White blood cells are part of the body's immune system, which keeps humans safe from pathogens such as viruses and bacteria that can cause infection. Unfortunately, sometimes the immune system gets confused, and treats harmless substances—including pollen and dust—as invaders. When this happens, the result is an allergy, complete with sneezing, itching, and watery eyes.
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The significance of an allergy is simply that it's an overreaction of the body's immune system to otherwise harmless substances. There are many types of white blood cells that work together in the body to produce a coherent immune system, notes Dr. Lauralee Sherwood in her book, "Human Physiology." One type of white cell, called a B-lymphocyte, produces antibodies that bind to pollen and other allergens and trigger reactions. Another type of white cell involved in allergies is called a mast cell.
Antibodies produced by B-lymphocytes serve important purposes in the body. They bind to viruses and bacteria, clustering the invaders together in easily removed packets and marking them for destruction. Without antibodies, a subset of white cells called killer T-cells wouldn't know the difference between invaders and body tissues. Mast cells, too, serve valuable purposes, notes Dr. Gary Thibodeau in his book, "Anatomy and Physiology." They line the mucous membranes of the nose, mouth and eyes, and protect these entry points from invasion.
In a 2006 article in the scientific journal "Methods in Molecular Biology," researchers note that the role of mast cells in allergic processes is relatively newly recognized. How, precisely, cells become confused as to what constitutes an actual pathogen and what is harmless isn't well understood at this point. What is clear, however, is that some harmless substances—like ragweed pollen—confuse white cells in a large portion of the population. Some individuals, however, are completely unaffected by the same substances that cause allergies in many others.
Mast cells produce allergy symptoms because when allergens bind to antibodies on the mast cells' surfaces, the cells release large quantities of a chemical irritant called histamine. The histamine increases blood flow to the area—ostensibly to bring in white cells to fight infection—which causes swelling. Histamine also causes fluid release, leading to runny nose and eyes, notes Dr. Sherwood. As mucus builds up in the nasal passages, it can irritate the throat, leading to soreness.
In the "Methods in Molecular Biology" article, the authors note that the mast cell and white cell confusion that leads to allergies may be the result of insufficient exposure to true pathogens at an early age. Allergies appear to be more common in highly industrialized nations that use more antibacterial soaps and keep surfaces cleaner. Researchers suspect that by failing to expose children and their immune systems to a variety of true pathogens, industrialized nations may crease allergies indirectly.