When fish are in facilities such as aquacultural ones, risk of exposure to diseases affecting skin, gills, and the digestive tract, increases. All of these areas of a fish have something in common; they are mucosal surfaces, or have a mucus-like layer to aid in protection. In order to come up with some form of a vaccine for these types of fish, researchers set out to discover how fish can respond and detect to different pathogens through these various mucosal surfaces.
It was found that fish induce production of a specific antibody in their gills when they sense there is a pathogen they have been exposed to. It was also discovered that the gills’ microbiota is covered with the same antibody. This is an immunoglobulin and is referred to as IgT. Up until this point, it was widely believed that mammals alone had this specific immune response involving mucosal surfaces.
Through earlier research it was found that IgT coats the commensal bacteria that are living on the skin and guts of fish. It is believed that this is done to help keep the microbes under control and to prevent them from causing illness.
Researchers examined the gills of these fish, since they are also considered to be a surface that is mucosal. The goal was to see if the same immune defense strategies were used there. To do this, they examined rainbow trout and found IgT to be present. Other immunoglobulins like IgD and IgM were also present, however the IgT was by far the most abundant.
To see why there was an abundance of the IgT immunoglobulin, the researchers exposed the fish to a parasite that is responsible for white spot disease. This is a very common disease in fish that are wild, farmed, or are pets.
A while after they had been initially infected, the fish were examined and found they were coated with a large majority of IgT. Only a few were coated with IgM, but there were none with IgD. If a fish survived, it was found that it had an increased amount of IgT-producing B cells in their gill areas. This indicates that the response associated with IgT was crucial to helping fight these types of parasites.
These findings help connect, with an evolutionary outlook in mind, that since both fish and mammals have these very well developed immune defense mechanisms, it must have developed very early on through the process of evolution.