A Qualitative Study on the Rejection Reaction of Human Skin to Metal
Human skin serves as both a physical and biological barrier, protecting the body from foreign substances and mounting immune responses against pathogenic microorganisms, exogenous macromolecular substances, and allografts. While the immune system typically targets exogenous cell-derived major histocompatibility complex (MHC) class-1 molecules and peptide-MHC class 1 complexes in the context of skin grafts, the rejection of inorganic substances by human skin remains poorly understood. This study explores a unique case in which a metal particle embedded in the skin was spontaneously expelled after nearly three decades, shedding light on the skin’s potential physiological function to reject inorganic foreign bodies.
The case in question involves the author, who, at the age of 15, experienced an incident where a metal particle was embedded in his left foot while hammering a piece of metal. The entry of the metal caused a 10-mm scratch but was not accompanied by pain or a foreign body sensation. Over time, the metal particle became visible as a small black object under the skin, located at a different site from the entry point. Remarkably, the skin remained intact and movable throughout this period. Seven years later, the author was able to feel the metal particle when it became enwrapped by the skin. At the age of 43, the metal particle, now corroded, was spontaneously discharged through a fistula on the skin surface. The diameter of the expelled metal particle was approximately 5 mm, and the fistula healed shortly after the expulsion. The exit point of the metal was 7 mm away from the original entry site.
The author conducted extensive research, including foot skin examinations and magnetic resonance imaging (MRI), to investigate this phenomenon. MRI scans taken three years after the expulsion revealed a fistula originating from the space between the skin and muscles in the foot dorsum, adjacent to the tibialis anterior muscle. The fistula measured 10 mm in length, and the skin thickness in the affected area was no more than 5 mm. These findings suggest that the metal particle resided in the space between the skin and muscles rather than being embedded within the skin itself. This is significant because the foot skin, which consists of the epidermis, dermis, and subcutaneous tissue, is rich in nerve endings and capillaries. Typically, foreign substances embedded in the skin would cause symptoms such as swelling, pain, and inflammation, but the author experienced none of these symptoms for nearly 30 years.
The study raises two key questions: (1) Where was the metal particle located after being embedded in the skin? and (2) Is the rejection of the metal particle due to a physiological function of the skin? Regarding the first question, the MRI findings and the absence of symptoms suggest that the metal particle was situated in the space between the skin and muscles of the foot dorsum. This location allowed the particle to remain undetected for decades without causing inflammation or necrosis. As for the second question, the rejection process appears to meet two prerequisites for being considered a physiological function: (1) the skin near the metal particle remained intact, and (2) the metal particle did not cause skin damage or infection. In contrast to other cases, such as the rejection of needles reported by Keum et al., where the possibility of pathophysiology could not be ruled out, the rejection in this study appears to be a purely physiological response.
The rejection process likely unfolded as follows: approximately 30 years ago, an iron particle was embedded in the skin, causing a 10-mm scratch and coming to rest in the space between the skin and the tibialis anterior muscle, 7 mm from the entry site. Over time, the metal particle became enwrapped by the skin and gradually moved toward the skin surface. Eventually, a fistula formed spontaneously, allowing the metal particle to be expelled. This process, which took several decades, is difficult to replicate in animal experiments, highlighting its uniqueness and the need for further investigation.
The study concludes that the skin’s ability to reject a metal particle over such an extended period must be correlated with a unique physiological function. The exact mechanism by which the skin enwraps the metal and forms a fistula to expel it remains unclear and warrants further exploration. This case provides valuable insights into the skin’s potential to reject inorganic foreign bodies, challenging the conventional understanding of skin rejection as a response limited to organic substances.
doi.org/10.1097/CM9.0000000000000865
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