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ImmunologyTHEME 106/Cells of the immune system.This includes leukocytes (white blood cells) and others like endothelial cells, adipocytes and epithelial cells. Immune cells are derived from stem cells in the bone marrow which differentiates under control of growth factors into pluripotent stem cells...

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Immunology THEME 1 06/02 Cells of the immune system. This includes leukocytes (white blood cells) and others like endothelial cells, adipocytes and epithelial cells. Immune cells are derived from stem cells in the bone marrow which differentiates under control of growth factors into pluripotent stem cells and then progenitor cells (myeloid and lymphoid). Myeloid progenitors give rise to- polymorphonuclear cells/granulocytes which have a strange shaped nucleus and make up 60-70% of the white blood cell population- these include: neutrophils, eosinophils and basophils. They also give rise to monocytes, megakaryocytes and erythrocytes. The development of progenitor cells into specific cell types depends on the secretion of hormones and growth factors (cytokines and colony stimulating factors) in the bone marrow and tissues. All of the cells are derived from the bone marrow. White blood cells move from the bone marrow into the tissues to differentiate and function. Mast cells are never found in blood and are instead found in exclusively tissues. Cells are identified by the specific receptors they express, under the cluster differentiation system (CD…). For example, all T cells express CD3. Myeloid- Granulocytes. Neutrophils:       

Large Live 2-3 days Make up 90% of the granulocytes The cytoplasm containing granules stains neutral (pink) Are phagocytes so kill bacteria by microbicidal mechanisms Are the most important cell in non viral infections Have receptors for antibody

Eosinophils:     

Make up 1-2% of the white blood cells The granules inside stain red with eosin and contain a cytotoxic core for parasites (eosinophil basic protein) Important in immunity to helminth infections (parasitic worms) Phagocytes Have receptors for antibody

Basophils:    

Make up 0.2% of white blood cells and only enter tissues during inflammation Stain darkly Promote inflammation when stimulated Important in allergies

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Have receptors for antibody

Myeloid others. Megakaryocytes:  

Stay in the bone marrow and give rise to platelets by cytoplasmic fragmentation Polyploidy- contain more than 2 sets of chromosomes

Platelets:    

Important in blood clotting Anucleated (contain no nucleus) Contractile Adherent to other cells and surfaces

Dendritic cells:     

Irregular shape (branches) Usually derived from myeloid progenitors but can also come from lymphoid progenitors Capture antibody when immature and move to lymphoid tissues to mature and present the antigen to T cells- they are professional antigen presenting cells (APCs) Secondary role is phagocytosis Subtypes- Langerhan cells (skin), interdigitating, plasmacytoid and follicular

Monocytes:      

Kidney bean shape Live 1-2 days Mononuclear (round nucleus) Phagocytes Become macrophages in tissue Have CD14 and CD15 molecules on the surface

Macrophages:       

Monocytes in tissue 10x larger than monocytes Can live for years Characteristics depend on the tissue they are in Phagocytes Adherent Types- Liver- Kupffer. Connective tissue- Histiocytes. Lung- Alveolar macrophages. KidneysMesangial cells. Brain- Microglial cells. Bone- Osteoclasts.

Monocytes/macrophages- have many microbicidal mechanisms including enzymes, secrete many growth factors, chemicals and activation factors. They are good at recognising foreign material using their surface receptors and engulfing it through phagocytosis and showing it to other immune cells through presentation. There are 2 major sub groups- M1 and M2.

Unknown. Mast cells:      

Are derived from an unknown progenitor cell Are immature cells in the blood until they enter tissues to mature Type of mast cell varies with their location and contents of granules Are found in connective and mucosal tissues Are important in allergy and release activating factors like histamine Have receptors for antibody

Lymphoid progenitors give rise to 20-30% of white blood cells- B lymphocytes, T lymphocytes, Natural killer cells and NKT cells. Lymphocytes have a small halo of cytoplasm surrounding a large round nucleus. Lymphocytes that are stimulated by antibody become effector cells or memory cells. Cells that pass into the thymus become T cells and cells that stay in the bone marrow become B cells.

