What is on my hand after gardening?

[PingaPenguin]

Slightly strange question I know.

Mowed the lawn then went round pulling out longer grass/weeds from the edge. Not long after felt a funny tickly/tingly feeling on my thumb. Look at my hand a little later and it's covered in this white stuff. What is it from?

Does it wash off? Surely it's just plant sap?

Do you think it's an allergic reaction? Euphorbia is often a culprit, although it wouldn't usually be a weed at the edge of the lawn.