1873: The Blackley Experiments, part IV

The results of a scratch test performed on a boy.

During the summer of 1865,  Charles Blackley performed perhaps his most famous experiment. He obtained pollen from Lolium Italicum and applied it to one forearm of a friend. He then scratched the skin over the pollen to introduce it to the skin. He did the same to his friends other forearm, although without applying pollen.

Soon thereafter he observed that the skin around the scratch where the pollen had been introduced to the skin tissue "raised a wheel such as is seen in uticaria or in stinging with nettles. In a few minutes, after the pollen had been applied the abraded spot began to itch intensely; the parts immediately around the abrasion began to swell, but this was not apparently due to any action on the cutis vera (skin)."(1, page 105) (2, page 818)

He observed that the rash was contained to the area around the scratch. He wrote:

The tumour increased in size until it measured two and a half inches in length, by one inch and a half in breadth, and was raised above the ordinary level of the surface nearly three quarters of an inch. No pain was felt in the limb, nor was there any heat or redness present at any time, beyond the very slight amount to which the abrading of the cuticle gave rise. The swelling attained its maximum in six hours, and then remained stationary for other eight hours; after this it gradually subsided, and in forty-eight hours it had entirely disappeared. The arm to which no pollen had been applied did not exhibit any sign of swelling or irritation.

Blackley performed this experiment to show that a rash was a specific result of pollen. This test has been modified some over the years, but it is still used to this day to test people for allergies. It is called the scratch test.

If a doctor suspects a patient has allergies, he can place a variety of allergens on the surface of the patient's skin, either on the forearm or back, and scratch the surface of the skin over each specific allergen. if a hive forms the patient is allergic to that allergen.  If no hive forms, the patient is not allergic to that allergen. The size of the hive also helps to reveal the severity of the allergy. For example, a large hive would be the result of a severe allergy to that substance.

He would continue this practice until he retired in 1894.  He passed away in 1900 at the age of 80.

References:

1. Blackley, Charles Harrison, "Experimental Researches on the Causes and Nature," 1880, 2nd edition, London, Bailliere, Tindall, and Cox
2. O'Sullivan, Dr. Stephen, "Charles Blackley and Allergy Research," New Scientist, June 28, 1973, Volume 58, No. 852, pages 818-819