Immunology seems to be one of those things that people either love or hate; I think it’s fascinating, but I know there will be a lot of people out there who approach the subject with a mixture of terror, frustration and loathing. To make the normal immune response less of a horrendous nightmare to learn, I made a summary diagram showing friendly, loveable cartoon immune cells doing what they do best . The immune system – Barrier mechanisms There are numerous potential ways for pathogens to enter the body. Humans have therefore evolved several physical and chemical barrier mechanisms to prevent the invasion of infective organisms: intrinsic epithelial barriers exist between the body and the outside world. Epithelial cell walls have very tight junctions between them, and are therefore hard to penetrate. Examples include the linings of the mouth, nasal passages, upper airways, lungs and GI tract. the continuous longitudinal flow of air or fluid through most body systems helps to create a flushing action which prevents situations in which bacteria could adhere to structures, proliferate and invade the movement of mucus by cilia in the lungs also helps prevent the stagnation of secretions and the adherence of inhaled droplets and particles. Mucus is moved upwards towards the pharynx, where it is then swallowed or coughed up. desquamation of skin and epithelial cells also prevents adherence of microorganisms natural acids persist in many parts of the body, for example fatty acids on the skin, lysozymes in saliva and hydrochloric acid in the stomach there are also many natural antibacterial peptides on the skin and the surface linings of the lungs and gut. These include cathelicidins, defensins, proteinase inhibitors and chemokines. normal bacterial flora colonising various parts of the body compete with infective microorganisms, and some also produce antimicrobial substances. For example, vaginal lactobacilli produce lactate, which creates an acidic environment and destroys many potentially infectious organisms. The immune system – Cells. There are many different groups of cells involved in the immune response. Depending on which medical school you’re at, you may be expected to be able to recognise them on microscopy, so I’ve included some pictures of actual real cells alongside my silly cartoon ones. GRANULOCYTES – a family of white blood cells containing granules in their cytoplasm - NEUTROPHILS - EOSINOPHILS - BASOPHILS MONOCYTES + MACROPHAGES – large cells involved in phagocytosis and antigen presentation - BLOOD MONOCYTES - TISSUE MACROPHAGES - DENDRITIC CELLS LYMPHOCYTES – small, specialised white blood cells with large nuclei and no granules - B CELLS - T CELLS - NATURAL KILLER CELLS . The immune response in a nutshell The normal immune response can be broken down into four main components: Part 1 – Innate Immune System This is the first line of defence against any infection. It is very fast – it is established within about 4 hours – but is non-specific and has no memory, so it is not strong enough to effectively tackle an infection on its own. It consists of a cellular response by the innate immune system, a chemical response by cytokines and complement, and the subsequent initiation of an acute inflammatory response. - INNATE CELLULAR IMMUNE RESPONSE Phagocytes identify pathogens by recognising pathogen-associated molecular patterns (PAMPs) using pathogen recognition receptors (PRRs). Toll-like receptors (TLRs) are an example of a PRR. once they have identified dangerous organisms, they internalise them, kill them and digest them down into their component proteins phagocytes then present the digested protein antigens to the cells of the adaptive immune system via major histocompatibility complexes (MHCs) on their surfaces. The MHC complex acts as a safety mechanism. It prevents the immune system from being activated too easily, as it ensures that T cells can only react to an antigen if it is presented within an MHC complex. This phenomenon is known as MHC restriction. when phagocyte PRRs are exposed to PAMPs, NFKB is activated. This is a transcription factor which results in the release of proinflammatory cytokines and the initiation of the inflammatory response. Natural killer cells unlike T cells, they do not require activation by specific antigens, which means they are able to respond immediately when exposed to a pathogen “self” cells are protected from the destructive action of NK cells by the inhibitory effects of MHC I, which is expressed on the surface of all nucleated body cells any cells without an identifiable MHC I are likely to be “non-self” and fair game for immediate annihilation – NK cells do this by releasing toxic granules to induce apoptosis normally, NK cells cannot attack healthy “self” cells. However, MHC I expression is often suppressed if cells are infected with viruses, or have become cancerous. NK cells can therefore perform additional vital roles in viral immunity and tumour rejection. - INNATE CHEMICAL IMMUNE RESPONSE Complement is a cascade of chemicals similar to the clotting cascade: There are three separate pathways which activate the complement system: classical pathway: activated by antibody-antigen complexes (aka immune complexes) on pathogen surfaces mannose-binding lectin pathway: activated when mannose-binding lectin binds to the carbohydrate molecule mannose on pathogen surfaces alternative pathway: C3 reacts directly with pathogen surfaces All three of these pathways act to generate the enzyme C3 convertase. This cleaves C3 into two parts (C3a and C3b) and activates the rest of the cascade. Proinflammatory cytokines are the second key component of the innate chemical immune response. They are