Fibrin is an elastic, insoluble, whitish protein produced by the action of thrombin on fibrinogen and forming an interlacing fibrous network in the coagulation of blood.
The Italian biologist Marcello Malpighi discovered fibrin in 1666 while examining washed blood clots under a microscope. In his publication he describes the washed (to the point that the water no longer turned red) blood clot as having a “fibrous texture, and a network of nerve-like threads, where small meshes and honeycomb-like interstices develop in which lie little pools of reddish ichor. When this ichor is washed out with water, it leaves the network looking grey, with the naked-eye appearance of a mucous membrane.”. These were his microscopic observations on fibrin. He also goes on to say, “I think the evidence of our senses is that this white network of the blood contributes the strength of the whole blood clot, giving it a firmer structure.” Here is an electron micrograph of a blood clot showing red blood cells entrapped in a fibrin mesh: