Paxillus involutus

The common or brown roll-rim, Paxillus involutus, also known as the poison pax, is a mushroom previously thought to be edible with some unusual recently-discovered poisonous properties. It can cause a haemolysis which can be fatal. It often grows near edible mushrooms as well which makes it harder to identify by amateur mushroomers.

Identification
The cap, initially convex then more funnel-shaped with a depressed centre and rolled rim (hence the common name), is brown in colour and 4-10 cm wide. The narrow brownish yellow gills are decurrent and forked, darkening when bruised. It has a mild odour and taste. The spore print is brown. The stipe can be crooked and brownish or greyish yellow in colour.

Habitat
The brown roll-rim can found growing on lawns and old meadows, as well as deciduous and coniferous woodlands in Europe and North America in summer and autumn. It is an uncommon mushroom.

Toxicity
Previously deemed edible after cooking (it can cause gastric upset when raw), Flammer in 1980 discovered an antigen within the mushroom which stimulates an autoimmune reaction causing the body's immune cells to consider its own red blood cells as foreign and attack them. This results in dangerous haemolysis, which has been fatal.

The use of corticosteroids may be useful in treatment.