Blood disorders are a range of diseases due to a problem with the red or white cells in the blood. These cells are made in the bone marrow and then travel to the blood where they circulate. Problems can arise when there are too many or too few of these cells, or when their function becomes uncontrollable.
HLH is a very rare group of diseases. It can be divided into genetic (inherited) and acquired forms. The frequency of inherited HLH is estimated at 1 child per 200,000 children. In the UK alone, there are approximately 15 new cases each year.
The incidence of acquired HLH is unknown but it is thought to be more common than the genetic type.
HLH is caused by an uncontrolled activation of white blood cells called lymphocytes and histiocytes which stop the body's immune system from working properly. This process can be likened to a very severe form of inflammation that the body is not able to switch off. Unfortunately, the immune system is overwhelmed by this activation and functions poorly, thus leaving the child susceptible to infection.
HLH often looks like a normal response to infection and it can take time to realise that the child's immune system is not functioning properly.
The different types of HLH are Genetic Haemophagocytic Lymphohistiocytosis, Familial HLH, Acquired Haemophagocytic Lymphohistiocytosis and Macrophage Activation Syndrome.
Both the acquired and genetic forms are triggered initially by infections, often viruses.
The prognosis for children with HLH has improved dramatically over the last 20 years and there is now a cure rate of more than 50%. Overall survival depends on the type of HLH and the response to treatment.