Can you spontaneously develop an allergy to something allergic the day before such as animal dander?

While it is possible to develop an allergy to something you were previously not allergic to, it is unlikely that you would develop a severe allergic reaction to animal dander the day after exposure. Allergies typically develop over time, with repeated exposure to an allergen. Symptoms of an allergic reaction usually develop within minutes to hours of exposure to the allergen, but can sometimes take up to a few days to appear.

If you are experiencing severe allergic symptoms, such as difficulty breathing, swelling of the face, throat, or tongue, or dizziness, it is important to seek medical attention immediately. These symptoms can be life-threatening and require prompt treatment.