COLLAGEN. COLLAGEN Collagen is the most abundant protein in the human body. It is the constitutive structure of connective tissue which assures nutritional supply to other body tissues and acts as an important immune defense system. Here fibers of collagen are shown in transversal sections (pink) and longitudinal oblique sections (dark pink). These fibers glide over each other very easily giving the tissue mechanical resistance and the capacity to absorb shocks. SEM, stained, x26000