Antigens are proteins or glycoproteins on the surface of cells or viruses capable of triggering an immune response.They have a specific three-dimensional shape and are either on the cell membrane or cell walls of cells, invading microorganism and abnormal cells such as cancerous cells.The initial response to invading pathogens is non-specific. The next stage is the specific response which confers immunity. This involves the immune system, principally B and T lymphocytes, being able to distinguish self cells from non-self or foreign cells and abnormal cells.A lymphocyte with a receptor on its cell surface which is complementary in shape to the antigen will recognise the antigen as foreign.