taken from the horses. At the point when the highest amount of antibodies were produced, five liters of blood, a tenth of the blood volume of a horse, were taken through a cannula. The blood was collected in a glass cylinder and brought to the laboratory in the Behring facilities. Above the rouleaux formation which contained the red blood cells, the serum was visible. The color of the serum varied from milky to brown. Concentration and sterility of the serum were checked carefully, and the serum was filtered many times. Protein content was decreased in order to use the serum for humans. After the blood sampling, the horses could rest for three to four weeks and received extra food to recover the blood loss. In this period the horses were especially weak and prone to disease and infection. Within a few years, with experience and observation of the horses, a rouleaux formation of the blood sample was placed back into the animal's body. This procedure is called plasmapheresis. References External links Blood Immune system