explain role of positive selection and negative selection in the immune system?
368912
Right, Posotive and negative selection occurs in the thymus.\n\nImmature T cells move from the Bone Marrow to the tymus where they are posotivly and negativly selected and this is the basis of Tolerance.  T cells that bind Too Weakly to the MHC antigen complex are killed by apoptosis.  T cells that bind too strongly are also killed by apoptosis.  Those that lie between are selected.  \n\nThis is important as T cell binding too weakly cant raise immune awarness and cells that vbind too strongly cause autoimmune diseases.\n\nLook up T cell anergy which is things like blocking of the receptor deletion of the T cell etc.  Try the book immunobilogy By c Janeway or "the immune system" by Peter Parham both published by garland science.  \n\nok this is a very basic overview as its a long subject and needs to be read rather than asked on this.\n\ngood luck
934896
The human genome is three billion base pairs.  There are four possible bases, so each base pair is two bits.  Six billion bits is less than one gigabyte, or a little more than a CD-ROM full.  MS Office takes up more disk space and doesn't do as much.\n\nDigital computers are easy.  Analog systems are far more complex.  You don't know exactly what will happen at any step.  Just a pretty good idea.  A short stretch of DNA may have many functions since the protein it encodes may do multiple things.  Take the proteins of the complement system.  Where antibodies attach to cells, they interact with the antibody and punch holes in the cell.  Some fragments that come off attract immune cells.  The immune cells have molecules that attach to the bound complement to help them ingest the attacked cells.\n\nAs for proof of God, it's not there.  Consider this: Microsoft has been around for 31 years.  What do you think they could develop in 500 million years?  Complement is complex, but its multiple functions all make sense from an evolutionary stand point.  Evolution favored creatures whose immune cells better responded to a defense protein system.
142278
The immune system is the system of specialized cells and organs that protect an organism from outside biological influences. (Though in a broad sense, almost every organ has a protective function - for example, the tight seal of the skin or the acidic environment of the stomach.) When the immune system is functioning properly, it protects the body against bacteria and viral infections, destroying cancer cells and foreign substances. If the immune system weakens, its ability to defend the body also weakens, allowing pathogens, including viruses that cause common colds and flu, to grow and flourish in the body. The immune system also performs surveillance of tumor cells, and immune suppression has been reported to increase the risk of certain types of cancer.
285327
Your argument has some assumptions, assertions, and logic that are not sound.\n\nFirst of all, you only multiply probabilities together when they are independent variables - it would be easy to argue that many of the items that you mentioned are interdependent.\n\nAlso, I think you misunderstand what evolution of species is proposing.  Evolution does NOT depend on luck - it is instead an observation that lucky organisms survive and pass on their properties to their descendants.\n\nConsider bacteria - almost all mutations are harmful but a few bacteria have lucky mutations that make them immune to penicillin.  If penicillin is used on bacteria, only these lucky ones survive - and most of their descendants inherit the the same immunity-mutation.  \n\nThe original mutation is luck - the environment then selects and a small set of traits due to beneficial mutations persists.  And THAT is not luck, that is selection.  That's why the process is called "Natural Selection".  \n\nSadly, this is why almost all bacteria were zapped by the early antibiotics, and now (only 60 years later) we have hundreds of resistant strains.
305237
Innate immunity - non-specific to a particular pathogen, macrophages important cells here.  Also involves inflammation and acute phase response\n\nAdaptive immunity - specific and has 'memory cells'. Involves humoral immunity and cell mediated responses.  Involves reactions to specific antigenic challenges.\n\nComparative immunity - signal transduction pathways used to invoke immunity that are similar across a wide range of organisms - vertebrates, insects and even plants!!!  Don't really know much about this found it in a book, see source below!!!
1062719
Due to differences of diversity of the human race, and the flora and fauna arond them, some diseases are localized only in certain areas, and due to genetic diveresity over time, people can develop immunities to certain diseases.\n\nFor example, it's been discovered that certain descendants of Scandinavians/Vikings are HIV immune.\n\nIt's for reasons like this that genetic diversity is such a good thing, and why imbreeding is so frowned upon.
336439
Mutation generates differences.  Most mutations are deleterious and natural selection tends to eliminate them quickly.  Many are neutral and are retained to some degree because there's nothing weeding them out.  A few are positive and variation is also lost in that case because selection will eliminate the other variations.\n\nThere is a process called frequency dependent selection whereby a trait is advantageous but only if its rare.  For example since hosts adapt to a certain kind of parasite, it is advantageous for a parasite to have rare characteristics.  That way the host is unlikely to have seen that model before and the parasite is likely to be successful.  In that case selection actually favors a lot of versions in a species because as soon as anything become really numerous it becomes deleterious.
