Final answer:
Allergic reactions do not typically occur the first time a person is exposed to an allergen; this initial exposure sensitizes the immune system. Subsequent exposures to the allergen can then cause allergic reactions. This statement is therefore false.
Step-by-step explanation:
The statement that allergic reactions generally occur the first time a person takes the drug is false. Allergic reactions are an example of type I hypersensitivities, which occur when an individual who is already sensitized to a specific allergen is exposed to it again. The first exposure to an allergen usually does not trigger a notable allergic response; instead, it sensitizes the immune system to the allergen through the production of IgE antibodies. It is upon subsequent exposure that these antibodies recognize the allergen and lead to an allergic reaction, which may include symptoms such as watery, itchy eyes, swollen tissues, and sneezing.
Allergies can manifest in both localized forms, such as hay fever or asthma, and in systemic forms, such as anaphylaxis, which can be severe and life-threatening. The efficacy of allergy shots in mitigating allergic reactions is due to the gradual desensitization of the immune system to the allergen.