THE ALLERGISTS The idea that men, other animals, and plants, could become infected by microbes which set up pathological changes, had been made acceptable by the analogy of sterile organic infusions that become infected with putrefying microbes. The idea became widely accepted during the latter half of the nineteenth century and, when once the cause of the common epidemic diseases had been established, advances in hygiene and therapy began to transform the social scene. Yet there remained some diseases for which no pathogenic or parasitic invader could be found. Some of these, such as pellagra and beri-beri, have now been traced to a variety of nutritional deficiencies. Another group, the so-called allergies, were at first difficult to grasp because a peculiar condition of the patient was a complicating factor. Allergic diseases, unlike those caused by invasion of the body by a pathogenic micro-organism, are due to a changed condition of an individual patient who has become sensitive and reacts adversely to substances, often in minute amounts, which normal individuals can tolerate. The substance or allergen can be taken into the body, for example in food, or by contact through the skin, or by inhalation from the air.
Hay fever was one of these puzzles. Long before Pasteur's epoch, hay fever had been attributed to inhalation of pollen; but it remained for Charles H. Blackley (1873), a Manchester physician, to prove by in halation experiments on himself and others that this guess was correct, and to demonstrate by trapping methods that pollen was at times present in the air in large quantities. Blackley first tried Pasteur's gun-cotton filters and obtained some pollens, but too few to satisfy him. Finally he used four sticky horizontal microscope slides exposed under a roof supported by a square central post. The slides were placed at `breathing level' (about 135 cm.), and he caught a maximum of 88o grains per sq. cm. per 24 hours on 28 June 1866. In 1867 his maximum was only 1o6, and in 1869 he placed his slides vertically in a vane shelter and gave no numerical data. He found that rain reduced the number of pollen grains caught to about 5 per cent of the number caught in dry weather. He ex plored the air above the ground up to 1,50o ft. by means of kites, and found that vertical slides facing the wind caught nearly zo times as much pollen at the higher altitude as at breathing level.
Blackley showed by means of his sticky slides that the air contains enough pollen during the grass-flowering season for large quantities to be deposited on exposed surfaces. He also gave himself an attack of bronchial catarrh by inhaling Penicillium and Chaelonaiuna spores—an experiment which he said was too unpleasant to repeat.
According to Durham (1942), after Blackley's pioneer work no progress was made with these studies until the period 1910-16, when fresh in terest was aroused by the discovery that injections of pollen extracts can be used to de-sensitize patients who are allergic to pollen.
When the study of airborne allergens was again taken up in the present century, it was unfortunate that the technique chosen should have been the so-called `gravity-slide' adopted by Blackley—a method which Pasteur had abandoned in 1861 and which Miguel had roundly con demned as `the simplest and most defective method' of collecting air borne particles.
By the early years of this century it became possible to assess the value of the ancient belief that the wind brings disease. Many diseases of crop, but very few diseases of man, have proved to be caused by minute particles carried on the wind. The particles are not some sort of invisible atoms as Lucretius thought; indeed, among the motes in the sunbeam, he may himself have been watching some of the baleful fungus spores and pollens which cause crop disease and respiratory allergy.
Meanwhile evidence was accumulating that these particles might be carried by wind to distances vastly greater than had been imagined by the ancients. In dust deposited after transport for hundreds of kilometres by sirocco and trade winds, Ehrenberg (1849, 1872, 1872a) found large quantities of protozoa and plant spores, and gradually he became con vinced that viable micro-organisms could survive transport through the atmosphere. When the Beagle was near the Cape Verde Islands, Darwin (1846) found the atmosphere hazy with dust from North Africa. In samples of this dust Ehrenberg found sixty-seven kinds of organisms— including freshwater infusoria and cryptogamic spores (Plate 2)—and Darwin at once grasped the importance of the phenomenon in the geographical distribution of organisms.