Human beta-defensin 3

Where it comes from

Made in

Keratinocytes, tonsil, airway, gingiva and placenta, induced by injury and infection

Sent out by

Wounding, bacteria and interferon-gamma switch it on

Acts on

• Staphylococcus aureus including MRSA
• Streptococci
• Candida
• HIV

What it does

• Kills Gram-positive bacteria at physiological salt, which most defensins cannot
• Blocks lipid II, the cell-wall building block, as vancomycin does
• Binds the melanocortin receptor MC1R, which in dogs sets coat colour

In medicine

• No medicine; copy number of its gene pair varies between people and is studied in psoriasis and Crohn’s disease ⁴
• It is the human defensin that keeps killing Staphylococcus aureus at the salt of body fluid, which is why antibacterial work starts here ¹

Beyond the body’s own use

• It blocks lipid II, the cell-wall building block vancomycin binds, so it is studied as a template for antibiotics against resistant Gram-positive bacteria ²
• Engineered versions are tested on skin and implant infections in animals; none has reached a human trial
• The dog version of this gene sets black coat colour through the melanocortin receptor, the clearest evidence that a defensin can double as a pigment signal ³