An intradermal injection introduces a substance into the dermis, the skin layer located between the outer epidermis and the deeper subcutaneous tissue. This technique is distinct from other methods that target muscles or blood vessels. The procedure requires precision because the dermal layer is very thin, and successful administration creates an observable reaction on the skin’s surface.
Applications and Anatomic Sites
The primary use for intradermal injections in research is to study localized immune responses. The dermis is populated with a high density of specialized immune cells, such as dendritic cells, making it an ideal location for applications like vaccine development, allergy testing, and immunology studies. These studies can include delayed-type hypersensitivity assays where localized inflammation is measured.
Two main anatomic sites are chosen for intradermal injections in mice: the dorsal flank and the ear pinna. The flank, on the mouse’s back or side, provides a large, flat surface area suitable for injections requiring slightly larger volumes. This site must be shaved before the procedure to allow for clear visualization of the needle insertion and the subsequent skin reaction.
The ear pinna, the flap of the external ear, is another frequently used site. Its thinness and translucency make it useful for studies where the inflammatory response, such as swelling or redness, needs to be easily and repeatedly measured. The choice between the flank and the ear pinna depends on the experiment’s goals; the flank is preferred for larger volumes, while the ear is better for monitoring visual changes.
Equipment and Subject Preparation
Performing an intradermal injection requires specific equipment to ensure accuracy. Researchers use low-volume syringes, such as 0.3 mL or 0.5 mL insulin syringes, for precise measurement. These are paired with small-gauge needles, from 27-gauge to 30-gauge. The fine diameter of these needles is suited for navigating the thin dermal layer and reduces potential injury at the injection site.
Proper preparation begins with safe and secure handling to minimize stress and movement. Depending on the injection site, additional steps are required. If the dorsal flank is the target, the fur must be removed using electric clippers or a depilatory cream. This provides an unobstructed view of the skin for correct needle placement.
Institutional guidelines may require the use of anesthesia, particularly for sensitive areas or to ensure the animal remains completely still. Anesthesia ensures the mouse does not experience distress and allows the researcher to perform the procedure with greater control. The mouse must be appropriately restrained with the injection site fully visible and accessible.
The Intradermal Injection Procedure
With the mouse prepared and restrained, hold the syringe at a shallow 10 to 15-degree angle, nearly parallel to the skin’s surface. The needle’s bevel, the slanted opening at the tip, must face upward. This orientation helps the needle slide smoothly into the dermis rather than piercing through to the subcutaneous tissue.
Gently insert the needle into the skin, advancing only 2 to 3 millimeters until the tip is within the dermal layer. The outline of the needle tip may be faintly visible through the epidermis. Hold the syringe steady, as movement could cause the needle to penetrate too deeply or exit the skin. Some techniques involve using forceps to lightly grasp and stabilize the skin for insertion.
Once the needle is correctly positioned, slowly depress the plunger to administer the substance. The volume for an intradermal injection in a mouse is between 10 and 50 microliters. Injecting the fluid slowly is necessary to allow the dermis to accommodate the volume without causing excessive pressure or leakage. After waiting a few seconds for pressure to disperse, withdraw the needle smoothly at the same shallow angle it was inserted.
Verification and Post-Procedure Care
A successful injection is verified by the immediate appearance of a small, well-defined, raised bump on the skin’s surface, known as a bleb or wheal. This translucent bleb is a visible pocket of fluid that confirms the substance was delivered to the intended dermal layer. If no bleb forms, it indicates the needle was inserted too deep, and the injection was delivered subcutaneously instead.
Following the procedure, the animal requires careful monitoring. If anesthesia was used, the mouse must be kept warm and observed until it has fully recovered and resumed normal movement. The injection site should be checked periodically for adverse reactions beyond mild inflammation, such as excessive swelling or ulceration. The animal’s general behavior, including activity and feeding habits, should also be monitored for signs of distress. Once fully recovered, the mouse is returned to its housing.