The MastAttack 107: The Layperson’s Guide to Understanding Mast Cell Diseases, Part 20

I answered the 107 questions I have been asked most in the last four years. No jargon. No terminology. Just answers.

29. Why do I swell up when I have a reaction? Where does the fluid come from and where does it go?
• Your body feeds its cells by keeping blood circulating. The blood passes by cells. The cells pull nutrients, oxygen, and other things they want out of the blood. In return, the cells release their wastes into the bloodstream to carry them away to a place where they can be broken down.
• Mast cells make and release many chemicals. These chemicals are commonly called mediators because they mediate many reactions in the body that affect the body in many different ways.
• Mast cell mediators do many things. They can make blood vessels get looser or tighter to help control heart rate and blood pressure. Their ability to make blood vessels relax is the important point when considering swelling. When mast cells release certain mediators, the vessels relax and get a little wider.
• Vessels are made of a complicated network of cells and fibers. It’s like an afghan blanket: some parts of mostly solid and in other places, there are holes you can put your fingers through. When the vessels relax, those holes get larger so it’s easier for things to fall through the holes. In this case, what falls through the hole is fluid from the blood.
• Keep in mind that blood is a mixture of many things. For right now, let’s divide it into cells and everything else. Everything else is a liquid with some stuff dissolved in it.
• When the blood vessels relax, that liquid from the blood trickles out of the blood vessel and goes into the tissue. In some cases, if the blood vessels relax enough, cells actually fall out of the bloodstream and end up in tissue, too.
• The problem is that once you fall out of the bloodstream, you can’t just turn around and go right back in. That’s why swelling takes longer to subside than other symptoms, even with appropriate treatment.
• Everyone is familiar with the bloodstream. Less familiar is the lymphatic circulatory system. Lymphatic circulation is how your body moves things that fall out of the blood back to the bloodstream. This process is slower than processes that can release other symptoms and can sometimes take days.
• When you swell up, fluid falls out of your bloodstream and gets stuck in your tissues. The cells nearby will absorb some of the fluid and take up molecules they can use. However, if there is a lot of extra fluid there, the cells nearby cannot take up all of the fluid. Gradually, your lymphatic system sucks up that fluid and brings it back toward the heart so it can get back into the bloodstream.
• When you get hives (urticaria), it happens because fluid falls out of the bloodstream in a layer of tissue in the skin called the upper dermis.
• When you get angioedema, it happens because fluid falls out of the bloodstream in lower portions of tissue in the skin called the dermis, subcutaneous tissue, mucosa, and submucosa.
• The fact that the process for hives and angioedema is so similar and really distinguished only by which tissue layer they affect is the reason urticaria and angioedema so commonly occur together and are discussed together.

For more detailed reading, please visit the following posts:
Chronic urticaria and angioedema: Part 1
Chronic urticaria and angioedema: Part 2
Chronic urticaria and angioedema: Part 3
Chronic urticaria and angioedema: Part 4
Chronic urticaria and angioedema: Part 5
The Provider Primer Series: Management of mast cell mediator symptoms and release