecule allowing MHC interaction
Describe the phases of T cell responses
- Antigens recognition in lymphoid organs
- T cell expansion and differentiation
- Differentiated effector T cells enter the circulation
- Effector T cells and leukocytes to site of antigen to encounter antigens in peripheral tissues
- Activation of effector T cells
Outline the 6 steps in cytotoxic T cell killing with perforin
- The Tc cell binds to the antigen on target cell MHC I which triggers Ca2+ inc
- Triggers exocyosis
- Granules fuse with the Tc membrane releasing perforin
- Ca2+ allows the perforin to inst in the target cell memebrane
- In the presence of Ca2+ perforin monomers polyamide forming pores
- Cells then release Ganymede B which triggers apoptosis
Outline cytotoxic T cells killing with Fas Fas-L
- FasL-Fas interaction activates FADD
- Pro- caspase 8 is activated during apoptosis
- A family of more than 12 caspases exit
- Results in orderly destruction of the target cell
Compare pro- inflammatory cytokines and anti- inflammatory cytokines
Both need to be tightly regulated
Pro have -Tumour necrosis factor alpha, Interleukin 1 and 6, MCP -1
Anti have- Transforming growth factor 1 and Interleukin 10
Describe chemokines
Type of cytokine that facilitates movement of cells
Produced during an immune response to recruit cells from blood to site of infection
Interact with G-protein- linked transmembrane receptors (chemokine receptors)
Describe growth factors
Proteins that regulate many aspects of cellular function including survival, migration and differentiation
What is DiGeorge Syndrome?
Suggested it may result from defect in development of 3rd and fourth pharyngeal pouches
At birth infants have a number of defects:
No T cells, no mediated immunity
Congenital cardiac abnormalities affecting major vessels of the heart
Unusual facial appearance
What is the complement system?
Protective system which is common to all vertebrae’s
Involved in both innate and adaptive immunity
Directly lyses shortens (innate)
Works alongside specific antibodies (adaptive)
Acts quickly, prior to any cells and can react to a pathogen
What is the standard abbreviation for the complement pathways?
Components are numbered 1-9 with a prefix of C
Numbers in the order they were discovered and classified
What are the 4 functions of the complement system?
- Lysis
- Opsonization
- Activation of inflammatory response
- Clearance of immune complexes
What are the 3 activation pathways?
- Classical pathway- requires Ig
- Alternative pathway- triggered by miroorganisms in the absence of Ig
- Lectin pathway- less important and doesn’t require Ig
Briefly outline how the classical pathway works
Major components- C1-9
Requires antibodies IgM and IgG
Specifically if they’re bound onto a solid antigen surface- bacterial cell walls/ membrane
Give the 5 steps of the classical pathway
- C1q binds to antigen- bound antibody, C1r activates auto- catalytically, which activated C1r, which activates C1s
- C1s cleaves C4 exposing a binding site for C2. C4 bind to the surface near C1 and C2 binds to C4, forming C3 convertase
- C3 convertase hydrolyses many C3 molecules. Some combine with it to form C5 convertase
- Cb component of C5 convertase binds to C5, permitting C4b2a to cleave C5
- C5b binds to C6, initiating the formation of membrane- attack complex
What is the major difference in the alternative pathway from the classical pathway?
Ig plays no part in alternative pathway, it’s antibody dependent
Describe the 5 stages of the alternative pathway
- C3 hydrolyses spontaneously, C3b fragment attaches to the foreign surface
- Factor B binds to C3a, exposes site acts on by factor D. C3bBb is generated, which has convertase activity
- Binging of properdin stabilises convertase
- Convertase generates C3b, some binds to C3 convertase, activating C5’ convertase. C5 binds to the antigenic surface
- Rest of the process is the same as the classical pathway
What makes the lectin pathway different?
