In this day in age, a majority of the population possesses some sort of allergy. Whether it be to penicillin, pollen, peanuts, eggs, or shellfish, the list goes on and on. Yet, what can be immediately noticed from this list of but a few allergens is that most are directed towards food. After all, food contains some of the most common "foreign entities" which enter our body, in the same ranks (kinda) as bacteria, viruses, and fungi. What's more is we let food in it willingly (why would we ever do such a crazy thing!). However, importantly, food does not often release immune danger signals into our body (unless it was those Mexican leftovers sitting in your fridge for one day too long).
A common form of allergy is immediate hypersenstivity, or type I hypersensitivity. This response to an allergen usually occurs relatively quickly (~ 30 mins) and can be very severe. Such a hypersensitivity (aka allergy) is mediated in two distinct steps. First is the sensitization stage where the allergen penetrates, most likely at a skin or mucosal interface, and is taken up by dendritic cells (DC), which are known for there extensive processes and ability to showcase a pathogen on their surface. Next, the DC activates a helper T cell and helps produce a polarized response, meaning the T cell develops into a Th2 subtype. A danger signal must be present in order for this type of activation to occur, but the whereabouts of such a signal is still under debate. Nevertheless, the Th2 cell, known for aiding in the humoral (antibody response) does its duty and activated B cells. These B cells ultimately undergo a process dubbed isotype switching where the type of antibody they produce is modified. This process leads to B cells producing large quantities of a type of antibody known as IgE, known for its role in allergies.
The next stage is the effector stage where if the allergen returns to the body it leads to an allergic response. The IgE antibodies can bind to the outer surface of certain leukocytes, such as mast cells, via Fc receptors and "arm" them. When the allergen returns then the mast cell is activated via the antibodies and degranulates, spewing toxic chemicals at the allergen and causing an immune response in the process.
In the recent work of Morin et al. (2011), the group examines the allergenicity of two proteins found in different fractions of cow milk, beta-lactoglobulin (BLG) and casein (CAS). Cow's milk allergy (CMA) is very common during early childhood and evokes a rapid response. However, CMA usually disappears , it remains in some (Sicherer et al. 2010). What is more dangerous about CMA is that it increases the child's susceptibility to other allergies (Saarinen et al. 2005). In this study, they examined the allergenic and immunogenic effects of these two proteins in two different strains of mice, one was germ-free (GF) and the other conventionally raised (CV). They attempted to complete this study in the absence of an adjuvant for the proteins. Adjuvants are commonly used to give the immune system a push, so to speak, helping to evoke an immune response to the injected compound. The downside of adjuvants in allergy studies is that they can evoke non-specific allergies to some of their own components, making it difficult to examine the specificity of a response. The GF mice have an immature immune system due to their lack of experience with many pathogens, so the authors hypothesized these mice would be more susceptible to an allergic response. Two sets of experiments were completed in each mouse, one measuring antibody type from blood samples and the other examining cytokine secretion from the isolated and reactivated spleens of the mice. Afterwards, they tried something novel and intraperitoneally administered a heated mix of the proteins, hoping to denature the proteins and open new epitopes for binding.
As described earlier, the authors first sensitized the mice two either BLG or CAS and upon second injection an immune response was mounted. The amount of IgE antibody in blood samples was determined because that is a prominent sign of an allergic response. The way this is done is by incubating a sample of blood into a well which contains the BLG tethered to the bottom and then determining the absorbance. As hypothesized, the GF mice had four times as much IgE secreted than the CV mice. However, when the same was done for CAS there was only an IgG response. This response is more common and is the sign of a simple immune response rather than allergy. The results were curious because they suggest that something about the protein structures themselves governed whether it was allergenic or not. When examining the cytokine secretion from the removed and reactivated spleen the GF mice had a much higher quantity of IL-4, IL-5, and IL-13 than the CV mice. These cytokines are typical of a Th2 response and further corroborated that GF mice evoked an allergic response to BLG. Once again, no allergic response was seen to CAS.
Next, the researchers mixed together BLG and CAS in the same ratio which would be expected in whole milk and heated up the sample before injection. For the most part there was no IgE response in either the GF or CV mice, only IgG (GF mice had a small amount of IgE but it turned out to be from non-denatured BLG in the sample). Interesetingly, the IgG response was actually higher in the GF mice this time around. The authors provide a suggestion for this in their discussion; they believe the aggregation of denatured BLG increased the number of binding sites, or avidity, of the protein and thus increased its immunogenicity, or propensity to evoke an immune response.
One of the main findings of this paper is that studying models of allergens do not necessarily require adjuvants. However, personally, I think there are many other noteworthy findingst. Firstly, the authors mentioned pasteurization of milk, where it is heated to a certain temperature and then rapidly cooled. Denaturation of food proteins decreased the allergenicity of the protein in milk and I am curious if this could occur with other common foods. For instance, do raw eggs evoke an allergic response more so than cooked eggs? Does allergic sensitization to a food protein require it to be in a certain state? They're both intriguing questions. Lastly, the most stimulating to me, was the idea of the food matrix, or how the environment and pass triggers can increase one's susceptibility to an allergy. Since the author's dealt with germ-free and conventional models it was expected that they mention gut microbiota. Even though the GF mice have an immature immune system, it's fun to speculate as to whether the bacterial environment of the gut has an effect on allergenicity versus immunogenicity. This idea could mean that the diet of certain people and cultures may make them more or less susceptible to certain food allergies!
Stéphanie Morin1 ,Hervé Bernard1 ,Laetitia Przybylski-Nicaise1, Gérard Corthier2, Sylvie Rabot2, Jean-Michel Wal1, Stéphane Hazebrouck (2011). Allergenic and immunogenic potential of cow's milk β-lactoglobulin and caseins evidenced without adjuvant in germ-free mice. Molecular Nutrition and Food Research. Volume 55. Issue 11. 1700-1707. November 2011.
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