Illustration of the main components of blood plasma, the liquid component of blood. At top left is a C1 complex from the complement system. The complement system is a part of the innate immune system that complements the action of antibodies and phagocytic cells. Immunoglobulin G (IgG, Y-shaped, centre left) and immunoglobulin M (IgM, bottom left) antibodies are proteins produced by B-lymphocyte white blood cells as part of an immune response. Also seen are fibrinogen (centre) and von Willebrand factor (centre right), which are proteins that facilitate blood clotting. Ferritin, a protein that acts as an iron store, is at top right. High and low density lipoproteins (HDL, purple and orange sphere at left, and LDL, purple and orange sphere at lower right) carry cholesterol around the body. The most abundant protein in the blood plasma is albumin (dark blue), which transports hormones, fatty acids (orange) and many drugs. For a labelled version of this images see F033/3377.