There are three well-known types of hay fever in the world. Together with grass pollen allergy in the UK, ragweed pollen allergy in the US, and cedar pollen allergy in Japan, they are known as the world’s three major types of hay fever. Hay fever first emerged in 1819 during the Victorian era under the name “summer conjunctivitis.” In England, attacks of “summer conjunctivitis” began during the haymaking season. Interestingly, this condition emerged with the Industrial Revolution and became particularly prevalent among clergymen and doctors, who valued cleanliness. This “summer conjunctivitis” did not occur among farmers, who were often covered in hay. In the US, the term “autumn conjunctivitis” was proposed because it occurs more frequently in autumn than summer.
Until about 50 years ago, the rate of parasitic infection among Japanese people was over 60%. This rate has now dropped to 0.01-0.02%. Since the rate of parasitic infections declined, various allergies, such as hay fever and atopic dermatitis, have rapidly increased. Why did people with parasites have a lower rate of hay fever? When cedar pollen enters the body, it recognizes it as a foreign substance. This causes the body to produce an antibody called IgE (immunoglobulin E) to attack the cedar pollen. The IgE antibodies attach to the surface of mast cells one after another. However, mast cells do not change if only one IgE antibody is attached. Symptoms only appear the next time the same cedar pollen enters the body. Only when the antibody that catches the second cedar pollen attaches to the surface of the mast cell does the mast cell rupture, releasing histamine and other substances that damage surrounding tissues, resulting in symptoms. Parasites act to prevent this second cedar pollen from coming into contact with the mast cell. The theory is that if the second factor is not present, symptoms will not develop.
Another reason seems to be a desire for cleanliness. Early hay fever was contracted by British clergymen and doctors, and the American leisure class. These wealthy people lived in clean places. Being in a sterile environment weakens the immune system, making people vulnerable to bacteria. People with diverse immune systems have the strength to survive. On the other hand, people in a sterile environment seem to be vulnerable to their environment. How we can develop diverse immune systems may be a test of modern wisdom.
