Some weeds are irritants - is that true?

Some weeds are a nuisance because they have thorns or prickles, some have chemicals that cause skin irritation and allergies or are hazardous if eaten, or have parts that come off and attach to fur or clothes. Examples of such weeds in Australia include Stinging Nettle, Wandering Trad, Black Nightshade, and Asthma Weed.