First off, a huge thank you to everyone who has read, shared, submitted questions to, and commented on this thus far. I really appreciate the support I have received.
This week’s question comes from Aerin, who asked, “I’m allergic to cats. Does that mean I’d be allergic to tigers?” Well, after some detailed exploration of this topic, I found that the answer to this is “kind of.” Obviously, this is not a particularly impressive answer, so I will definitely explain what I mean in a little bit. But, before I get into that, this question brings up the larger question of how we get allergies in the first place. So I wish to begin with an overview of what allergies are and how they are produced in the body before then exploring Aerin’s question more in-depth.
Allergies are the body’s response to something foreign, called an allergen (or, more generally, an antigen). The dictionary defines allergens as things that cause allergies (shocker!). While not a particularly helpful definition, it is more telling that it seems on the surface. This broad definition demonstrates the wide variety of environmental elements to which the body can react. In theory, the body could produce an allergy response to just about anything, from avocados to pet dander to bee pollen to the detergent you use. Even foods you’re normally not allergic to may present allergens if they have been cross-bred or genetically altered. Some responses may be mild, others catastrophic.
Antibodies are protein structures secreted by immune cells called B cells that seek out specific epitopes (the part of the antigen that the antibody can bind to). B cells come in billions of varieties, producing a correspondingly wide array of antibodies to allow the body to recognize a large number of possible antigens and epitopes. There are five different types of antibiodies (or immunoglobulins, abbreviated Ig): IgA, IgD, IgG, IgM, and IgE. The particular one that concerns us here is IgE due to its role in the formation of the symptoms we most readily associate with allergies. More on that in a bit.
Once released by the B cells, antibodies themselves “tag” the antigen when they bind to said epitope, thus marking it for the body to destroy or at least attempt to eliminate. They can also be used to disable antigens in their own right (which is the foundation of monoclonal antibody therapy), but that is outside the scope of this discussion. These tags, if they successfully attach to an antigen, prompt the division of the successful B cell in preparation for the possibility of an antigenic invasion of some type as well as the stimulation of other immune cells to flock to the area. Once the reaction has subsided, the B cells created for this purpose largely die off, except for some, known as “memory B cells,” which stick around in the body in a state of ready alert to ward off any previously encountered threat. This is the basis of the “immunity” one gets from vaccines. Unfortunately, antibodies are not particularly selective in terms of their target. If the protein it seeks is found locally, antibodies can initiate a damaging response to the body itself. The exact mechanism of how this comes about is debated (B cells that damage the body aren’t supposed to survive), but this is thought to be the main source of autoimmune diseases like rheumatoid arthritis.
IgE itself is an important immunoglobulin because it is the only immunoglobulin that stimulates mast cells and basophils, cells that contain and release histamine. As you may have realized if you take Benedryl or Claratin (over the counter antihistamines), histamine is responsible for a vast majority of the physical symptoms felt during an allergic reaction, including runny nose, watery eyes, tightness in the chest, flushing of the skin, hives, and swelling. It causes the swelling by making the capillaries less water tight (increasing their “permeability”) as well as dilating (opening up) the blood vessels feeding them, which allows for immune cells in the blood to cross through more easily and in greater numbers in response to the antigen. This leakiness also allows fluid to flow from the capillary to the surrounding tissue, resulting in the characteristic swelling. If this release of fluid happens to an extreme degree, anaphylaxis can occur, causing throat swelling and shock (when blood pressure drops to dangerously low levels due to a loss of blood volume). Epinephrine, the medication contained in an Epi-Pen, acts to reverse the constriction of the lungs and dilation of blood vessels. This can quickly reverse the symptoms as the fluid drains back into the bloodstream.
So now that we have a general idea of how the allergy response works, let’s dive into Aerin’s question. As you may or may not know, “big” cats like tigers and lions are more distantly related to house cats than is commonly believed. Although house cats look very much like miniature, less ferocious versions of big cats, they are actually only related at the family level (Felidae). That means they aren’t just separate species, but in separate genera (Panthera for big cats and Felis for house cats) as well. This is a fair evolutionary spread, which contributes to the ambiguity of the question’s answer.
Just about the only conclusive journal article I could find that tackles this issue head on comes from the July 1990 issue of The Journal of Allergy and Clinical Immunology. In this article, the researchers investigated if the main house cat protein known to cause allergies in humans (Fel d I) is found in big cats as well by examining Fel d I-specific IgE response (as well as the more general IgG response) to big cat dander. Their results were mixed. Although they found the IgE reacted with the proteins found in the big cat dander, the amount of reaction was nowhere near that of Fel d I itself. The tiger dander specifically was found closely in line with the other big cats. Based on that data and the authors’ conclusions, it appears that tigers can prompt an IgE response in house cat-allergic people, however not with the same vigor. In other words, being allergic to house cats means you’ll likely feel something if you come in contact with big cat dander, but the severity will likely differ from the original allergy.
Thank you for the question, Aerin! Hope this response helps! If you want your question answered too, you can submit yours directly to me by selecting the “Submit Your Question” tab at the top of this page.
Till we meet again next week, “Live Long and Prosper.”
Thank you to MCAT studying and biology classes for endowing me with the knowledge to explain this without significant help. Who knew you’d actually be useful?