B lymphocytes:      

Stay in the bone marrow to differentiate and are taught not to recognise themselves Have millions of copies of identical antibodies on their cell membrane to form the specific B cell receptor (BCR) Express CD19 and CD21 markers Become memory cells or plasma cells which secrete antibody when activated May become suppressor cells 2 subgroups- B1 and B2

T lymphocytes:   

Move into the thymus from the bone marrow to be educated not to react to peptides from self molecules Have a specific T cell receptor made of an alpha and beta chain on their surface to bind foreign antigens Have CD3 receptors, but helper cells have CD4+ and cytotoxic cells have CD8+

Helper T cells:   

Express CD3 and CD4+ Secrete cytokines when activated to help the immune response or become memory cells 2 sub groups- Th1 and Th2

Cytotoxic T cells:   

Express CD3 and CD8+ Kill infected targets when activated or become memory cells Release toxic granules or induce apoptosis in target cells

Regulatory T cells:  

Mainly CD4+ T cells able to affect immune responses by supressing them or activating them through direct contact or secretion of cytokines 2 types- natural or inducible

Gammaγdeltaδ T cells:  

Made of gamma and delta chains instead of alpha and beta Recognise lipid antigens

Large granular lymphocytes:     

Make up 4% of white blood cells Play a role between innate and specific immunity Like lymphocytes but large with a granular cytoplasm Kill some tumour cells and virally infected cells Main group is natural killer cells, and also natural killer t cells

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Destroys cells by perforins (cytotoxic molecules) and granzymes leading to necrosis or induction of apoptosis

Natural killer cells:     

Has no t cell receptor but has others that recognise self and non self structures Receptors for growth factors, antibody and specialised activatory/inhibitory receptors to recognise infected cells without MHC Has the same progenitor as T cells but do not depend on the thymus, and branch off early Action is enhanced by cytokines from CD4+ T cells Express some of the same T cell/monocyte/granulocyte markers

Natural killer t cells:    

Share properties with natural killer cells and t cells Make up 0.2% of white blood cells Have a t cell receptor as well as some of the natural killer cell receptors Make lots of growth factors (cytokines)

09/02 Tissues of the immune system. Lymphoid tissues are places where most lymphocytes are found, when they are not in the blood. There are primary, secondary and tertiary tissues. 1. Primary tissues- are involved in development and differentiation of lymphocytes. Includes the thymus (containing T cells), foetal liver and adult bone marrow (containing B cells).

Thymus- is the first organ to produce lymphocytes and T cells mature and are educated there: T cells are positively selected to bind to MHC molecules and are negatively selected if they bind self peptides, this takes 1-3 weeks. The thymus has 2 lobes split into lobules by connective tissue walls (trabeculae) and has an outer cortex and inner medulla. Cells move from the cortex into the medulla when differentiating into T cells. In the medulla and corticomedullary junction where most blood vessels are found- there are interdigitating cells. The thymus shrinks/involutes with age. Thymic nurse cells affect thymocyte development. Cells not leaving the thymus undergo apoptosis. When thymocytes enter the thymus they do not express CD4 or CD8 but go through a stage of expressing both, and then decide whether to express CD4+ or CD8+. Expression of CD3 and TCR increases. T cell phenotype changes during maturation- they go through double negative, double positive and single positive stages for CD4 and CD8. 2. Secondary tissues- allow the antigen to accumulate, present the antigen to lymphocytes.  Lymph nodes- are small bean shaped tissues at junctions of major lymphatic vessels, and become larger during infection. Lymph nodes are made up of 3 areas- the cortex, paracortical area and medulla and is surrounded by a capsule and supported by a reticulum. Antigen is carried in the lymphatic system from the tissues into the lymph nodes so the immune cells can join with the antigen to start the specific immune response. When antigens enter the node, lymphocyte turnover is increased. Cells enter into the medulla and drain into the efferent lymphatic (leaving) at the hilus. Lymphocytes from the blood enter the lymph nodes at high endothelial venules (HEV) and return to the blood via the thoracic duct. The outer cortex contains B cells which form follicles where they aggregate. Primary follicles are dense and uniform, secondary follicles contain larger cells and form germinal centres. The paracortical areas contain T cells, large lymphocytes and activated cells- BLAST cells. The medulla contains plasma cells secreting antibody.

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The spleen

Lymphocytes enter and leave the spleen mainly through the blood stream. The spleen is a capsule with septae (fibrous partitions). It contains red pulp tissue where damaged or aged red cells are filtered (non immunological role). White pulp tissue develops immune responses. Periarteriolar lymphoid sheaths (PALS)- cells and tissues surrounding the major arterial branches of the spleen are called periarteriolar. The sheath is associated clusters of lymphocytes (follicles or nodules). PALS contain T cells. Lymphoid follicles contain B cells.

