Certain types of grass like Congress grass (or Parthenium hyterophorus) among others, are known to cause instant reaction in the form of intense itching and rashes over the body, specifically in the areas of direct contact, in people who are sensitive to them. Irrespective of the cause, these symptoms usually subside with a dose of anti-histaminic and do not reoccur unless there is a future contact. The anti-allergic therapy may be continued for at least 3 days (72h) regardless of the symptoms, to avoid a late phase reaction.
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