The tuberculin skin test, also called the Mantoux test, is the most well-known example of an in vivo immunological test. It is performed by injecting a small amount of protein derived from tuberculosis bacteria directly into the skin of a living person, then reading the body’s immune response 48 to 72 hours later. Other in vivo immunological tests include the skin prick test for allergies and patch testing for contact dermatitis. All of these share a defining feature: the immune reaction happens inside the patient’s body, not in a laboratory dish.
What “In Vivo” Means in Immunology
In vivo literally means “within the living.” An in vivo immunological test uses the patient’s own immune system as the detection tool. A substance is introduced into the body, usually through the skin, and the clinician observes whether the immune system mounts a visible reaction at the site. This is the opposite of in vitro (“in glass”) tests like blood-based assays, where a sample is drawn and analyzed in a lab.
The distinction matters because in vivo tests depend on a functioning immune system. If someone’s immune defenses are suppressed, the test may produce a falsely negative result even when an infection or allergy is present.
The Mantoux Test: The Classic Example
The Mantoux test is the textbook answer when exam questions ask for an in vivo immunological test. A healthcare worker injects 5 tuberculin units of purified protein derivative (PPD) just under the skin of the forearm. If the person has been previously exposed to tuberculosis bacteria, their memory T-cells recognize the injected protein and trigger a localized immune response called a Type IV delayed-type hypersensitivity reaction. Over the next two to three days, immune cells migrate to the injection site, producing a firm, raised area of skin called an induration.
The test is read by measuring the diameter of that induration in millimeters, not the redness around it. What counts as “positive” depends on the person’s risk profile. According to CDC guidelines, an induration of 5 mm or more is positive for people with HIV, organ transplant recipients, and those on immune-suppressing medications. The threshold rises to 10 mm for people born in countries where TB is common, those with diabetes or kidney disease, young children, and people living in congregate settings like shelters or correctional facilities. For people with no known risk factors, 15 mm or more is the cutoff.
One quirk of the Mantoux test is the booster phenomenon. In people who were infected years ago, immune memory can fade enough to produce a false-negative result on the first test. But that initial injection can “wake up” the dormant immune memory, so a second test given later shows a stronger, positive reaction. This isn’t a new infection; it’s the immune system being reminded of an old one.
How It Compares to Blood-Based TB Tests
The main in vitro alternative to the Mantoux test is the interferon-gamma release assay, or IGRA. Instead of injecting anything into the body, a blood sample is drawn and exposed to TB-specific proteins in a lab. If the patient’s T-cells release interferon-gamma in response, the test is positive.
A large diagnostic meta-analysis found that the Mantoux test has a sensitivity of about 72% and a specificity of 79% for active TB. The IGRA performs somewhat better, with sensitivity around 79% and specificity near 86%. The gap widens dramatically in immunocompromised patients: the Mantoux test’s sensitivity drops to just 23% in that group, while the IGRA maintains about 66% sensitivity. This makes sense because in vivo tests rely on a healthy immune system to generate the skin reaction, and people with weakened immunity simply can’t mount a strong enough response.
Skin Prick Testing for Allergies
Allergy skin prick testing is another common in vivo immunological test, though it measures a different type of immune reaction. Small drops of allergen extracts are placed on the skin, usually the forearm or back, and a tiny lancet pricks through each drop. If the person is allergic, their immune system releases histamine and other chemicals at the site, producing a raised, itchy bump called a wheal within 15 to 20 minutes. This is an immediate hypersensitivity reaction (Type I), driven by antibodies called IgE, rather than the delayed T-cell response seen in the Mantoux test.
A wheal with an average diameter of 3 mm or larger is generally considered a positive result. To improve accuracy, the test always includes a positive control (histamine, which should produce a wheal in everyone) and a negative control (saline, which shouldn’t). Some clinicians calculate a ratio between the allergen wheal size and the histamine control wheal to correct for individual differences in skin reactivity.
Patch Testing for Contact Dermatitis
Patch testing identifies substances that cause allergic contact dermatitis, like nickel, fragrances, or preservatives. Small chambers containing suspected allergens are taped to the patient’s back and left in place for 48 hours. After removal, the skin is examined for redness, swelling, or tiny blisters. Like the Mantoux test, this is a Type IV delayed hypersensitivity reaction driven by T-cells.
A single reading at 48 hours isn’t enough. Research has shown that about 30% of relevant allergic reactions that are positive at 96 hours appear completely negative at the 48-hour reading. For this reason, a second reading is typically done between day 3 and day 7 after the patches were first applied.
Why In Vivo Tests Carry Unique Risks
Because in vivo tests introduce a substance directly into the body and rely on a real immune reaction, they carry a small but real risk of triggering a systemic response. Skin prick testing occasionally causes reactions beyond the test site, and intradermal testing (injecting allergens deeper into the skin) is performed with sequentially increasing concentrations specifically to minimize the chance of a severe reaction.
Drug provocation testing, where a patient takes a suspect medication under observation, is considered the gold standard for confirming drug allergy but is not recommended for patients who have previously experienced anaphylaxis because it could trigger another life-threatening episode. In those cases, in vitro blood tests offer a safer way to investigate the allergy without exposing the patient to the substance again.
Quick Comparison: In Vivo vs. In Vitro Tests
- In vivo tests (Mantoux, skin prick, patch test): performed on the patient’s body, produce a visible skin reaction, require a functioning immune system, give results in minutes to days depending on the type
- In vitro tests (IGRA, specific IgE blood tests, ELISA): performed on a blood sample in the lab, do not require the patient’s immune system to react in real time, generally carry no risk of allergic reaction, and are preferred when in vivo testing is unsafe or unreliable
In vivo tests remain widely used because they are inexpensive, produce results without specialized lab equipment, and in many settings provide sufficient accuracy. The tradeoff is that their reliability depends on the patient’s immune status, and they expose the patient, however briefly, to the substance being tested.