small messenger proteins released by immune cells in response to evidence of infection, which interact to mediate the acute inflammatory response (see Part 2). There are a huge number of different cytokine molecules, including whole families of interleukins, tumour necrosis factors and chemokines. Some important examples include: IL-1 – causes fever and activates lymphocytes IL-6 – causes fever, stimulates the liver to produce acute phase proteins such as CRP, activates lymphocytes and promotes antibody production Part 2 – Inflammatory Response It acts as a bridging mechanism to localise and contain the infection in the period from about 4-96 hours after its onset, when the innate immune system is running out of steam and the specific cellular immune response is still trying to get going. The main features of this process are: vasodilation and increased blood flow – this leads to erythema (“rubor”) and warmth (“calor”) increased vascular permeability – this allows an inflammatory cell infiltrate to extravasate and reach the site of infection, and also causes tissue oedema and swelling (“tumour”) release of inflammatory mediators such as bradykinins and prostaglandins which increase pain sensitivity and cause hyperalgesia in the infected area (“dolor”) neutrophil chemotaxis – neutrophils migrate to the site of infection and begin their clean-up operation, phagocytosing pathogens and debris microvascular coagulation – this is induced by local tissue damage, and acts to confine the infection and prevent its spread systemic features such as fever and raised inflammatory markers such as CRP and ferritin – this produces unpleasant “flu-like” symptoms such as hot flushes, sweats, chills, rigors, headache, nausea, myalgia, arthralgia and fatigue. upregulation of costimulatory molecules such as MHC-II and B7 to encourage activation of the adaptive immune system Part 3 – Antigen Presentation The innate immune system and inflammatory response can only hold off an infection for so long – ultimately, a specific immune response needs to be activated. This is done via antigen presentation to the adaptive immune system. There are two main protective mechanisms which prevent this from happening by controlling the activation of the adaptive immune system: MHC restriction ensures that only antigens presented within the context of MHC complexes are able to trigger the immune response in order to become fully activated by their specific antigen, naive T helper cells also require a second signal from antigen presenting cells. Dendritic cells are able to provide this in the form of B7 proteins (CD80 or CD86) which bind to CD28 receptors on T cell surfaces. expression of second signal molecules is increased by the presence of an inflammatory response, increasing the likelihood of T helper cell activation The combination of the right antigen, an MHC II and a B7 second signal gives the green light for naive T helper cells to get going. The next step is for them to differentiate into either TH1 cells, which promote cytotoxic T cells and cell-mediated immunity, or TH2 cells, which promote B cells and humoral immunity. Part 4a – Humoral Immunity Humoral immunity is the term for a specific adaptive immune response activated by TH2 cells, which leads to the production of B cells and antibodies. This immune response is designed to fight extracellular infections, including most bacteria andfungi, protozoans such as Giardia, and parasitic worms such as Schistosoma. - ANTIBODIES Antibody molecules are essentially secreted B cell receptors which provide an antigen-specific action. They are Y-shaped molecules with a complex structure comprising: There are five antibody classes or “isotypes”. These are dictated by the structure of the heavy chain constant region IgM – this has a pentameric structure. It is expressed on B cell surfaces and produced early in the immune response whilst IgG is being generated. IgG – this has a monomeric structure and provides the majority of antibody-based immunity. It is found mainly in circulating blood and tissues, and also crosses the placenta to provide passive immunity to the fetus. IgA – this forms a dimeric structure once it reaches its target tissues. It is found in mucosal areas such as the GI, respiratory and urinary tracts. It is also secreted in saliva, tears and breast milk. IgE – this has a monomeric structure. It binds to allergens and mediates allergic reactions, as well as providing immunity against multicellular organisms such as parasitic worms. IgD – this has a monomeric structure and is rather mysterious. It is found in very low levels in the serum, and appears to interact with basophils and mast cells. As a result, a range of mechanisms have evolved to allow B cells to manipulate their own DNA and produce billions of different variable region structures: antibody variable region genes are coded in three parts: V (variable), D (diversity) and J (joining) segments. RAG proteins allow B cells to shuffle these gene segments around during their maturation and recombine them in millions of different ways. This is known as VDJ recombination. junctional diversity is produced by imprecise joining of VDJ segments during maturation, as the variable overlap of genes results in the gain or loss of a few nucleotides “looping out” and rejoining of gene segments produces variations in genomic structure genetic diversity is also increased by the addition of random nucleotides called N regions when mature B cells are activated by their specific antigen, they start to produce IgM antibodies, and also undergo isotype class switching to produce different types of antibody adapted for various locations within the body B cell activation also promotes