Clinical medicine--including the specialty of allergy-- is about sight, touch, smell, and hearing--not just about the latest medical article we've read in the Annals of Allergy or JACI.  One thing we've tended to underemphasize  in our profession is late-phase skin test reactions--something we can see and touch hours after the test has been applied--if we just look for them.  But there is another  item that I've never seen described or documented in the lilterature--and that is the curious phenomenon of intradermal skin test "recall" days or even months after intradermal skin testing was done.  Under certain occasions, it seems as if the site where a skin test was formerly applied  retains a "memory" for furher reaction  when a similar antigen is encountered in our environment many days later. 

What do I mean?  For example, I've seen patients receive intradermal skin tests for molds, and end up with strong delayed rections to them.  Upon getting a subsequent  airborne mold exposure many days later, (for example, mowing the lawn), the patient may note pruritis and swelling at the site of his former tests.  If you ask patients about this phenomenon, they will frequently volunteer that it does indeed occur.  Interestingly, I had a chance to examine this phenomenon first-hand in my office one day, in  a patient who I had previously tested for mold allergy.  She had had strong delayed reactions to mold when I initially tested her. .  When she saw me, she had just had a major symptomatic mold exposure the day before.  On her arm, there were faint areas of erythematous swelling and puriritis where I had previously tested her to mold on an earlier occasion.   

I'd like to call this phenomenon "Eaton's Sign", named in memory of the late Dr. Keith Eaton, M.D. 

I remember meeting Dr. Eaton in Manchester, England, when he excitedly came up to me and asked if I thought that heavy mold exposure could trigger depression in susceptible individuals.  He was one of the earliest members of the BSACI (British Society for Allergy and Clinical Immunology), and a student of Professor Jack Pepys. He was a prolific writer, publishing some 80 papers, and specifically wrote about the delayed reaction to molds on intradermal testing,  and described it thoroughly in his publications.  He felt the delayed mold reaction, although obscure in cause, was "not without biological significance".  In retrospect, his interest in mold was probably stimulated by his wife Susan's serious illness from mold, and a serious case of "dry rot" in his house!  He was a consumate clinician and researcher, who tragically passed away with pancreatic cancer.  Dr. David J Freed has this to say about him in his memorium: