innate immune system: types of initial response
- first barrier against infection
- can be mechanical, chemical, or microbiological
mechanical:
- longutidunal flow of air
- skin and gut
- constant flow –> bacteria cannot settle in one place - movement of mucus by cilia
- lungs
- bacteria trapped in mucus, cilia transport mucus into throat, swallow mucus
chemical:
- skin, gut, eyes and nose
- breaks down bacteria in various ways
- fatty acids, enzymes, low pH
microbiological:
- skin and gut
- bacteria must out-compete normal flora for resources
innate immune system: immune cells
next level of defence after initial barrier
immune cells come from bone marrow (hematopoietic stem cells)
—> lymphoid lineage (T/B/NK cells)
—> myeloid lineage
—> granulocyte/megakaryocyte
acute inflammation mechanism
blood vessels during inflammation
macrophages sitting underneath skin
macrophages recognise pathogens
—> sends out chemical signals
—> phagocytosis of pathogens
chemical signals effect blood vessels, binding to receptors on epithelial cells
—> blood vessels dilate = more blood flow
—> inside wall of blood vessel releases adhesion molecules to become stickier = immune cells stick
—> tight junctions btw cells loosen up = cells and fluids move out into tissue
neutrophils leave blood stream and attracted to tissue sight
—> phagocytosis of pathogens
changes to blood vessels in inflammation:
- dilation –> increased blood flow –> redness
- changes in adhesion molecules –> blood sticks to site –> accumulation of cells –> swelling
- increased permeability –> blood cells move into tissue –> blood sent direct from heart –> heat –> blood cells interact with nerves –> pain
how are pathogens recognised by phagocytes?
PAMP receptors = pathogen associated molecular pattern
binding of PAMPs to receptors results in activation and secretion of inflammatory mediators
types of macrophages in the body (5)
- microgilia:
- found in brain
- phagocytose dying neurones - alveolar macrophages:
- found in the lungs
- respond to local irritants via cytokine release - spleen macrophages
- found in the spleen
- immune function
- phagocytosis of naturally dying cells, clearance of agents, etc - kuppfer cells
- found in the liver
- exposed to gut microbial products - synovial A cells
- found in joints
- cytokines in arthritis etc
stages of phagocytosis
(1) Binding of pathogen to surface receptors e.g. PAMP receptors
(2) Engulfment into vacuole/phagosome
(3) Fusion of phagosome with lysosome
(4) Killing and degradation of bacterium by lysozyme, proteases, acid hydrolases, free radicals
secreted factors after macrophage activation (3)
macrophages ingest and degrade bacteria and are activated by LPS to secrete cytokines
*IL = inter-luekin, transferred between leukocytes
1. IL-1 local effects: - activates vascular endothelium - activates lymphocytes - increases access to effector cells systemic effects: - fever - production of IL-6
2. IL-6 local effects: - activates lymphocytes - increases antibody production systemic effects: - fever - acute protein production
3. TNF-alpha local effects: - activates vascular endothelium and increases vascular permeability - results in increased entry of IgG - increased fluid drainage to lymph nodes systemic effects: - fever - mobilisation of metabolites - shock
- fever for ~24hours is good –> fever burns brown fat –> decrease rate of bacterial growth
neutrophils
main line of defence against invading bacteria –> first cells to bind to inflamed tissue
primary function is phagocytosis and killing of pathogens
neutrophils must gain access to tissues from the bloodstream –> move via chemotaxis
complement system
complement cascade pathways
set of plasma proteins that act together as a defense against pathogens in extracellular spaces
functions:
- inactive enzymes float in cytoplasm –> attach to bacteria to alert immune system –> recruitment of inflammatory cells
- kill pahogens
- coats microbes with molecules (opsonins) that enhances their phagocytosis
complement cascade:
- classical:
- antibody attaches to bacterium
- recruits complement protein t surface
- first complement protein binds, chopped to become active enzyme
- active complement protein chops next in line
- accumulate onto bacteria surface - mb-lectin:
- proteins bind to sugar groups uniquely found on pathogen
- activates complement cascade - alternative
- pathogen surfaces recognised by complements
mast cells
primarily responsible for type 1 hypersensitivity (immediate)
certain allergens invoke an IgE response –> then bind to mast cells (and basophils)
Fc~RI are high affinity receptors for IgE on the surface of mast cells
when cross-linked by allergen-antibody complexes, mast cells respond by degranulation
natural killer (NK) cells antibody dependent cell-mediated cytotoxicity (ADCC)
develop in the bone marrow from common lymphoid progenitor cells.
