Many other common diseases are caused by autoimmunity such as juvenile diabetes and low thyroid disease. It has been recently discovered that some persons who suffer with hives or angioedema also have an autoimmune disease. In these cases, autoantibodies have been formed that bind to the Fc-receptor on mast cells.
Lesson Summary. Chitin is a primary component in the exoskeletons of arthropods and crustaceans and is also found in the cell walls of fungi. It's a polysaccharide, and it's excreted by the epidermal cells in arthropods. But, it can't grow with the animal, so shedding becomes necessary.
Chitin was probably present in the exoskeletons of Cambrian arthropods such as trilobites. The oldest preserved chitin dates to the Oligocene, about 25 million years ago, comprising a scorpion encased in amber.tructure of the chitin molecule, showing two of the N-acetylglucosamine units that repeat to form long chains in Î²-1,4 linkage. A close-up of the wing of a sap beetle; the wing is composed of chitin.