Anaphylaxis is the immune system's alarm going off everywhere at once
Its name combines Greek words for up and protection, yet anaphylaxis is defence gone badly wrong. Within minutes of meeting a trigger such as a peanut, a bee sting or penicillin, immune cells dump inflammatory chemicals across the body, affecting skin, airways, gut and circulation, often several systems at once.
Most cases run through an antibody called immunoglobulin E. It latches onto the offending substance, and the bound pair then activates receptors on mast cells and basophils, a family of white blood cells packed with histamine and other mediators. Released in a rapidly escalating cascade, those chemicals tighten airway muscles, widen blood vessels and let fluid leak from them, dragging blood pressure down. Some substances skip the antibody step entirely and burst mast cells directly; contrast dyes, opioids, heat, cold and even vibration can do this. Curiously, a different antibody, immunoglobulin G, usually protects against such reactions by competing for the allergen.
Skin is involved in 80 to 90 percent of episodes, with hives, flushing or swelling, and breathing in about 70 percent. When food is swallowed, symptoms tend to begin within up to 2 hours; when a trigger enters a vein, typically within 5 to 30 minutes. Some people describe a strange sense of impending doom. In a small minority, the heart slows rather than races as pressure falls.
Triggers vary with age and place. Food dominates among children and young adults, while medications and insect stings matter more later in life. Western reactions often involve peanuts, wheat, shellfish, milk and eggs; sesame is common in the Middle East, and rice and chickpeas in Asia. Many children grow out of it: by 16, 80 percent of those who reacted to milk or eggs tolerate them. Penicillin causes a reaction roughly once in every 2,000 to 10,000 courses. In 32 to 50 percent of cases no cause is ever found.
Somewhere between 0.05 and 2 percent of people are estimated to experience it during their lives, most often the young and women. Emergency care centres on an epinephrine injection, and about 99.7 percent of people hospitalised with it in the United States survive.
Source: Anaphylaxis
