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.
Os tibiale navicular refers to an extra bone found in the foot. An accessory bone is a bone that is not normally found in the average human, but in most cases is not considered abnormal. This condition represents a secondary ossification center (growth center) of the navicular bone.It is present from birth.The navicular bone is found on the inside part of the foot.ccessory navicular syndrome as it is called can result from a number of causes: 1  excess or overuse syndrome as seen in an athlete. 2  trauma to the foot as in an ankle sprain or direct trauma to the navicular bone. 3  chronic irritation from shoes rubbing against the extra bone, over time, may cause pain.