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Mucosa-associated lymphoid tissue (MALT) Some lymphoid tissue in the mucosa lining of the gastrointestinal, respiratory and urogenital tract. Lymphocytes are also found in the intestinal epithelium, intestinal lumen and lamina propria.

GALT- gut associated lymphoid tissue. GALT is made up of Peyer’s patches and some follicles in the colonic submucosa. Peyer’s patches are collections of lymphocytes- B cells in a central follicle surrounded by T cells and macrophages, they have no afferent lymphatics but efferent lymphatics drain into the mesenteric lymph nodes. They are covered by microfold (M) cells which are specialised lymphoepithelium that take up antigen in the gut and deliver them to lymphoid follicles, along with dendritic cells (afferent transport).

BALT- bronchus associated lymphoid tissue- has aggregates of B cells and follicles with some germinal centres. BALT contains a network of capillaries, arterioles, venules and efferent lymphatics so it may sample antigen from the lungs AND the systemic circulation.

3. Tertiary tissues- all the remaining lymphoid tissues, e.g. the skin, don’t contain many lymphocytes but during inflammation memory lymphocytes may enter.

B cells are derived from the bone marrow and stay in the bone marrow and mature by growth factor influences. They are educated and are negatively selected if they bind to self molecules with BCRs but are positively selected if they respond to survival factors. 90% of B cells do not leave and undergo apoptosis, and the others are released as functionally immature naïve cells.

Lymphatic system. Components of blood from capillaries enter tissues which make extracellular fluid that provides food and gas exchange. The extracellular fluid drains into lymphatic vessels. Lymphocytes and other white blood cells use the vessel system to move around the body. Movement is regulated by the expression of adhesion molecules in endothelial cells that lymphocytes can bind to. Lymphocytes recirculate between tissues, primary and secondary lymphoid organs via the blood and the lymphatics.

The lymphatic system movement works by smooth muscle cell movement, it also uses arterial pressure as it is close to blood vessels and has valves to prevent backflow.

Homing. Leukocytes have homing receptors that allow them to bind to the endothelium of blood vessels so they can move from the blood into the lymph, into the tissues and back into the blood. Lymphocyte homing is the name of the process of this non random movement of cells to specific areas of the body.

THEME 2 Innate immunity- non specific and ready at birth, it is the first line of defence against pathogens and is fast to react. Tissue damage from trauma or infection causes chemical and cellular events which trigger innate immunity and aims to limit the spread, eliminate the pathogen and repair damage. Phagocytes- neutrophils, monocytes and macrophages, B cells, mast cells and eosinophils. Antigen presenting cells- monocytes and macrophages, dendritic cells. Other innate immune cells- natural killer cells, NKT cells, eosinophils, B1 cells, gammadelta t cells. Main defence barriers: 1. Anatomical- skin is a mechanical barrier to prevent microbe entry, dead skin sloughs off, skin has an acidic pH of 3-5 to prevent bacterial growth and commensal bacteria secrete bacteriocins and metabolites like lactic acid to kill other bacteria. Mucosal surfaces compete for attachment and nutrients and entrap pathogens, cilia propel it along.

2. Physiological/chemical- temperature of 37 inhibits growth of pathogens and raising it with a fever makes it more effective. Low pH in the stomach kills pathogens. Chemical mediators like lysozyme, lactoferrin, S100 proteins, defensins, cathelicidins, surfactant proteins and complement. 3. Phagocytotic/endocytic- involves uptake of extracellular material to get nutrients, sample the environment and for defence. All cells can take up extracellular material, for example by using osmosis, diffusion, ion pumps or endocytosis. Phagosome= membrane bound vesicle that engulfs a particle by pseudopodia forming around it. The force of fluid pushes pseudopodia out and around the particle. The eR is involved in the formation of phagosomes. Phagocytic and endocytic membranes are recycled. Digestion- some products are reused, some are exocytosed and some are stored until death. Oxygen independent:  Acidification- phagolysosome acidifies when it fuses, to enhance the enzymes activity.  Lysozyme- digests gram positive bacterial cell walls, but it works better with other enzymes.  Enzymes- acid hydrolases e.g. phosphatases, sulphatases, glycosidases, deoxyribonucleases. Lipases e.g. phospholipase A2. Neutral proteases e.g. collagenases, elastase, cysteine proteases.  Defensins- positively charged polypeptides that electrostatically bind negatively charged PAMPs like LPS or LTA. They aggregate to form pores in the cytoplasmic membrane, and activate complement in the classical pathway. It is the most common protein in neutrophil granules.  Lactoferrin- binds to essential nutrients to inhibit bacterial and fungal growth.  Cationic proteins- mainly in neutrophil granules, and some in eosinophils. Has a high molecular weight and is most active at alkaline pH. They damage bacterial membranes and proteins e.g. elastase, cathepsin G, serine proteases.  Tumour necrosis factor alpha (TNFa)- is mainly secreted in macrophages and is cytotoxic to tumour cells.

