complex and meaning that the antigen is now


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php References:                                       Tonsils/ thymus- Make antibodiesLymph nodes-Lymph vessels- Carry lymph fluid around the body, one of three types of bloodvesselAs each pathogen is specific to a particular victim and therefore notall organisms are susceptible to the same pathogens. For example, humans cannot catch the viruses that cause canine distemper, feline leukemia, and mousepox. Similarly, the virus that causes AIDS in humans does not infect animalssuch as cats and dogs.

 Theimmune system is a composite arrangement of organs, containing cells thatrecognise foreign pathogens entering the body and destroy them, via theprocess of phagocytosis (the engulfment of foreign pathogens). Phagocytosis Sometimes the immune system can malfunction. It may react to the wrongthing (autoimmunity), overreact (hypersensitivity), and in some cases it maynot react at all (immunosuppression and immunodeficiency).

  Natural immunity is where you become immune to a diseaseonce you have already caught it. For example, if you catch chicken pox as ayoung child, the risk of catching it again as an adult is minimal. B lymphocytes are activated, resultantly secreting antibodies,which move through the blood stream, binding it to the foreign antigen anddeactivating it, forming an antigen-antibody complex and meaning that theantigen is now unable to bind to the host cell. T lymphocytes  Adaptive immunity is the third line of defence and refers tothe type of immunity in which mechanisms have become specialised to respond toa specific pathogen. This type of immunity is often developed after the firstexposure to a disease, remaining ‘on standby’ in case of further infection. Adaptiveimmunity involves two types of response (cell mediated and humoral). In cellmediated response, T helper cells are produced in the thymus and haveantibodies on their surface which are specific to a certain antigen which bindtogether, forming an antigen-antibody complex via a process known as clonalselection and therefore activating the T helper cell. After this, clonalexpansion will occur, a process whereby the T lymphocyte will divide mitoticallyto produce clones of itself, causing the activation of T regulatory cells, Tkiller cells and macrophages.

T regulatory cells suppress immune response and Tkiller cells attack their own cells when they have been antigenically alteredas well as unicellular parasites. They do this by severing their cell membranes,causing the contents to overspill.   The second line of defence is referred to as innate immunity and is thenext step to prevent disease, should the first line of defence have beenineffective. Innate immunity is immunity which is non- specific and presentfrom birth, effective against a wide variety of pathogens and foreignsubstances via physical, chemical and cellular defences. It provides a quickresponse, however it does not distinguish between different pathogens so is onlya basic defence mechanism. Macrophages carry out a process called phagocytosiswhich is essentially the engulfment of pathogens. Each pathogen will have anantigen on its surface which is specific to that particular pathogen. Duringthe phagocytic process, the phagocyte will recognise the pathogen, promptingthe cytoplasm of the phagocyte to enclose itself around the pathogen, assistedby opsonins in engulfing it.

 As a resultof this, the pathogen will now be contained within a phagosome (a type of vesiclewhich is a fluid filled sac, found within the cytoplasm of a cell and responsiblefor the transportation of substances into and out of the cell). The phagosomewill then fuse with a lysosome, the digestive enzymes within, breaking down thepathogen. Lastly, the macrophage will become an antigen-presenting cell,meaning that it will present the antigens of the pathogen which will stick toits surface which then will activate other cells found within the immune system. Blood clotting is also part of the first line of defence.

It works byhost cells releasing histamine which increases the permeability of the bloodvessels at the site of damaged tissue, causing acute inflammation. Inflammationcauses an increase in blood flow to the affected area due to vasodilation sothere will be an increased concentration of white blood cells which will helpexpel pathogens from the infected site. Signs of inflammation include pain,redness, heat and swelling. A blood clot will form to prevent continuedbleeding; this occurs due to the damage to blood vessels exposing collagenfibres which activate platelets and therefore start the blood clotting cascadereaction, resultantly forming a temporary plug. The platelets activate thesoluble enzyme, fibrogen in the presence of vitamin K and calcium ions and arereinforced by substances known as clotting factors. This causes the formationof fibrin (a soluble protein produced by the liver and found within bloodplasma at the wound due to the action of the clotting enzyme, thrombin) whichforms a platelet plug at the site of the damaged tissue, holding platelets andclotting factors together.  Within the body, there are three lines of response to pathogens, thefirst of these being physical and chemical barriers. The skin is the largest organin the barrier and provides a physical barrier which pathogenic microorganismsare unable to penetrate.

