Immune system. Conceptual computer artwork of a man standing with his arms outstretched surrounded by components of the immune system. The skeletal system is seen, with the joints highlighted in red. The immune system defends against foreign organisms such as viruses and bacteria. There are various types of white blood cells (leucocytes, blue and purple). The leucocytes seen are a neutrophil, eosinophil, basophil, granulocyte and a monocyte. Also seen is a macrophage (phagocyte, lower left). Platelets (thrombocytes, pink), which are involved in blood clotting, and a red blood cell are at top left corner.