Today was the last Immunology lecture and was looking at hypersensitivity reactions in animals. Hypersensitivity is when the immune system exagerates the response to allegens (which is the name for antigens involved in hypersensitivity reactions) and is the cause of most allergic responses. There are four different types of hypersensitivity reactions…
- Type I – Reaction mediated by IgE – atopy
- Type II – Reaction mediated by IgG and IgM -cytotoxic
- Type III – Reaction mediated by immune complexes
- Type IV – Reaction mediated by cells – delayed type hypersensitivity DTH
Now type 1 hypersensitivity is usually caused by genetic or environmental factors, with the levels of IgE (a immunoglobulin responsible for the immune response) being higher than normal. The reactions are usually localised and form your general allergic reactions such as hayfever, asthma, conjunctivitis, food alergys and atopic dermatitis. This is usually tested for using intradermal tests where small amounts of the substance are placed under the skin and the results monitored or by using serum tests in a laboratory.
Type 2 hypersensitivity usually involved IgG and IgM and the complement pathways (the cell interactions for destroying cells). The IgG and IgM cells are produced that act against the bodies own cells, with the activated complement pathway which then destroys the cells. This often occurs with transfusion reactions or against red blood cells in new born animals such as foals which are born healthy and then sicken several hours after suckling for colostrum. This is tested for by a direct or indirect coombs test which works by testing the blood against the antibodies showing agglutination (cells sticking together) when it is positive.
Type 3 hypersensitivity involves soluble immune complexes, the complement system and neutrophils. These soluble immune complexes are deposited in tissues and organs which then activates the complement pathways causing inflammation and distruction of tissues. It is often responsible for pneumonitis, rheumatoid arthritis, rheumatic fever and associated conditions. There are ELISA tests available to test for this in addition to testing of the levels of the immune complexes or the presence of antibodies.
Type IV hypersensitivity (also called Delayed Type Hypersensitivity) involves Th1 memory lymphocytes, macrophages and cytokines. As it is cell mediated it has a prolonged onset (hence the “delayed type”). Generally this is contact allergy where as exposure to a substance for a second time causes a exageratted inflammatory response. The most well known of these is the response to TB, if the body has been previously exposed to TB, then when the skin test is taken an abnormally large response is recorded.
I am at the stage now where I can see the stuff I am learning being directly beneficial to me when I reach practice and start seeing animals which is pretty cool. Animals are fun when they are well, however sometimes they are not. This evening (well more like 10pm) I helped a friend wash her cat after he laid down in his own urine. Most cats would try to kill you if you washed them, however this guy is so tame that he barely objected, and most definately enjoyed being toweled dry…