A connective tissue disease is any disease that affects the parts of the body that connect the structures of the body together. Connective tissues are made up of two proteins: collagen and elastin. Collagen is a protein found in the tendons, ligaments, skin, cornea, cartilage, bone, and blood vessels.Elastin is a stretchy protein that resembles a rubber band and is the major component of ligaments and skin. When a patient has a connective tissue disease, the collagen and elastin are inflamed.ollagen is a protein found in the tendons, ligaments, skin, cornea, cartilage, bone, and blood vessels. Elastin is a stretchy protein that resembles a rubber band and is the major component of ligaments and skin.