Uncircumcised men have soft foreskin around the head of the penis containing many cells that are easily infected by HIV, according to epidemiologist Robert C. Bailey of the University of Illinois at Chicago. These cells, called Langerhans' cells, "tend to be close to the surface," he says. Once infected, he adds, "they carry the virus deeper," to the immune system T cells that HIV most commonly infects.
Circumcision removes the foreskin. During healing after the procedure, the protein keratin toughens the skin of the penis, which reduces HIV penetration there, Bailey says.
The trial examining the question in South Africa was called off three months early because the benefits of circumcision were so clear.