Tears, mucus and saliva contain lysozyme, an enzymewhich destroys bacteria via the destruction of their cell walls by breakingdown the bonds holding together the peptidoglycans that the cell wall iscomposed of. Cilia, lining the trachea and the lungs move mucus and otherparticles which may become trapped away from the lungs, pushing them back outand urinary flow flushes pathogens out of the bladder area. The acidic natureof the stomach ensures that any bacteria and parasites which may have beeningested are destroyed.                        Anexample of   Parasitesthat are required to exploit more than one host species for the completion oftheir life cycle are referred to as having an indirect lifecycle, whereas thoseinfecting only one species are said to have a direct life cycle.  Aparasite is an organism which obtains its nutrients from a ‘host’organism.Parasites can be classified into the categories of endoparasites and ectoparasitesas well as protozoa.

There are several defining characteristics of the twotypes of parasites; endoparasites live within the host’s own body and areusually permanent, respiring anaerobically. They are also usually obligateparasites, meaning that they are unable to complete their entire life cyclewithout the exploitation of a host organism. On the other hand, an ectoparasitelives on the surface of the host and has the capacity to be either temporary,intermittent or permanent. Some are also obligate parasites, however some arefacultative (does not depend upon its host for continuation of life cycle).Ectoparasites also respire aerobically, which occurs within the mitochondria ofits cells where adenosine triphosphate is broken down in to adenosinediphosphate by breaking the bond connecting one of the phosphate groups to therest of the molecule, therefore releasing energy.  Protozoa are unicellular, eukaryotic organismswhich Rabies is an example of a viralinfection that can be contracted in animals and is neurotropic, (attacks thebody’s nerve cells). It is of lyssavirus genus, meaning that it is oficosahedral shape, approximately 180 nanometres in length and of therhabdoviridae family, meaning that it consists of a helical ribonucleoproteincore situated within a surrounding lipoprotein envelope. Five proteins areencoded by the rabies genome- nucleoprotein (N), phosphoprotein (P), matrixprotein (M), glycoprotein (G) and polymerase (L).

In order to infect its host,the rabies virus first binds to the receptor before entering the host cell viathe endosomal transport pathway via a process called endocytic recycling, whichregulates the composition of the proteins found within the plasma membrane. Thedisease is usually passed on when an infected animal bites another infectedanimal, as the virus is carried in saliva. Rabies has a relatively longincubation period, with symptoms usually taking approximately three- eightweeks to present in dogs and two to six weeks to present in cats. Clinical signsof the disease vary, however there are three main stages which may occur,although not every infected animal will necessarily present with every stage.The first stage is called the ‘prodomal phase’ and lasts for approximately two-three days in dogs and a day or two in cats. Signs that they are within thisphase include repetitive licking of the site of infection and a fever.

Erraticbehaviour often occurs, with otherwise friendly animals becoming shy orirritable whilst unaffectionate animals may become affectionate and docile.They may also display signs of nervousness, anxiety, isolation and trepidation.The second stage is known as the ‘furious phase’, which cats are more prone todeveloping than dogs and lasts approximately one- seven days. Restlessness andirritability occurs, followed by hyperresponsiveness to both visual andauditory stimuli. Animals within the furious phase become highly agitated,becoming more and more vicious and irritable, potentially biting and attackingpeople/ other animals around them and their surrounding environment.Disorientation sits in and seizures occur, resulting in death of the animal.Lastly, the paralytic phase (which occurs after either the prodromal or furiousphase) affects the nerves in the head and throat, causing salivation anddifficulty swallowing. There is potential for facial paralysation due deep,laboured breathing and a dropped jaw.

