Antigen presentation. Coloured scanning electron micrograph (SEM) showing the interaction between a macrophage (top) and a T helper lymphocyte two components of the body's immune system. Both are types of white blood cell. Macrophages are antigen-presenting cells (APCs). They present antigens (fragments on the surface of pathogens or foreign objects) to T lymphocytes, activating them. Each T lymphocyte recognises and binds to a specific antigen. Binding of the lymphocyte to the antigen presented by the macrophage activates the helper cell. This leads to its proliferation and the activation of other immune cells that eliminate the antigen. Magnification: x3000 when printed 10cm high.