Well lemme give you a bit more info if you’d like: Essentially, allergies is a inappropriate response by the immune system against a non-pathogen. For a molecule to become a possible allergen it has to be large and common enough for the body to process them. That’s why no ones allergic to elements by themselves like oxygen, sodium or carbon (etc). Usually, a molecule has to be a protein to cause a reaction (which have chains of amino acids that connect together and bind with other chains and so on). Best analogy I’ve heard, is alphabet is the amino acids which come together to form words which then form sentences which then form paragraphs(quaternary proteins). The body then will take that big molecule, break it down and make antibodies to identify and fight the “pathogen” (antigen). Some allergies like peanut allergies, directly cause a reaction (due to the family of Ara proteins). There are many different ways which this may occur. For instance, penicillin reaction is not the usual. Penicillins by themselves in your body aren’t big enough to trigger a reaction. They are haptens, which means they are too small for your body to consider a threat by themselves, yet we know people are allergic to penicillin. This occurs when the penicillin binds with another protein in your body (albumin, which helps carry molecules around in your body), which then makes a big enough molecule for your body to analyze and make antibodies against. If you’re allergic, rubbing penicillins on your arm won’t do much, taking it may kill you tho.
This goes along with how vaccines are designed. We want to introduce a potential pathogen to your system so that your body has a chance to analyze it and make antibodies. From analyzing to antibody production, it’s bout 7 days. So they idea is if we vaccinate you, and then more than 7 days later you’re introduced to the pathogen (I.e influenza virus) your body will already have the antibodies to fight the infection. The problem (and cause of anti-vaxxers) is the adjuvant. There are essentially 3 parts to a vaccine: 1) the pathogen (killed, modified living or broken down), adjuvant and preservatives (low amount to keep the proteins from breaking down). If I was to inject you with a vaccine that only had a killed, broken down or mod. Living (meaning it’s alive as a cell but stressed to the point where it can no longer cause disease) pathogen and no adjuvant, your body would breakdown and absorb it as it was not causing an issue (small dose of protein, carbs and lipids). The adjuvant is the critical part, it is a chemical that agitates the tissue around the injection enough to cause your immune system to come running which then leads the immune system to process the large molecules (the pathogen), blame it for the problem and create antibodies. Essentially, the adjuvant frames the pathogen for a small problem to keep it from causing a much bigger problem. This immune response is what causes you to feel sick sometimes. The immune response causes a lot of the same symptoms. Histamines released cause fever, flush face, runny nose, watery eyes, inflammation. That’s why regardless what you get sick with (as far as upper respiratory infections) you get similar symptoms. The virus/bacteria isn’t actually causing you to have a runny nose, that’s your body’s response to pushing a pathogen out of your nasal cavity, increased secretions, increase blood flow (to bring in your white blood cells) cause inflammation and flush areas.