A choking sound will occur, as thoughthere is something lodged in the animal’s throat, proceeded by the occurrenceof respiratory failure and eventually death. The optimum temperature range forthe growth of viruses is around 37 degrees Celsius, with the ideal pH rangebeing a neutral condition of around 7-7.4. In order for viruses to grow, Oxygenis not needed and they reproduce within the body’s own cells; thereforeantibiotics are ineffective on viruses.

 Complex: Structuresare formed from a combination of helical and icosahedral shapes and they oftenhave a complicated outer wall, incorporating head-tail morphology in certainviruses and usually only affect bacteria. Helical: Consist ofproteins forming a circle around a capsid with a cavity in the middle which isfilled with nucleic acid. 15-19nm is the standard width, with 300-500nm beingthe general range of length.

 Envelope: Surrounded bya lipid bilayer membrane, encasing the virus. As the virus exits the cell viabudding, the envelope is formed. Examples of these include hepatitis C andInfluenza.  Icosahedral: At adistance, icosahedral viruses appear spherical to the naked eye, however uponcloser inspection they are not strictly spherical- they are composed ofequilateral triangles, fused together to form a sphere. Icosahedral viruses arereleased into the atmosphere when the cell dies, breaks down and lyses, thuscausing viral diseases at this point.

 Viruses are extremely smallorganisms, ranging from just 20-750 nanometres, composed of a capsid (proteincoat) surrounding a coiled up strand of nucleic acid. There are four differentshapes of which a virus can take form:Aspergillosis is a fungal diseasewhich is prevalent in mammals and birds. The respiratory tract is the mostcommonly affected area (nasal aspergillosis); however it has also been known toaffect the ears, mouth, liver and throat (systemic aspergillosis). The nasalform of aspergillosis is contracted via airborne transmission as aspergillusfungi shed conidia (microscopic spores) which are very easily inhaled as theyfloat around the air. A healthy immune system will prevent these spores fromentering the body, however if the immune system is compromised, it will be ableto find a way in so some animals are more vulnerable to the disease thanothers. It is not yet known how the systemic strain of the disease iscontracted. In both forms of the disease, the fungus aspergillus causesnecrosis and the formation of small abscesses which grow out spore producinghyphae, thus causing the further spread of infection.

Symptoms are different,dependent upon which organs and species of animal is affected, varying betweennasal and systemic. Chronic nasal discharge with a strong odour is usuallyprevalent in the nasal strain of the disease and will not subside withantibiotics, as it is not a bacterial infection. Intermittent nosebleeds and ulcersaround the nostrils are also often common. Systemic aspergillosis symptomsdepend upon which organs are affected however may include lameness, weakness,lack of appetite, weight loss and lack of coordination. It can also lead to thedevelopment of uveitis and holes with pus or bloody discharge may be present,however most animals with systemic aspergillosis will not show any nasalsymptoms. Aspergillosis is a nasty disease with reference to the fact thatclinical signs take a long time to present themselves so it is usually at acritical stage before diagnosis.

 There are several fungal routesof transmission, one of these being via direct contact (tactile) transmissionwhich is how lymphangitis is contracted. Epizootic lymphangitis is a fungaldisease which is most common in equine organisms, however can also occur withincattle. It is caused by the fungus Histoplasma farciminosus and causes chronicinflammation and suppuration of both the cutaneous and the subcutaneouslymphatic vessels and glands. It enters the body via a wound or an abrasion onthe skin or of a mucous surface and is usually spread via fomites such as tackor grooming brushes which have come into contact with an already infectedanimal. The incubation period of lymphangitis is relatively long (on average 6-8 weeks) and the disease’s spread is slow and subtle, causing gradually increasingpain.

 The function of fungi is tosurvive and reproduce and they have several methods of reproduction and may beclassified according to how they reproduce. There are two asexual methods ofreproduction and one sexual method of reproduction. Zygomycota reproduceasexually when nuclei fuse within a thick-walled zygospore when certain hyphaegrow towards each other and join together. The second asexual method ofreproduction within fungi produces a group called ascomycota. These organismscan produce asexually or sexually, taking their name from the sac-like ascusformed during sexual reproduction, however also reproducing asexually viafission, spores or budding. Lastly, basidiomycota reproduce solely sexually viaspores, for example mushrooms and toadstalls. They produce basidia upon afruiting body and develop basidiospores, enabling them to reproduce.

