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.
Location/Articulation. The navicular bone is one of the tarsal bones found the foot. Its name derives from the bone's resemblance to a small boat, caused by the strongly concave proximal articular surface. The term navicular bone or hand navicular bone was formerly used for the scaphoid bone, one of the carpal bones of the wrist.