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Transcript
Igs and the Immune System – a Short Primer By the Health and Education Sub-Committee I n most discussions of animal immunity and vaccinations you will come across references to ‘Ig’. This is an abbreviation for ‘immunoglobulin’. An immunoglobulin, or Ig, is a protein molecule that is produced by an animal’s immune system to help recognize invading organisms (bacterial, viral, fungal, and parasitic worms) and mark them for elimination by other cells of the immune system. Igs are more commonly known as ‘antibodies’. Measuring the Ig level, or quantity of antibodies, in the blood therefore provides a good indication of the health and capabilities of the immune system. A B cell is triggered when it encounters its matching antigen. The B-cell engulfs the antigen and digests it, then it displays antigen fragments bound to its unique MHC molecules.. This combination of antigen and MHC attracts the help of a mature, matching T cell. Cytokines secreted by the T cell help the B cell to multiply and mature into antibody producing plasma cells. newborn cria within the first 12 hours after birth. Absorption of antibodies in the gut is only possible in the first 24 hours after birth. Thereafter components in the colostrum will still have a local protective action in the gut of the cria, but the antibodies will no longer be absorbed into the blood of the cria. If the baby does not get any colostrum, or only a small amount of colostrum, it is very vulnerable to infection and is much more likely to die. A plasma transfusion can save such an at-risk cria by providing it with the antibodies of an adult animal. Vaccination presents the immune system with a sample of what some of When a baby alpaca or llama the common and deadly Released into the blood, is born, it has no antibodies infectious organisms look antibodies lock onto matching at all, and is therefore very like. This induces the antigens. the antigen-antibody complexes are vulnerable to infection. The immune system to create then cleared by the complacent cascade or by mother’s initial colostrum ‘memory cells’, which are the liver and spleen. milk contains a large number antibody (Ig) producing cells of antibodies (that the mother that circulate in the blood has produced) to be passed and watch for more invaders. on to the young. These antibodies provide protection in the The memory cells can last for months or years. By giving weeks and months after birth while the cria’s own immune periodic booster vaccinations, we refresh the population of system is developing. The stronger the mother’s immune memory cells, keeping the immune system primed to fight off system, the better the protection that the baby receives via invaders. the colostrum. This is why it is recommended to give the mother a vaccine booster shot a few weeks before birth, as The primary components of the immune system are the it enhances her antibody levels against common paddock leukocytes, or ‘white blood cells’, that are manufactured in the diseases. This improved protection is passed on to the cria bone marrow and released into the blood stream. Leukocytes via the colostrum. come in many varieties, but they are all designed to work together to identify foreign invaders and destroy them. The It is essential that the colostrum reaches the gut of the combined mass of cells of the immune system is about the 22 same as the liver, making the leukocytes, as a group, one of the largest ‘organs’ in the body. A subtype of the leukocyte is the lymphocyte. Broadly speaking, there are two different classes of lymphocytes, the T-lymphocytes (T-cells) and B-lymphocytes (B-cells). The T-cells are involved in what are known as ‘cell mediated responses’. They respond to cells which have foreign bodies displayed on their surface and can eliminate virus-infected and cancerous cells. T-cells can also respond to chemical signals to activate and search for invading bacteria. The B-cells are involved in the production, presentation, and retention of antibodies. Immunoglobulins are the primary tools of the B-cells. Specialized B-cells wander the body looking for cells or particles that are foreign. Finding an invader is only the first step. The rest of the immune system must be notified and mobilized. Almost any large molecule (protein, carbohydrate, fat) can induce an immune response. These molecules are known as antigens, which stands for ‘antibody generators’, and a single bacterium or virus may be covered by thousands of them. The antigens are chopped up into small pieces, and those pieces are then presented to the antibody-producing cells. Every day alpacas produce millions of different antibodyproducing cells in the bone marrow. Each of these cells produces a different, unique antibody. The end-tips of the antibodies have what is known as a ‘variable region’ that is different for every antibody and forms a unique threedimensional shape. If that antibody finds an antigen that has the mirror inverse of the shape at the variable-tip, then the antibody and antigen will bind to on another like a key fitting into a lock. When that happens the B-cell that makes that particular antibody knows it has a ‘winner’ and starts to mass-produce that antibody. An invader coated in antibodies can be easily identified and eliminated. The antibodies coating the surface of invading bacteria or virus often have an additional effect of preventing them from attacking the body or reproducing, speeding their elimination. A single infectious organism can trigger the production of dozens or even hundreds of different antibodies against it, as there will be lots of different ‘fits’ since each organism has many different antigens on its surface. These first antibodies vary in their efficiency for targeting the foreign invader, but as time goes on the response will ‘mature’ as the immune system adapts and produces antibodies better able to target the invading antigens. Vaccines are mixtures of antigens that can activate the immune system without causing an infection. One common tool for making a vaccine is to use heat-killed bacteria or viruses. The immune system is thus presented with the shell of the invader, and can start producing antibodies against those antigens. Because we are exposed to millions of different antigens, the immune system has a method to distinguish occasional threats from persistent threats. If the immune system wasted all its efforts mass-producing antibodies against infections it would never experience again, it might not be ready for the more dangerous diseases. For this reason there are memory cells. When an infection runs for a long time, or if a creature gets infected by the same disease again and again, some of the best antibody-producing cells will convert into memory cells. These are antibody-producing cells that are effectively in storage. They drift through the bloodstream, often for years, waiting and watching for the invader for which they optimized to reappear. If that invader comes back, the memory cell immediately kicks into gear, makes thousands of copies of itself, starts cranking out antibodies, and mobilizes the rest of the immune system against the attacker. This speeds the response time of the immune system from weeks to days, or even hours. Vaccinations are typically given in two doses four to six weeks apart because this is the best method to activate the immune system (first dose), and then cause a maturation of the response and the production of memory cells (second dose). Booster shots cause the memory cells to activate and reproduce, increasing their numbers in the bloodstream. This is necessary because the memory cells do eventually die, so without periodic boosters the immunity would gradually fade. For most human vaccines we know how long the memory cells against various antigens last, and thus doctors prescribe different periods for booster shots (3 years, 10 years, etc.). In camelids, the lifespan of the memory cells is not known, but as the common vaccines are known to only provide about 12 months of protection in cattle and sheep it is recommended that booster shots be given at least once and preferably twice per year to ensure protection. If you ever give a new type of vaccine (such as switching from 5 in 1 to 10 in 1) you need to give two doses, about six weeks apart, to trigger the production of the memory cells. New Zealand Alpaca acknowledges the input of CVG (the Camelid Veterinary Group) in the development of this article. 23