larger than T cells with distinctive cytoplasmic granules
recognise infected cells or tumour cells and destroy them.
- -> form synapses with infected cell
- -> -degranulate cytotoxic components across synapse
- -> these kill infected cell via apoptosis
activation state controlled by +/- signals on their cell surface (inhibitory dominates over activation)
ADCC:
- antibody binds antigens on surface of target cell
- fc receptors on NK cells recognise bound antibody
- cross-linking of fc receptors signals NK cell to kill target cell
- target cell dies via apoptosis
T cells: MHC1 vs MHC2
T cells recognise a combination of peptide (sampling of inside cell) and the MHC (not foreign) –> gives very high specificity
MHC1
- T-cells with short peptides bind MHC1
- MHC1 derived from cytoplasm
- Viral proteins invoke MHC1 response
- MHC1 = aplha 1, 2, 3 + beta2
- CD8 stabilises
MHC2:
- T-cells with long peptides bind MHC2 (ends open to fit longer peptide in groove)
- MHC2 derived from extracellular/vesicle proteins
- Extracellular bacteria invoke MHC2 response
- MHC1 = aplha 1, 2 + beta 1, 2
- CD4 stabilises
Describe how T-cells are activated
1) T-cells develop in thymus as naive T-cells
2) A cell digests bacteria and loads polypeptides onto MHC1 or MHC2
3) Once naive T-cell receives both signal 1 (TCR engagement) and signal 2 (MHC), becomes effector T-cell
4) Active effector T-cell releases growth signal to induce proliferation and differentiation
5) Active T-cell triggers effector function at infection site
T-cell selection
STRONG AFFINITY for self peptides —> too autoreactive —> negative selection —> undergoes apoptosis
WEAK AFFINITY for self peptides —> high affinity for foreign peptides —> positive selection
VERY WEAK AFFINITY for self peptides —> death by neglection
CTL killing mechanism
facilitated by CD8 + T-cell (MHC1)
kill virally infected cells by secreting cytokines
PRIMARY MECHANISM
- T-cell receptor (TCR) triggering leads to directed secretion of preformed lytic granules
- Lytic granules contain two main proteins:
- —–> perforin: polymerises to form pore in target cell membrane
- ——> granzymes: >3 serine proteases activate apoptotic pathways in target cell cytoplasm
- Lytic granules target cell for death
SECONDARY MECHANISM
- TCR receptor causes T-cell membrane to express Fas-ligand
- Fas crosslinks on target cell and triggers apoptosis
helper T-cells: CD4 + TH-1 mechanism
ACTIVATES MACROPHAGES (detects inside cell)
1) Dendritic cell in tissue takes up antigen and migrates to lymph node
2) Now matured dendritic cell interacts with CD4 Th-1 cell (MHC2 to TCR). Activates T-cell by secreting IL-12
3) CD4 Th-1 cell proliferates / differentiates in lymph node, then migrates to inflammatory site
4) Th-1 cell interacts with infected macrophage (MHC2 to TCR).