somatic hypermutation of variable region genes to produce ever-so-slightly different versions of the same specific antibody. These are “tested” to find the best match using clonal selection, and the ones with the highest possible affinity for the antigen are encouraged to proliferate in a process called affinity maturation. Antibodies fight extracellular infections in a number of ways: they neutralise toxins by directly binding to them they bind to antigens on pathogen surfaces. This agglutinates them to impair their mobility, and also opsonises them to enhance phagocytosis. the binding of antibodies to antigens to form complexes activates the classical complement pathway they also directly activate effector cells such as dendritic cells, NK cells and cytotoxic T cells - THE HUMORAL IMMUNE RESPONSE Humoral immunity and antibody production are dependent upon T helper cells activating B cells: once naive TH0 cells have been activated by their specific antigen, they differentiate into TH2 cells TH2 cells locate their corresponding B cell counterparts by identifying the correct antigen within an MHC II on the B cell’s surface they then provide the B cell with a second signal, in this case CD40 ligand which binds to CD40 on the B cell surface they also release cytokines such as IL-2, IL-4 and IL-5 which promote B cell development Activated B cells mature into plasma cells and start to make antibodies: initially plasma cells produce IgM antibodies, then isotype class switching produces different types to cover different areas of the body clonal expansion of antigen-specific plasma cells is followed by somatic hypermutation, clonal selection and affinity maturation to ensure the production of the best antibodies for the job, which are released to tackle the infection Once the infection has been cleared, some plasma cells will remain as dormant “memory” B cells: only the most highly-antigen specific B cells produced by affinity maturation will be selected to become memory cells the presence of memory cells means that immediate plasma cell proliferation and antibody production can occur at the time of the next infection the number of surviving memory cells increases after each reinfection, so the more times you are exposed to a particular pathogen, the better your immune response to it becomes Part 4b – Cell-Mediated Immunity Cell-mediated immunity is the term for a specific adaptive immune response activated by TH1 cells, which leads to activation of antigen presenting cells and a cytotoxic T cell response. This immune response is designed to fight intracellular infections, including viruses, some bacteria and fungi, and protozoans such as Plasmodium and Toxoplasma. - T CELL RECEPTORS. All immature T cells undergo a rigorous “education” in the thymus gland before they are released into the bloodstream, but this process is particularly important for cytotoxic T cells due to their destructive nature. They are “tested” with a variety of self cell antigens, and any cells which have generated a receptor that reacts to these undergo negative selection and are destroyed. This essential mechanism prevents the immune system from reacting to the body and is known as immunological tolerance or self-tolerance. In order to graduate successfully from the thymus, T cells must also express CD3 and CD4 or CD8 (but never both), and bind to self MHC complexes (but not too strongly). Only about 1% of T cells generated in the bone marrow actually make it through this process alive! - ACTIVATION OF ANTIGEN PRESENTING CELLS The first step of the cell-mediated immune response is the activation of antigen presenting cells: a TH1 cell encounters an unhappy infected antigen presenting cell, and recognises the MHC II-restricted antigen on its surface it then “activates” the APC by providing a CD40 ligand second signal and secreting interferon gamma (IFNγ), a cytokine which is essential in stimulating the immune response to intracellular infections once activated, APCs are able to increase their production of nitric oxide and superoxide radicals, which optimises their killing mechanisms and allows them to destroy ingested pathogens much more effectively - THE CYTOTOXIC T CELL RESPONSE. The next step is the activation of an antigen-specific cytotoxic T cell response: activated APCs present their antigen to the specific cytotoxicT cell receptor within an MHC I, along with a variety of second signals, including B7 + CD28 and/or 4-IBB + 4-IBBL this process is helped along by the secretion of IL-2 – a potent T cell growth factor – by TH1 cells and the cytotoxic T cells themselves Once activated, the cytotoxic T cells are very keen to get out and start hunting and killing things. They identify infected cells by recognising the antigen displayed within MHC I on their surfaces. They then destroy these cells using one of several mechanisms: they classically form an immunological synapse with their target cell – this just means the cell membranes touch – and release a substance called perforin to make a hole in the cell wall. They then use this hole to release granzymes and granulysin into the cell, which induce apoptosis and DNA fragmentation. Fas ligand interactions between the cell surfaces can also produce apoptosis of the infected cells via the aptly named death-inducing signalling complex (DISC) cytotoxic T cells can also release interferon gamma (IFNγ), whichhas an interesting role in viral infections, as it is able to block intracellular viral replication without killing the cell itself. This effect is very useful, as killing and lysing virally infected cells would simply let all the baby viruses out and cause the infection to spread itself even further, which is clearly