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.
The causes of AAI are not fully understood but they may include abnormalities of the ligaments that maintain the integrity of this joint between the first (C-1) and second (C-2) cervical vertebrae or be related to bony abnormalities of either or both of these vertebrae. AAI can be so mild as to not cause symptoms. It is only discovered as an incidental finding on an X-ray. The instability is recognized through lateral neck radiographs (films taken from the side) in which the excessive mobility of C-2 relative to C-1 results in an abnormally large distance between these two vertebrae.