However, severe forces or repetitive force over time can cause tiny microtears in tendon tissue, leading to injury. Tendinitis is inflammation (redness, swelling, heat, soreness) that occurs when the body's immune system detects an injury and responds. In the case of tendinitis (also spelled ‘tendonitis’), the body increases the flow of blood and infection-fighting substances to the injured tendon. Tendinosis is a degenerative injury to the tendon that doesn't provoke an immune response. It occurs when repetitive, unrelenting stress over time causes the breakdown of collagen, growth of abnormal blood vessels, and thickening of the tendon's sheath (covering).