Category B - full version without watermark 720x576 (check terms and conditions at www.teledesign.de) The embed version with watermark is free of charge. Description: When a macrophage has eaten an invader, it presents an antigen fragment from the invader on its own surface, a process called antigen presentation. When the receptor of a helper T cell recognizes the antigen, the T cell is activated. Once activated, helper T cells start to divide and to produce proteins that activate B and T-killer cells. The killer T cell is specialized in attacking cells of the body infected by viruses and sometimes also by bacteria. The B lymphocyte cell searches for antigen matching its receptors. The second time an intruder tries to invade the body, B and T memory cells help the immune system to activate much faster.