Mycosis fungoides is due to invasion of the skin by T cells.
These are normally a form of white blood cells called lymphocytes
that are involved in the immune system but in this condition are
behaving abnormally.
The condition starts as inflamed (erythematous) scaly patches on
covered areas of the skin, the buttocks, thighs or trunk. These may
last for several years. Untreated they may form tumours from which
the name 'mushroom like' fungoides come. This is, however,
extremely rare these days.