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.
Because most navicular fractures do not cause the wrist to look broken and many cause only minor symptoms, it can be hard to know if your navicular bone is broken. If the bone is broken, you may have: 1  Pain, tenderness, or swelling on the thumb side of your wrist.2  A hard time grabbing or gripping things or moving and twisting your wrist or thumb.f the bone is broken, you may have: 1  Pain, tenderness, or swelling on the thumb side of your wrist. 2  A hard time grabbing or gripping things or moving and twisting your wrist or thumb.