5) Th-1 cell secretes IFN-gamma
6) Activates macrophages to kill intracellular pathogens
helper T-cells: CD4 + TH-2 mechanism
INITIATES ANTIBODY PRODUCTION (cannot see inside cell)
1) B-cell interacts with CD4 Th-2 cell (MHC2 to TCR)
2) CD4 Th-2 cell secretes IL-4 and IL-5 to B-cell
3) IL-4 and IL-5 induce proliferation, differentiation, Ig production, Ig class switching ----> antibody factory (IL-4 ---> IgG1 and IgE. IL-5 ---> IgA)
antibody structure and antibody types
HEAVY CHAIN
—> particular protein pairs joined together by disulphide bonds
—> heavy chain determines the class or isotype of antibody
LIGHT CHAIN
—> protein pairs attaching to heavy chains via disulphide bonds
—> Light chains are either kappa or lambda
VARIABLE REGION
—> encoded by block of genes
CONSTANT REGION
—> encoded by different block of genes
—> different constant regions can be swapped out (isotype switching)
—> constant region of heavy chain determines class
ANTIBODY ISOTYPES
- IgM = main antibody in PRIMARY immune response
- IgG = transport across the placenta
- IgD = no known function
- IgA1 = transport across epithelium, main antibody found at mucosal sites and mucosal secretions
- IgE = sensitization of mast cells, involved in allergy
- ** IgM and IgA can form multimers with 10 binding sites
B-cell development
1) B-cells generated in bone marrow
2) B-cell precursor rearranges Ig genes to produce IgM and IgD –> only time two Ig’s produced in same cell
3) B-cells can sometimes generate autoimmune receptor. B-cells test receptors with free floating proteins. If B-cell has high affinity for self proteins –> negative selection –> receptor deleted (prevents reactions against own antigens)
4) When BCR recognises antigen, turns into mature B-cell by secreting plasma cells and memory cells (travel back to bone marrow)
how do antibodies become so diverse?
1) variable region production
- variable region —> the antigen binding area
- V, D and J blocks, each containing multiple genes
- a random single chain from V, D and J genes fuse via recombinase enzyme to create variable region —> huge variety/specificity
2) junctional diversity
- errors can be made in base pairs —> even more diversity —> “junctional diversity”
3) pairing of heavy + light chains
- in light chains, only V and J associated with variable region —> smaller variety
- however different light and heavy chains paired —> even more diversity !!!!!!
4) somatic hypermutation
- mutations cause diversity
(1-3 occurs in T-cells)
primary immune deficiency
in primary immune deficiency, the deficiency is the cause of the disease
may be hereditary (DNA mutations –> common) or aquired (very rare)
primary immune deficiencies can result in absence of T or B-cells —> highly susceptible to disease
secondary immune deficiency
in secondary immune deficiency, the deficiency is the result of another disease/condition (e.g. virus causes problems in immune system). this then generates further problems
for example: burns: - chemicals suppress inflammation i.e. temporarily immune deficient leukaemia: - malignant cells replace T/B cells - immune deficient state chemotherapy: - toxic to bone marrow - can’t make immune system deliberate immunosuppression of transplant recipients: - immune system reacts to transplant - immunosuppression drugs for rest of life certain infections: - HIV
allergies: hypersensitivity
TYPE 1 HYPERSENSITIVITY:
- IgE responds to antigen
- IgE binds mast cells with high affinity to activate them
- mast cell produces immune response
TYPE 2 HYPERSENSITIVITY:
- IgG responds to antigen
- antigen found on cell surface or in cell matrix
TYPE 3 HYPERSENSITIVITY:
- IgG responds to non-surface bound antigens
TYPE 4 HYPERSENSITIVITY:
- known as delayed-type hypersensitivity (DTH)
- not dependent on antigens
- antibodies formed against material —> forms immune complexes —> macrophages interested in immune complexes to start inflammation
mechanisms of immune tolerance (6)
(1) Central tolerance/Negative selection
- occurs in thymus
- autoimmune T-cells deleted
- some can escape
(2) Antigen segregation
- physical barrier —> no access
- e.g. immune system cannot access eye unless damaged
(3) Peripheral anergy
- no co-stimulation
- removes signal 2 —> cell death
(4) Regulatory T cells
- produce T-cells which suppress immune responses
(5) Cytokine deviation
- autoimmune response requires certain type of T-cell
- immune system deviates to other types of T-cells
(6) Clonal exhaustion
- apoptosis of cells after continuous stimulation