Comment of the innate immune system
Doesn’t require the elements of the adaptive immune response
Antibody dependent
Doesn’t involve C1
Activated by Mannose Binding protein (MBP)
Outline how the lectin pathway works
When MBP has bound to the surface of pathogens, serine proteases binds to the MBP
MASP 1 &2 (MBP Associated Serine Protease)
Complex of MBP and MASP molecules cleaves C4 and C2 , as in the classical pathway
Generated C4b attacthes to the cell wall/ membrane and Remainder of the sequence is as for the Classical pathway
What is transplantation?
Transfer of cells, tissues/ organs to another site
Define autograft
Transplantation of self tissue to another site
Define isograft
Transplantation between genetically identical individuals (identical twins)
Define Xenograft
Transplantation between different species
Define allograft
Between non-identical members of same species, conventional transplant surgery
Which type of graft is most likely o trigger an immune response?
Xenograft
Describe tissue typing
The human leukocytes antigens of an individual are identified and characterised by tissue typing
Disparity at these sites between donor and recipient will cause strong rejection mechanisms
Microcytotoxicity tests are carried out using blood cells from potential donors and the intended recipient to test for both Class I and Class II compatibility
Describe the process of a mediated lysis assay
Lymphocytes from donors and the recipient are plated out into separate microtiter plate wells and probed with antibody for different HLA antigens
Complement is added to all wells, if antibody has bound the cell will become porous
Dye can be added which will be taken up only by porous cells
Therefore if the antibody specific for the HLA on the cell is added to a well the cells in the well will take up dye
What is the test used to determine the degree of class II compatability between donors and recipients?
Mixed Lymphocyte Reaction assay
Lymphocytes from potential donor are irradiated and used to stimulate lymphocytes from the recipient
If T cells are recognised by recipient= Proliferation Proliferation detected by uptake of thymidine into cellular DNA
On average how long does it take for an auto/ isograft to be fully accepted?
12-14 days
Describe 1st set rejection in terms of mice
Mouse from inbred mouse strain A given a skin graft from mouse strain B
Immune system is naive s immune response is delayed
The graft is rejected within 14 days
Describe 2nd set rejection in terms of mice
The same mice are used again in a subsequent experiment
Immune system has already been primed and memory cells have formed
Graft is rejected quicker in aprox 6 days
List the 4 signs of rejection on the skin
- Accumulation of neutrophils around the new blood vessels
- Macrophages and lymphocytes then infiltrate, eventually moving through the entire graft
- Capillaries supplying the graft are damaged by destruction of the endothelium
- Blood then clots within the vessel, the graft becomes ischeamic and tissue dies
What are the 4 steps / signs of rejection in the kidney
- Entire graft is infiltrated with mononuclear cells which cause damage to the endothelial lining of the blood vessels
- Leads to thrombosis of the vessels causing tissue destruction
- Blood flow is interrupted, this can cause necrosis of some tissue but can also lead to haemorrhage
- Causes death of the grafted kidney
What are the main processes of graft rejection?
