The immune system is the body's defense against infectious organisms and other invaders. Through a series of steps called the immune response, the immune system attacks organisms and substances that invade body systems and cause disease.
When the humidity is high it feels oppressive outside because sweat doesn't evaporate and provide cooling. When the humidity is low you feel cooler, but your skin dries out and you get dehydrated more easily because more moisture is being evaporated from your body. The same thing applies to crops in high and low humidity. A high water vapor content is necessary to produce clouds and precipitation.
The Effects of Humidity on the Human Body. As discussed in Home Humidity and Allergies, humidity directly affects the amount of allergens in the indoor environment. In particular, high humidity causes both dust mite populations and mold colonies to grow, thereby greatly increasing allergy sufferers' total allergen load.