 Fungi are multicellular,eukaryotic organisms. The main body of fungi is composed of a fine, branchedstructure called hyphae which intertwine to form mycelium. Generally the cellwall is composed of chitin, however a few types of fungi have cellulose cellwalls, in the way that plants do.

Fungi are heterotrophic organisms, absorbingnutrients from living or dead organic matter on which they grow. They thrivewithin a slightly acidic environment and require very little moisture for theirgrowth. The optimal conditions for growth of fungi vary, with most survivingbest in a temperature range of 21- 32°C, however somespecies of fungi can be found below freezing and above 65°C, so it is largelyvariant. Another species of bacteria whichcan have a negative effect upon animalistic organisms is bacillus anthracis,which causes a disease called anthrax, carried by both wild and domesticanimals in Africa, Asia and certain parts of Europe. There are three types ofanthrax- cutaneous anthrax begins as a bump on the skin and gradually expands,ulcerating and is not a serious illness. On the other hand, inhalationalanthrax is far more severe and can even be fatal. Far less common, the victimwill start out with flu-like symptoms which gradually progress to pneumonia,respiratory failure and eventually septicaemia, ultimately leading to shock andin some cases, death.

The rarest form of anthrax is intestinal anthrax,resulting in fever and severe gut disease. There are four types of reproduction within bacteria. Firstly, binary fissionis a type of asexual reproduction, based around cell division.

During theprocess of binary fission, the cell will grow to a point where it appears to bedouble its initial size, before splitting. Conjugation is a form of sexual reproduction within bacteria. In orderfor it to occur, direct cell-to-cell contact must occur and genetic material isexchanged between cells. Another type of bacterial reproductionis transformation, where a homologous gene from the environment binds to andreplaces the matching gene in the bacteria, resultantly meaning that one typeof bacteria is transformed into a different type of bacteria. Lastly,transduction is where DNA from one bacterial cell is transferred into anotherbacterial cell, with the help of a bacteriophage- a virus which parasitizes abacterium by infecting it and reproducing inside it.  Although there are many types ofbacteria which are beneficial towards animals, there are also many types whichmay cause illness and disease. For example, E. Coli is a type of bacteria whichis usually found within the intestine of animals and is usually harmless,however certain types of E.

Coli easily cause intestinal infection withinorganisms. For example E. Coli 0157:H7 causes symptoms such as diarrhoea,abdominal pain and fever. In severe cases, it can cause dehydration orpotentially even kidney failure, affecting the very young, the elderly andpeople with weakened immune systems the most severely.  There are three main shapes whichbacteria may form. The first of these is called the bacillus (rod) and can beeither obligate aerobes (require oxygen for respiration) or facultativeanaerobes (do not require oxygen for respiration) and have the ability toreduce themselves and lie dormant for an extremely long period of time.

Thesecond of these is the coccus (sphere) and this refers to any bacteria with aspherical, oval or generally round shape, however they can appear flattenedwhen viewed next to one another. Last but not least, spirilla (twisted)bacteria are curved bacteria which can range anywhere from a gentle curve to atight, corkskrew-like spiral, many of which possess the ability to move. Bacteria may be classified according to how they obtain their nutrients.Autrophic bacteria gain their nutrients by building their own organic food viaphotosynthesis and turning it into organic nourishment. The opposite of anautrophic organism is a heterotrophic organism; this means that they useorganic carbon as food, in the same way as fungi and animals (includinghumans).  In order for bacteria to thrive,the conditions must be just right. Warm, moist conditions are essential forbacterial growth, with the optimum temperature range being from 15-20°c for psychrophilicbacteria, 30-37°c for mesophilic bacteria and 50-60°c for thermophilicbacteria.

The ideal pH range for most bacteria is in neutral conditions ofaround 6.5-7; however some types of bacteria can withstand extreme values,although it may not be the optimum conditions for growth.  Bacteria are prokaryotic,unicellular photosynthetic organisms which are generally only a few micrometresin length.

They do not contain a nucleus or other membrane-bound organelles;however they do contain a cell wall which provides rigidity to the cell.