Figure A.  6 hours after administration of tuberculin, some reaction can be seen.  It is thought that this does not reflect a specific reaction.

Figure B.  After 24 hours a clear reaction is seen, but the demarcations are unclear.

Figure C.  After 48 hours a central induration is now clearly present. It is thought to be the result of release of cytokines by specified cells that activate the local macrophages, resulting in the typical induration.

Figure D.  After 72 hours the result can be read equally well.  The recommendation is to read the tuberculin test after 48 to 72 hours.

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