Delayed type hypersensitivity
Cell- mediated cytotoxicity
Describe the sensation stage
Some CD4+ and CD8+ host T cells will recognise the graft MHC
Tolerance will not remove cells that bind non-self MHC (as its based on -ve selection)
Both types of T cells become activated by antigen and proliferate
Mainly donor dendritic cells display MHC complexes
Antigen presentation is initially in the spleen
MHC I on all graft cells will present peptides produced within the graft cell
MHC II on any donor APC’s within graft present peptide produced outside the cell
Any host CD4+ T cells which can recognise the donor MHC- antigen complex are activated
Host APCs can also migrate into the graft and present donor antigen (minor and major compatibility antigens)
Describe the effector stage
Most common- Cell mediated mechanism such as delayed type hypersenitiyvity and CTL-mediated cytotoxicity is caused by influx of T cells and macrophages
Less common- ADCC (antibody- dependent cell- mediates cytotoxicity
Classical complement cascade mediate cell lysis
Delayed type hypersensitivity- CD4+ T cells are activated by the graft antigens and proliferate
They secrete cytokines that recruit immune cells, partic macrophages – Activated macrophages release cytotoxic mediators and lytic enzymes that causes cell damage
CTL mediated cytotoxicity- FA’s ligand on CD8+ T cells interacts with FA’s on graft cell including apoptosis
Perforin and granzyme released
Antibody- dependent cell- mediates cytotoxicity and classical complement cascade mediated cell lysis:
Both rely on antibody binding to host antigens
List the 3 types of rejection that occur post transplant surgery
- Hyper acute rejection
- Acute rejection
- Chronic rejection
Describe hyperacute rejection
Occurs rapidly (minutes after transplantation)
Mediates by pre-formed antibodies and complement
Result in vascular damage and thrombosis
Easy to revert by cross-matching
Rarely encountered
Describe acute rejection
Episodes occur in most patients during first 3 months after transplantation
T-cell mediated response -CD4+ and CD8+ T cells
Targeted at histocompatabiliy antigens
Can be managed successfully in most cases by immunosuppression
Describe chronic rejection
Occurs months and years after transplantation
Most common cause of graft failure
Generally observed as a fibro-proliferative disease
Variable organ sensitivity- lung is v susceptible to obliterative bronchiolitis
Give 4 common drugs used to prevent rejection
Inhibit T cell activation
High-dose steroids
Anti-proliferate drugs
Anti T-Cell antibodies
Why is pregnancy unusual in term of immunology?
Unusual case of immune system attatcking non-self tissue
Foetus contains paternal MHC and minor histocompatability antigens that differ from the mother
Mother can successfully carry several pregnancies from the same father
But make antibodies against paternal antigens
List the primary lymphoid organs
Bone marrow
Tymus
Gut associated Lymphoid organ
List the secondary lymphoid organs
Mucosal associated lymphoid tissues
Spleen
Tonsils
List the immune organs in the endocrine system
Pituitary gland
Hypothalamus
Thyroid
Give the 2 types of bone marrow and their functions
Red- active hematopoiesis
Yellow- inactive hematopoiesis
What is stroma made from?
Cells derived from mesenchymal stem cels
Define a stem cell niche and give its function
Spatial structure ins which HSC’s are housed by allowing self renewal in absence of differentiation
Function- inhibition of differentiation
What is a vascular niche?
Specific site where endothelial cell, fibroblasts and adipocytes are found
Describe the location and structure of the thymus
Location— superior mediastinum, above heart
Structure- Stromal cell network composed of epithelia, dendritic cells and macrophages which helps to produce T cell repatoire
What is diffuse lypmphoid tissue?
Non encapsulated accumulations of lymphocytes and other free cells (plasma cells, eosinophils, fibroblasts)
Initial immune response
Located in the lamina propria (subepithelial tissue)
Mucosa Associated Lymphatic Tissue (MALT) includes BALT & GALT
Site for initial immune response characterised by proliferation of B cells and development of lymphatic nodules
What is nodular/ follicular lymphoid tissue?
Protects body from antigens
Characterised by nodules/ follicles
NOT encapsulated
Found in wall of the GI tract
What are the GC?
Germinal Center
Specialist microoenviroment where B cells undergo activation proliferation and differentiation
Paler zone of lymphoid nodule when stained under a microscope
What is the mantle zone/ corona?
Outer darker staining region surrounding the GC
Resting cels are displaced towards edge to form a mantle zone
Describe the location and function of the spleen
Left- posterior abdominal wall
Function- Filter blood by removing aged blood cells and recycling iron in the bone marrow
What is splenomegaly?
Pathologic enlargement of the spleen
List some hypothalamic disorders
Obesity
Diabetes
Anorexia
Bulimia
What is the role of the thyroid and the parathyroid?
Thyroid- Regulates rate and control calcium levels
Parathyroid- Regulate calcium and phosphorus levels
What is hypoparathyroidism?
Low circulating of parathyroid hormone
Results in low Ca levels and inc phosphate levels in blood
