Horse serum farm at Elstree, 1915. One unusual contribution of the horse to the war effort was as a supply of serum which acted as an antidote to tetanus poisoning, also known as lockjaw. Serum, or blood fluid with the clotting agents removed, is collected from the blood of specially selected horses, who have carefully infected with the disease, and so built up an immunity to it. The article accompanying this picture concludes, "The regiment of healing horses, through others of its recruits