Plasma cell. Coloured transmission electron micrograph of a plasma cell. Plasma cells are mature B lymphocytes (white blood cells) that produce and secrete antibodies during an immune response. The cell's nucleus (black) contains dense chromatin (green), a complex of DNA (deoxyribonucleic acid) and proteins. In the cell's cytoplasm (green) is an extensive network of rough endoplasmic reticulum (RER, dark lines). RER manufactures, modifies and transports proteins, in this case antibodies. There are also a number of mitochondria (blue, oval) in the cytoplasm, which provide the cell with energy. Magnification: x4000 when printed 10 centimetres wide.