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Oxygen dependent: Activated phagocytes produce reactive oxygen and reactive nitrogen intermediates- free radicals (unpaired electrons). The ROI and RNI are very unstable and damage proteins, lipids, DNA and cell membranes. Oxygen radicals are short lived but superoxide and hydrogen peroxide released in tissues persists to cause tissue damage. RNI may act with ROI to give a full effect. Catalase, superoxide dismutase and glutathione peroxidase are free radical scavengers. There is a rapid increase in oxygen consumption, involving cytoplasmic and membrane associated enzymes. Oxygen is converted to peroxide, hydroxyl or superoxide anion with unpaired electrons. eNOS and nNOS (endothelial and neuronal nitric oxide synthase) are expressed and active in vasculature and neurones- for tone and as a neurotransmitter. Inducible NOS is activated by microbes and some cytokines, and oxidises L-arginine into L-citrulline and nitric oxide. In phagocytes, there is a large oxygen burst and lots of nitric oxide is produced with strong antimicrobial activity, which can combine with superoxide to be stronger. Most microbicidal mediators are produced in professional phagocytesmacrophages/monocytes (made in the lysosome) and neutrophils (made in the granules).

Neutrophils are more likely to kill ingested material as they have higher respiratory burst and more defensins. This does not kill all pathogens though.

General endocytosis is where the cell internalises extracellular material through invagination of the plasma membrane, but it can be done by different mechanisms: Pinocytosis- cell drinking to get nutrients and allow the cell to regulate osmotic pressure. The plasma membrane invaginates non specifically and depends on the external concentration. Most cells use pinocytosis. Macro-pinocytosis- takes larger gulps of nutrients and may recycle membrane receptors, as well as processing antigens. Some viruses use macro pinocytosis to infect host cells. Receptor mediated endocytosis- the specific uptake of a ligand, growth factor, hormone or immune complex. This happens when a ligand binds to a membrane receptor and moves to a clathrin coated pit that forms and endosome where the ligand goes to the golgi to be processed and receptors are recycled back into the membrane. Most cells use receptor mediated endocytosis. Phagocytosis- specialised cells internalise particulate matter and destroy it. For example, monocytes/macrophages, neutrophils, dendritic cells, eosinophils and B cells/mast cells phagocytose. The plasma membrane expands around the particulate to form phagosomes (large vesicles 10-20x larger than endocytic vesicles). The phagosome fuses with a lysosome to digest the material and this is released by exocytosis. Phagocytosis is caused by clustering of cell surface receptors and requires rearrangement of the cytoskeleton and energy.

Recognition allows the phagocyte to bind to foreign molecules on pathogens. Phagocytes bind to pathogen associated molecular patterns- PAMPs or damage associated molecular patterns- DAMPs where self cells signal danger or damage, these are important in tissue and limb development and damaged and dying cells. Recognition can be direct where surface receptors bind directly to molecules on the particulate matter. Or it can be indirect, where surface receptors bind to opsonised (coated) particles. PAMPs- flagellin on bacteria, glycoproteins on viruses, helminths and fungi.

DAMPs- phosphatidylserine, RNA, HDL and vitronectin on the surface of damaged necrosing/apoptotic cells. Direct: C type lectin receptors. Mannose receptor: binds to mannose on the surface of most macrophages and dendritic cells. It has 8 extracellular domains capable of binding large repeated structures, and a cytoplasmic tail that is able to trigger internalisation. Dectin-1: binds beta1-3 glucan and is expressed on many myeloid cells. It contains a cytoplasmic signalling motif. Scavenger receptors. These receptors bind altered self molecules on the surface of necrotic or apoptotic cells and recognise some PAMPs. SR-A- is found on all macrophages and some endothelial cells and binds modified LDL e.g. oxidised LDL. SR-B includes CD36 found on the endothelium, dendritic cells, platelets, monocytes and macrophages. CD36 is involved in helping cells interact with each other and cell lipid transport. Toll-like receptors. The toll gene was identified in Drasophilia. There are 10 TLRs in humans and each is specific for...