Also known as leukocytes. Cells of the immune system, which are mainly present in the blood and the lymph, and originate in the blood marrow. There are several types of white blood cells (WBC) which have different functions: antibody production, recognition and destruction of pathogens, release of histamin. If a horse suffers from an infection, the number of WBCs in its blood will increase to fight the pathogen. In blood tests, the WBC count is therefore an important measure for infection and the detected types of WBC can give information about the pathogen the horse is dealing with, as some of them are very specific. Unlike red blood cells, WBCs have a cell nucleus but do not contain haemoglobin, hence the name.