Which injections are most commonly administered intradermally?

• A. Intramuscular vaccines
• B. Subcutaneous injections
• C. IV infusions
• D. TB skin test, allergy tests, local anesthetics

Answer: (D)

Injections placed into the dermal layer are chosen because the dermis contains many immune cells that produce a visible, localized reaction when antigen is introduced. This makes it ideal for skin testing. The TB skin test relies on this mechanism: a tiny amount of antigen is injected intradermally to provoke a measurable induration in the skin. Allergy skin testing uses the same principle, injecting small amounts of suspected allergens into the dermis to observe wheal-and-flare reactions. These reactions are easiest to detect and quantify when the injection is just beneath the epidermis, which is why intradermal administration is the standard for these tests. Other routes—into muscle, subcutaneous tissue, or a vein—serve different purposes and do not produce the same skin-test reactions. Local anesthetics can be used for superficial numbness in some contexts, but the hallmark intradermal applications are the TB test and allergy testing.