HLH and its causes

What is HLH?

Haemophagocytic lymphohistiocytosis (HLH) is a rare immune disorder where
the body reacts inappropriately to a ‘trigger’, usually an infection. Specialised
white blood cells (known as T-cells and macrophages) become over-activated,
causing severe inflammation and damage to tissues such as the liver, spleen and
bone marrow.

HLH is a rare disease and it can be difficult to diagnose because many of the
symptoms can mimic severe infection or other conditions.

What causes HLH?

There are two main types of HLH: primary (‘familial’) and secondary. Primary HLH is caused by a genetic mistake (mutation) that can be passed on from parent to child. In primary HLH, mutations usually affect the ability of white blood cells to clear infections.

It can be inherited either in an autosomal recessive manner, which means that a child has to inherit the faulty gene from both parents to develop the condition,  or
in an X-linked manner, which means that the faulty gene is carried on the X chromosome and only affects boys.

Sometimes mutations can just happen, by chance, and are not inherited from parents. More information about genetics is available in our separate leaflet:
www.piduk.org/static/media/up/geneticaspectsofpid.pdf

Secondary HLH is not inherited but rather develops as a result of another illness,
usually a widespread infection or immune disorder.

Primary HLH usually shows up within the first year of life, while secondary HLH
typically develops later in childhood, although it can appear at any age. Both
types can present in the same serious and life-threatening way.