In immunology, an antigen (Ag) is any structural substance that serves as a target for the receptors of an adaptive immune response, TCR or BCR or its secreted form antibody.he term originally described a structural molecule that binds specifically to an antibody. It expanded to refer to any molecule or a linear molecular fragment that can be recognized by highly variable antigen receptors (B-cell receptor or T-cell receptor) of the adaptive immune system.
Definition. A navicular fracture is a fracture of the navicular bone of the foot, a bone on the top of the midfoot. Athletes are particularly susceptible to fractures of the navicular bone.(There is also a navicular bone in the wrist.).A navicular fracture can be caused by a fall, severe twist, or direct trauma to the navicular bone. It can also be caused by repeated stress to the foot, creating a fracture not due to any acute trauma (a stress fracture).efinition. A navicular fracture is a fracture of the navicular bone of the foot, a bone on the top of the midfoot. Athletes are particularly susceptible to fractures of the navicular bone.
What Causes Avascular Necrosis? There are two major forms of AVN, traumatic or non-traumatic (meaning no injury). Hip fractures or hip joint dislocations, partial or complete, are the most common causes of AVN. Direct damage to the hip blood vessels can occur with these injuries, resulting in loss or compromise of blood flow to the femoral head. The mysterious loss of blood flow to the hip in the non-traumatic form has been associated with many risk factors, but the exact cause is not always well understood. In fact, about 25% of patients with AVN of the hip develop the disorder for no apparent reason.