Immune response to bacteria, artwork. Invading bacteria (yellow) are phagocytosed (engulfed) by macrophages, a type of white blood cell. Fragments of protein (antigen) from the bacteria are displayed on the macrophage's surface. The antigen is recognised by helper T lymphocytes, which secrete chemicals such as cytokines (pink dots) to activate other immune cells, including B lymphocytes. The B lymphocytes differentiate into plasma cells, which produce large numbers of antibodies (orange) that recognise the antigen. The antibodies either neutralise the pathogen or flag it for destruction by other cells.