With the coronavirus on everyone’s mind these days, the slightest sniffle, sneeze, and cough can be alarming. In today’s article, we aren’t going to dive into the science of viral infections. Instead, let’s take a look at another source for runny noses, scratchy throats, and red eyes…histamines!

 

What is Histamine?

Histamine is a molecule within our bodies that serves several essential functions. It can dilate, or widen, the capillaries, the smallest blood vessels in the body that transfer nutrients in the blood to individual cells. It can also increase heart rate and cause involuntary muscles, such as those in the lungs and digestive organs, to contract (1). 

 

What is Histamine’s Role in Allergy and Immunity?

Though histamine is produced, stored, and used throughout the body, it is utilized frequently by mast cells and basophils as part of the immune response. Basophils are white blood cells, while mast cells are hybrid cells that serve a local immune function in specific tissues. Both types of cells use histamine to dilate capillaries in an infected area. This allows additional white blood cells to more easily enter an affected area and initiate an immune response. 

The most common response to histamine is itching, due to local immune reactions near the surface of the skin. An increase in histamine can also cause a runny nose and watering eyes; as capillaries dilate, fluid can more easily pass into cells, including those in the nose and eyes (which have numerous mast cells). This excess fluid can leak into the nasal cavity or tear ducts. These three effects (runny nose, watering eyes, itching) are the hallmark symptoms of hay fever, a common allergic reaction to pollen, dust, pet dander, and other environmental triggers. 

 

How Do Allergens Trigger Histamine Release?

In a healthy immune response, mast cells and basophils produce antibodies and continuously “screen” the blood and local tissues for virus and bacteria particles, often proteins (2). When a foreign substance is identified, these cells initiate a series of chemical signals that act to remove the invading particles.

One very common response is for mast cells and basophils to release histamine, allowing additional white blood cells from the immune system to cross into the infected tissue. 

In an allergic reaction, a similar process occurs, only the body reacts to a neutral environmental particle rather than an infectious one. For example, when someone with a pollen allergy breathes in summer air in a park or field, pollen particles will enter the respiratory tract. Here, mast cells in the eyes and nose will “recognize” the pollen as an invader and signal an immune response (3).

The mast cells will send a signal to alert white blood cells to the presence of the foreigner and release histamine to allow the white cells to clear out the infected tissue more easily. The action of the white blood cells, combined with the release of histamine, causes the symptoms of hay fever, also called rhinitis. 

 

How Do Antihistamines Work?

Antihistamines, as you may have guessed, act by blocking the action of histamine. Because histamine is involved in a variety of other processes in addition to immunity, there are actually several different types of antihistamines. The effects of histamine are caused by its action on a receptor, a site on the surface of a cell that reacts to specific molecules or proteins.

There are four main types of histamine receptors, called H1, H2, H3, and H4. Each receptor serves a particular function, and sometimes, multiple functions. 

When histamine reacts or binds to the H1 receptor, it causes local immune responses, including rhinitis (4). Therefore, the majority of antihistamines block the H1 receptor, preventing histamine from reacting at the site. Although antihistamines bind to the H1 receptor, they don’t create the same set of signals that leads to hay fever. Thus, antihistamines effectively prevent allergic reactions. 

However, if an allergic reaction has already occurred, antihistamines are not as effective; that’s because histamine has already reacted with the H1 receptor, and antihistamines don’t flush it out; instead, they prevent it from binding in the first place.

For this reason, it’s much more effective to take antihistamines prior to an allergic reaction. For example, if you know you have an allergy to pet fur or dander, and you’re going to visit a friend or family member who owns a dog, it’s best to take antihistamines before you go to their house. That way, when you arrive, the H1 receptor is already blocked, and the pet won’t bother you. 

 

Antihistamines and Drowsiness

Earlier, we mentioned that histamine is a versatile substance that plays many roles in the body, one of which is to mediate the immune response. Histamine also plays a role in the contractions of involuntary muscles in the heart and digestive tract. It also acts as a neurotransmitter, a signaling agent in the brain.

The four histamine receptors each serve a different function, allowing specificity of the response. For example, if a pathogen (or allergen) enters the nasal passage, the release of histamine will cause the blood vessels to dilate as it acts on H1 receptors. But, the lack of H2, H3, and H4 receptors ensure that histamine doesn’t interfere with other bodily functions. 

However, H1 receptors are also present in the brain. This usually doesn’t create any complications; rarely do infectious agents reach the brain, so histamine typically doesn’t act on H1 receptors in blood vessels and nerves at the same time. Antihistamines, on the other hand, can do just this.

When you take an antihistamine, it enters your bloodstream and interacts with H1 receptors throughout your body, including those in your brain. That’s why taking an antihistamine can cause you to feel drowsy; it interacts with H1 receptors in the brain that help to signal sleepiness. 

 

Not sure if you’re dealing with allergies or a viral infection?

The best place to start is to call your primary care physician and see what they recommend you do next.  For situation updates, ways to prevent illness, and common questions regarding the coronavirus, visit the CDC’s website!

References 

1. Dorland’s Illustrated Medical Dictionary. Philadelphia, PA: Elsevier Saunders; 2012: 861-862. 
2. da Silva EZM, Jamur MC, Oliver C. Mast cell function: A new vision of an old cell. J Histochem Cytochem. 2014;62(10):698-738. DOI: 10.1369/0022155414545334. 
3. Allergies. Mayoclinic.org. https://www.mayoclinic.org/diseases-conditions/allergies/symptoms-causes/syc-20351497. Accessed March 10, 2020. 
4. Canonica GW, Blaiss M. Antihistaminic, anti-inflammatory, and antiallergic properties of the nonsedating second-generation antihistamine desloratadine: A review of the evidence. World Allergy Organ J. 2011;4(2):47-53. DOI: 10.1097/WOX.0b013e3182093e19.