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What is Histiocytosis?

Histiocytosis can be defined as a proliferation of a certain type of white blood cell, known as a histiocyte. These histiocytes gather in certain bones and/or organs of the body and cause inflammation and damage (lesions) to the area affected. The cause, or trigger of this proliferation, in the case of Langerhans cell histiocytosis, remains a mystery.

However, there is no mystery as to the origin of histiocytes themselves. We all have them. They play a very important role in our immune system. They are part of a group of white blood cells known as monocytes.

White blood cells, or leukocytes, are classified into three groups:
1. Granulocytes - these include neutrophils, eosinophils and basophils
2. Lymphocytes -these include B lymphocytes (or B cells) and T lymphocytes (or T Cells)
3. Monocytes

Monocytes are identified as the largest of the white blood cells, and circulate in the bloodstream. Histiocytes are derived from monocytes which have been pre-programmed to migrate to tissue where they take up residence. Once resident they no longer circulate in the bloodstream, but can still travel along the body's secondary circulatory system, the lymphatic system.

Histiocytes are known as antigen presenting cells (APCs). This means that they take up, process, and hand over the antigens to T lymphocyte cells (T cells) in a way that the T cells can recognize them. The T cells will then ingest, degrade, and destroy the antigen. An antigen is a foreign protein that enters the body via an infection. Bacteria and viruses are not themselves antigens, but contain antigens.

This defense mechanism can go terribly wrong when, for no apparent reason, these white blood cells start to replicate or proliferate and there is no obvious disease, or virus present.

What is Langerhans Cell Histiocytosis?

The disease has become known as Langerhans cell histiocytosis because a cell that has properties similar to a Langerhans cell is found in these proliferating histiocytes.

Langerhans cells are dendritic cells, that is they have hair-like structures jutting out of the cell body.

Langerhans cells are found in the base of the skin layer (the epidermis). They are the frontier guards of the immune system. As they are stationed at the outermost parts of our bodies, their job is to act as the first line of defense (second, if you include the skin itself) to protect against invading bacteria and/or viruses. Langerhans cells are professional antigen presenting cells and express a CD1a+ (CD1a positive) antigen on their surface to attract a T cell response.

A distinctive feature of a Langerhans cell is tennis racket shaped granules within the cell itself, called Birbeck granules. These Birbeck granules, or something like them, are found in the proliferating histiocytes of Langerhans cell histiocytosis.

A diagnosis of Langerhans cell histiocytosis (LCH) is made if Birbeck granules and/or CD1a+ antigens are found to be present in the damaged area (lesion).

Birbeck granules: the tennis racket structure is clearly seen in this photo.

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