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.
Causes and Risk Factors. Some CSF leaks occur spontaneously and the cause is unknown, while others are a result of trauma such as a head injury, brain or spinal surgery, an epidural, a lumbar puncture (spinal tap) or skull base tumor.