579669
The 'modern' human experiment could be seen as an answer to your question.  We now, for our 'comfort' do things which are certainly to be considered 'adaptive selection of species'.  The way this results in 'natural selection of a population' can be felt and seen and even quantified, if one is sentient and humble enough.  Glad to help!
121182
Part of the cell mediated antigen-antibody defense system.
314496
Good and bad only exists in human mind. Spread of a virus is good for that virus, and for whatever thing that fish ate. It also helps that one little & weak fish that just happened to have natural resistance to the virus - it would re-populate the pond with it's offspring.\n\nNature is about balance of many things that fight for their own survival against everybody else. If the balance gets upset by some force, some species benefit from it, and some loose.
574003
In human anatomy, the thymus is an organ located in the upper anterior portion of the chest cavity. The thymus plays an important role in the development of the immune system in early life, and its cells form a part of the body's normal immune system. It is most active before puberty, after which it shrinks in size and activity in most individuals and is replaced with fat.\n\nThe thymus is also present in many other animals. When animal thymus tissue is sold in a butcher shop or at a meat counter, thymus is known as sweetbread.\n\n\nFunction:\nThe function of the thymus in the immune system was determined by Jacques Miller. In the two thymic lobes, lymphocytes precursors mature into T cells (where T stands for “thymus”)[1]. The key function of the thymus is the selection of the T cell repertoire that the immune system uses to combat infections. This involves selection of T cells that are functional (positive selection), and elimination of T cells that are autoreactive (negative selection). Positively-selected cells will be taken care of by specialized Nurse Cells. In order to be positively-selected, pre-T cells will have to interact with several cell surface molecules, MHC/HLA, to ensure reactivity and specificity. Cells that pass both levels of selection are released into the bloodstream to perform vital immune functions. Negative selection is not 100% complete. Some autoreactive T cells escape thymic censorship, and are released into the circulation. Additional mechanisms of tolerance active in the peripheral exist to silence these cells such as anergy, deletion, cross-tolerance and regulatory T cells. If these mechanisms also fail, autoimmunity may arise.\n\nFurther info: \n\nhttp://en.wikipedia.org/wiki/Thymus
810506
Its a.) marker proteins.  They let the other surrounding cells know what that cell is and that it is not a forign body which should be destroyed.  Lets all be thankful for the marker proteins protecting our cells from being attacked by our own immune system.
69326
Innate immunity is what you inherhit through your genes that provides an immediate defense against pathogens.\n\nI'm not sure of your question regarding how it affects sex and age, but the link below can provide more info.
438015
Certain bacteria have evolved into a symbiotic relationship with their human host.  In exchange for producing a vitamin that humans are unable to synthesis on their own, the body’s immune system does not mount a strong immune response against the bacteria.\n\nThis was advantageous for the host (humans) in areas where diets rich in vitamin K were rare or seasonal and beneficial for the bacteria.\n\nOr if you prefer Creationism to evolutionary theory… “Because God said so”
655371
I think that the correct word you are looking for is IMMUNOGEN: "any substance or organism that provokes an immune response (produces immunity) when introduced into the body"\n\nNot all immunogens are antigens (which means "substance capable of raising antibodies against it") nor allergens ("responsible of causing allergy, a particular kind of immune response.")
67893
Well, not to sure about your question... so I'll do my best.\n\nPositive interactions between microbes would be ones that cause a general benefit.  Like a bacteria excreting a certain amino acid that another bacteria would need in order to survive.\n\nNegative interaction would be microbes causing a detriment to each other.  Pencillium notatum generates an antibiotic naturally, causing a harmful environment to some bacteria.\n\nSymbiosis is two organisms living together in harmony.  If you refference Margulis's endosymbiont hypothesis, a large microbe, say an amoeba engulfs a photosynthetic bacteria.  The amoeba provides a safe location for the bacteria, and the bacteria provides energy to the amoeba.\n\nAntagonism... not to sure about this one.  Probably something of a parasitic interaction.
328841
Natural Selection is a process where the weaker individuals are "culled out" and only the stronger ones survive.  A good example of this concerning bacteria is the overuse of antibiotics.  Eventually, the resilient individuals will survive antibiotic treatment and become resistant to that particular antibiotic.  Eventually, that population will grow enough and be untreatable with that particular treatment and another antibiotic will have to be used.  Some speculate (myself included) that this is how Bird Flu was created except on a viral level instead of bacterial.  In summary, new environment (heat, cold, antibiotics, etc.) weaker bacteria die off, strong ones survive and propagate and therefore, live on. . . thus adaptation.  Hope this helps.