• Question: What causes an itch?

    Asked by brooket__ to Franco, Koi, Mark on 23 Jun 2016.
    • Photo: Franco Falcone

      Franco Falcone answered on 23 Jun 2016:

      Hi Brooket
      another interesting question. There are nerve endings in your skin which when stimulated by the release of a small molecule called histamine, signal to the brain and your brain interprets this as ‘itch’. The histamine comes from cells also found in your skin called mast cells. If you are allergic to something, when your skin (or mucosa, e.g. eyes or nose) gets in contact with the substance you are allergic to, the mast cells release the histamine which they store inside themselves in little granules. Then the histamine activates the nerve endings which signal to the brain.

      We think that one of the reasons why this mechanism has evolved is not to cause allergy, but it is there to alert animals to the presence of exoparasites such as ticks, fleas, lice which can then be removed before they infect you with the viruses or bacteria that they carry.

      However some parasites such as ticks have evolved strategies to make sure that you feel no itch, by injecting substances that neutralise the histamine, and by injecting pain killers so that you don’t feel any pain when they cut a whole in your skin, then inject susbtances that keep your blood flowing and prevent it from clotting.

      So although you might not have thought that this would be the case, the itch and parasites are linked, and the link is the molecule called histamine.

    • Photo: Mark Booth

      Mark Booth answered on 23 Jun 2016:

      Hi Brooklet

      Yet again Franco has produced an excellent answer. Histamine has evolved specifically to deal with parasites. It is secreted by specialised immune cells as part of the immune response to parasites. These cells – called Mast cells, contain the histamine until they are triggered by signals from other cells that they need to release their histamine. This finely controlled system can go wrong in allergy and is part of the ‘arms race ‘ between parasites and their hosts. It is also worth noting that many parasites have co-evolved with their hosts so that the parasite can live in the host without the host having to commit to trying to always get rid of it. The host suffers something, but accepts a trade-off and compromise.

    • Photo: Arporn Wangwiwatsin

      Arporn Wangwiwatsin answered on 23 Jun 2016:

      from itch to histamine to allergy to parasites!!
      I’m learning something here too!

      (シ_ _)シ a bow with respect to Franco and Mark (シ_ _)シ

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