A tuberculin skin test is read two days after the injection. What accounts for the delay before the reaction appears?

1. Delayed hypersensitivity, or type IV, is basically a standard cellular immune response and is exclusively a T cell phenomenon. 2. The first exposure to the antigen is called sensitization. 3. On re-exposure, a secondary cellular response secretes cytokines that recruit macrophages and other phagocytes to the site. 4. The time it takes to build that cellular infiltrate accounts for the 24 to 72 hour delay in development. 5. In the tuberculin test, a positive result is a raised red area that is hard to the touch, called an induration, made of accumulated activated macrophages. 6. Histamine release and IgE coating drive immediate type I reactions that occur within just a few minutes, so neither explains a two day wait. 7. Complement proteins are constitutively found in the blood plasma already, so nothing has to be manufactured before the test can turn positive. _Source: OpenStax Anatomy and Physiology (CC BY 4.0), Ch 21 "The Lymphatic and Immune System", section 21.6 Diseases Associated with Depressed or Overactive Immune Responses_