Illustration of a coronavirus particle in blood plasma showing immunoglobulin G antibodies (IgG, light blue) bound to the coronavirus' spike proteins (red). IgG antibodies are Y-shaped proteins produced by B-lymphocyte white blood cells as part of an immune response. Also present in the plasma are immunoglobulin M antibodies (IgM, light blue) and components of the complement system. The complement system is a part of the innate immune system that complements the action of antibodies and phagocytic cells. When stimulated by one of several triggers, the end result of complement activation is the stimulation of phagocytes to clear foreign and damaged material, inflammation to attract additional phagocytes and activation of the cell-killing membrane attack complex. Other components seen in the blood plasma include fibrinogen and von Willebrand factor, which are proteins that facilitate blood clotting. Blood plasma contains ferritin (red, smaller spheres), a protein that acts as an iron store. High and low density lipoproteins (HDL and LDL, orange and blue spheres) carry cholesterol around the body. Also shown is albumin (dark blue), the most abundant protein in the blood plasma.