I wish I could develop an immunity, but it seems I become more sensitized with each exposure.
My understanding of the toxin is that it is an oil. Your skin has a thin layer of keratin rich dead cells, underwhich is a fatty layer. The oils are only stopped by the top level of the epidermis. Once they reach the stratum corneum they are easily able to diffuse through. Any sort of scratching or abrasion speeds delivery of the oils. It is important to use soap and water to emulsify these oils. Water alone will not remove them easily.