suboptimal. After the infection has been dealt with, the most antigen-specific cytotoxic T cells will remain behind as dormant memory T cells. The principles of T cell memory are essentially the same as B cell memory. during a reinfection, only the first signal (MHC + antigen) is required to activate the cytotoxic T cell response; no second signal is necessary this means that any antigen presenting cell (not just dendritic cells) can activate cytotoxic T cells directly, reducing the need for TH1 cell help and resulting in a much swifter and more efficient cell-mediated immune response. Summary of the Immune Response We’ve now covered everything in the diagram in detail – hopefully it seems a lot less scary now! Responses to Different Infections It is useful to be able to apply your knowledge of the immune response to different types of infection, especially when it crops up in exam questions. The most important differentiation to make is whether the infection is intracellular or extracellular, as this generally dictates which branch of the adaptive immune response will be activated: extracellular infections –> TH2 –> humoral immune response with B cells and antibodies intracellular infections –> TH1 –> cell-mediated immune response with activated APCs and cytotoxic T cells Some types of pathogens can only exist as either extracellular or intracellular organisms, whilst other types can vary depending on the individual species. There are also unique variations in aspects of the immune response for some organisms. - BACTERIA bacterial infections trigger the classic immune response as described in the main article above bacterial infections are usually extracellular however, some bacteria do choose to exist as intracellular organisms; examples of these include Neisseria, Salmonella, Chlamydia and Mycobacteria - VIRUSES viral infections are intracellular and therefore handled by cell-mediated immunity interferons are a family of cytokines which act as the equivalent of complement in viral immunity, and also have additional unique functions. For example, cytotoxic T cells release interferon gamma, which inhibits viral replication within infected cells without damaging the cells themselves. new baby viruses are released from cells as part of the spread of a viral infection, and viral antigens are also expressed on the surfaces of infected cells. This means that some aspects of humoral immunity are also useful in viral infections. Antibodies are able to bind to viral antigens in order to neutralise and opsonise the baby viruses after they are released, limiting the spread of infection. natural killer cells also play a vital role in viral immunity, as they recognise and destroy infected cells which have either expressed antigen on their surfaces or lost their inhibitory MHC I - FUNGI the normal immune response is able to deal with fungal infections very swiftly and effectively fungal infections are usually extracellular and therefore dealt with by humoral immunity less commonly, fungi opt to “break the mould” (sorry) and become intracellular – examples of these include Histoplasma, Cryptococcus and Pneumocystis, all of which are well known to cause opportunistic infections in immunosuppressed patients lacking the cell-mediated immunity vital in tackling such organisms macrophages and other phagocytes are very important in fungal immunity - PROTOZOANS our immune response to protozoa isn’t that great. This is probably because many species have evolved hundreds of clever protective mechanisms which turn the immune system to their advantage, such as resistance to phagocytosis and complement lysis, antigen variation, antigen shedding and even direct modifications of host immune mechanisms. examples of extracellular protozoa include Giardia, which infects the intestines and may have caused many of you the joys of traveller’s diarrhoea; Entamoeba, which causes dysentery and nasty amoebic liver abscesses; and Trypanosoma, which causes sleeping sickness and featured in an excellent episode of House. other protozoa are intracellular organisms. Examples include Plasmodium, which occupies red blood cells and liver cells to cause malaria; Leishmania, which survives inside phagocytes after being ingested, and affected many soldiers during the conflicts in Iraq and Afghanistan; and Toxoplasma, which lives in many body tissues and can cause “crazy cat lady syndrome” amongst other things. - WORMS (aka HELMINTHS) worms are very big compared to other infective organisms, and are obviously always extracellular – common examples include Schistosoma, which spreads worms through the bloodstream to vital organs; Onchocercha which causes river blindness; and Taenia tapeworms which can cause malnutrition and cysticercosis TH2 cells and humoral immunity form the basis of the body’s immune response to parasitic worms eosinophils and IgE are also very important in killing helminths, as alongside promoting a powerful inflammatory response, they appear to bind to the opsonising antibodies on the worm’s skin in order to subsequently dissolve it and kill it I hope you found this guide helpful and fun. I certainly enjoyed creating the little immune cells and telling their story (perhaps a bit too much actually). It was a mammoth undertaking to write this thing, and while I have tried to be as thorough and accurate as possible, if any of you clever folks out there have noticed any mistakes/miscommunications/other cock-ups please do let me know so I can correct them for the benefit of everyone else – in return you shall receive the reward of your name immortalised below and a Geeky Medics gold star… " many thanks to Colin Hill for helpfully highlighting a mix-up in Part 3! " Source
