Having a hypersensitivity means that someone’s immune system has reacted to something in such a way that it ends up damaging them, as opposed to protecting them. There are four different types of hypersensitivities, and in the first type or type one, the reactions rely on Immunoglobulin E, or IgE antibody, which is a specific type of antibody - the other major ones being IgG, IgA, IgM, and IgD. So because IgE is involved with type one hypersensitivity reactions they are also called IgE-mediated hypersensitivities. This type of reaction is also sometimes called immediate hypersensitivities, because the reaction happens super fast—on the order of minutes. So most allergic reactions are IgE-mediated, and therefore most allergies are type I hypersensitivity reactions. “Allergy” comes from the Greek Allos which roughly means “other” and ergon which means “reactivity”. Essentially, allergies are reactions to molecules from outside your own body that most people don’t react to—and these are specific molecules from things you might breathe or take in like foods, animal dander, bee stings, mold, drugs or medications, and pollen. You can also mount an allergic reaction to things you come in contact with on your skin like latex, lotions, and soaps. These specific molecules are also called antigens, and when they cause an allergic reaction, they’re called allergens. An allergic reaction happens in two steps, a first exposure, or sensitization, and then a subsequent exposure, which is when it gets a lot more serious. People that react to these allergens usually have a genetic predisposition to having over-reactions to unknown molecules or allergens. This means that these people have certain genes that cause their T-helper cells to be more hypersensitive to certain antigens. Since the production of these T-helper cells is genetically linked, allergies to things tend to run in families. So let’s say this person breathes in some ragweed pollen, that person happens to have T-helper cells that can bind to a specific molecule on the pollen, making that molecule an allergen. First off, that antigen gets picked up by immune cells hanging out in the membranes along the airways, which then grab the molecule and migrate to the lymph nodes, which happens regardless of if the person is allergic or not. These cells are antigen-presenting cells, since they carry the antigen to the lymph nodes and present it to the T-helper cells living there. Dendritic cells and macrophages are examples of antigen-presenting cells. When the person is allergic, the antigen presenting cell will also express costimulatory molecules, which are needed to mount an effective immune response. Before the T-helper cell sees the antigen though, it’s called a naive T-helper, since, even though it’s built to recognize the antigen, it hasn’t actually seen it before. When the T-helper gets its hands on the antigen though, and also binds the costimulatory molecule, it’s now been primed, and the naive T-helper changes into a different sort of T-helper cell. Usually in type I hypersensitivity it differentiates into a type 2 T helper cell, or just TH2 cell, and this step happens in response to various small proteins or interleukins that might be floating around at the time. Some interleukins that sway the T-helper cell into turning into a TH2 cell are interleukin 4, interleukin 5, and interleukin 10, and these are all cytokines - and they’re given numbers because it’s easier to keep track of them that way. At any rate, the excited TH2 cells release a bit of their own interleukin 4 and get the B cells to undergo antibody class-switching, and so the B cell switches from making IgM antibodies to making IgE antibodies which are specific to ragweed pollen in our example. TH2 cells also release some interleukin 5, which stimulates production and activation of eosinophils, a granulocyte, which is a type of white blood cell that degranulates or essentially releases a whole bunch of toxic substances that can damage both invading cells and nearby host cells.
These ragweed-specific IgE antibodies have a high affinity for, or basically really like Fc epsilon receptors on mast cells, another type of granulocyte, so they quickly attach themselves to the surface of mast cells. These antibodies are also called cytotropic antibodies, since they can bind to cell surfaces. At this point it’s like the mast cell’s been geared up for combat, and is ready for action, and therefore we’re finished with the sensitization phase.
Now let’s say that that same person breathes in the ragweed pollen again, maybe a few months later - a second exposure. Well, the suped up mast cells, using their coat of antibodies, binds to the antigen. Actually, it takes two or more bound antigens to cross-link the IgE antibodies, which signals the mast cell to degranulate and release a bunch of pro-inflammatory molecules called mediators that ultimately causes the effects seen in an allergic reaction.
One of the major mediators released in an allergic reaction is histamine. Histamine binds to H1 receptors and causes the smooth muscles around the bronchi to contract, which means the airways get smaller, making it more difficult to breathe. It also causes blood vessel dilation and increased permeability of the blood vessel walls, meaning that, while blood vessel diameter increases and blood flow to the affected area increases, fluid is allowed to more easily leak out the blood vessel walls and get into the interstitium, the spaces between cells, which causes edema and swelling, and urticaria, or hives. In addition to histamine, mast cells release other pro-inflammatory mediators including some that activate eosinophils and proteases which chop up large proteins into small peptides. The effects of these molecules are called “early phase reactions”, and they happen within minutes of the second exposure. There are also “late phase reactions” though, which happen 8-12 hours after that second exposure, where even more immune cells like TH2 cells, eosinophils, and basophils, yet another type of granulocyte, are recruited to the site where the allergen is located because of the cytokines and pro-inflammatory molecules produced during that early phase. These include some of those same interleukins again, interleukin 4, interleukin 5, and interleukin 10, but also leukotrienes which are smaller molecules made out of fatty acids and facilitate communication between a local group of cells. Two leukotrienes in particular, LTB4 and LTC4, can not only cause smooth muscle contraction and damage to the epithelium like histamine, but they can attract immune cells - like neutrophils, mast cells, and eosinophils to their location even after the allergen is long-gone. A lot of people with allergic reactions experience mild symptoms, like hives, eczema, allergic rhinitis—which is inflammation of the nose, as well as asthma. Certain people though, when exposed to a large load of specific allergens, like bee stings, seafood, or peanuts, can have a really severe and potentially life threatening allergic reaction. The increased vascular permeability, along with the constriction of airways can be severe enough such that the body can’t supply the vital organs—like the brain, with enough oxygen-rich blood, a condition known as anaphylactic shock.
Treatment for type one hypersensitivity can involve a variety of medications. Antihistamines, act to block the effects of histamine, which reduces vascular permeability and bronchoconstriction. Also there’re corticosteroids, which can be used to reduce the inflammatory response, as well as epinephrine, which is sometimes given during severe reactions via intramuscular injections through an EpiPen or intravenous injection. Epinephrine can help constrict blood vessels and prevent anaphylactic shock. If there’s ever a serious type one hypersensitivity reaction that requires something like steroids or Epinephrine, it’s super important to get medical attention because type 1 hypersensitivity reactions can be serious and can sometimes get slightly better before getting worse again.