A rough guide to the immune system
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A rough guide to the immune system
Dr Adrian PalfreemanSenior Clinical Scientist MRC CTU
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Stem cells
Stem cells –why they matter
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Neutrophil (polymorph)
• Multi-lobed nucleus.• Commonest leucocyte (2500-7500/mm3 of
blood).• A short-lived phagocytic cell whose granules
contain numerous bactericidal substances.• leave the blood to go to tissues where
infection or inflammation is developing
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Eosinophil
• A leucocyte whose large refractile granules contain a number of highly basic or ‘cationic’ proteins,
• possibly important in killing larger parasites including worms.
• Bind avidly to IgE-coated particles (i.e. Helminthic parasites) .
• Abundant at sites of allergic reactions.
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Components of the immune system
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• Monocyte– The largest nucleated cell of the blood (16-20μm– diameter), developing into a macrophage when it– migrates into the tissues.
• Macrophage (A professional antigen presenting cell)– The principal resident phagocyte of the tissues.– Strongly phagocytic of particles and microbes.– Has receptors for Ig and complement.– CNS – “microglia”– Liver – “Kupffer cells”– Lungs – “alveolar macrophages”– Bone – “osteoclasts”
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T lymphocyte (T cell)
• A thymus-derived (or processed) lymphocyte.• 1500 - 4000/mm3 blood• 6-15μm diameter (red blood cell 7.2μm
diam.)• 2 main subdivisions – CD8 (cytotoxic T cells)- CD4 (helper T cells)
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• B lymphocyte– A bone marrow- (or in birds, bursa-) derived
lymphocyte, the precursor of antibody-forming cells. In foetal life, the liver may play the role of ‘bursa’.
• NK (Natural Killer) cells– do not have to recognise a specific antigen
before acting against it– are effective against a wide range of infectious
microbes.
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The 2 arms of the adaptiveimmune response
1. Humoral immunity (antibodies)2. Cellular immunity (T-cells)• Sub-divided into T helper cells (CD4+) and• Cytotoxic T cells (CD8+)
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CD4 Lymphocytes (T helper cells)
• coordinate much of the immune response to micro-organisms
• help B-cells respond to foreign proteins• secrete substances that enable CD8 T-cells to
proliferate• activate macrophages so that they can kill
certain organisms, including some organisms associated HIV infection.
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CD8 Lymphocytes (Cytotoxic T cells)
• kill cells in the body identified as abnormal or foreign
• tumour cells • cells that have been infected by viruses.
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How does HIV reduce CD4 Cells?
• Increased turnover of cells in response to infection
• Trapping of HIV in lymph nodes• Shortened survival of CD4 cells• Reduced production of new cells• Reduction of T cell progenitor production
from bone marrow
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Sites of theprincipal
lymphoid tissueswithin the human
body.
Primary lymphoidorgans
Secondary lymphoidorgans
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Human lymphoid organs
Primary lymphoidorgans
Secondary lymphoid
organs
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• Lymphoid tissues– Immune system compartmentalised into
organs/tissues.– Funtionally unified via blood and lymph systems.– Lymphocytes recirculate.– In total, equivalent in weight to brain or liver.
• Primary lymphoid organs– Bone marrow where T and B lymphocytes are made.– Thymus where T lymphocytes mature/are selected.
• Secondary lymphoid organs– e.g. spleen, lymph nodes and Peyer’s patches.– Contain T cells, B cells, antigen presenting cells (APCs)
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T cell precursors(thymocytes) migrate
from the bone marrowto the thymus to mature.
Mature T cells leave thethymus and migrate tosecondary lymphoid tissueswhere they may encounterforeign antigen.
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Thymus
Lobules show - a lymphocyte-dense outer cortex- an inner lighter-staining medulla.Stromal framework with specialised epithelial cells, DCsand macrophages (APCs).T cell precursors arrive from the bone marrow.Cortex and medulla ‘educate’ thymocytes into mature,competent T cells (1 to 3% of T cells survive education).Mature T cells are released into the peripheralcirculation.
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THYMUSDevelopingthymocytesoccupy the
interstices ofan extensivenetwork of
epithelial cells
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Clusters of Differentiation (CD)
• CD3 T cells• CD4 Helper T cells• CD8 Cytotoxic T cells• CD16 Macrophages• CD19 B cells
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Maturation of T lymphocytes in the thymus
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Circulatinglymphocytes
meetlymph-borne
pathogensin draining
lymph nodes.
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Lymph node
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Cytokines
• Il2 stimulates division of B and T cells and killing of HIV infected cells by NK cells
• IL2 levels reduced in HIV infection
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IL2
• Does administration of IL2 help?• Raises CD4 numbers• Significant side effects• Injectable • Short term benefit in clinical trials• Does it reduce mortality and morbidity in the
long term?
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Summary • B cells recognise antigens (antigenic epitopes) via their monomeric IgM
receptor• T cells recognise antigens (small peptides)via the T cell receptor (TCR) which
is always associated at the cell surface with CD3 11• The monomeric B cell receptor (and, in fact, all antibodies) recognise
antigens in solution – in their native (folded) state• The TCR does not recognise soluble antigens but only small antigenic
peptides associated with the Major Histocompatibility (MHC) molecules I & II
• For a T cell or B cell to be activated 2 appropriate signals are always required
• T cells need binding of the TCR to peptide/MHC plus specific cytokines from the APC (notably IL- 1 and IL-2) and interaction between B7 and CD28
• B cells need binding of mIgM surface receptor plus signals from TH cells (notably IL-4 and IL- 10) and interaction of CD40/CD40L.
• This process has evolved to prevent unwanted activation of immune cells which can lead to harmful responses such as allergies and autoimmunity