DDR1 and rheumatoid arthritis: Table 1 lists some functions of these five types of collagen. Collagen type I is the main component of the calcified tissue of teeth and bone and is present in skin, tendons, vasculature, lungs, and heart. It can be a ligand for receptor-mediated signalings such as integrins, OSCAR, GPVI, G6b-B, DDR1 and 2, and LAIR-1 of the leukocyte receptor complex [12]. Collagen type II is an abundant matrix molecule of cartilage and is associated with many diseases such as skeletal dysplasias, rheumatoid arthritis (RA) and osteoarthritis (OA) [13,14,15].