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Fleiss ''et al''. (1998) have listed numerous immunological functions of the foreskin that help to protect the human body against infection. The sub-preputial moisture contains cathepsin B, chymotrypsin, neutrophil elastase, cytokines, and lysozyme, which has the capacity to desttroy the cell walls of bacteria. The preputial muscles keep the tip of the foreskin closed to keep out pathogens.<ref name="fleiss-hodges-vanhowe1998"/>

Although claims have been made that the presence of the foreskin increases the risk of infection with [[Circumcision and Sexually Transmitted Diseases (STD)| human immune deficiency virus]] (HIV), that is not correct. The claims are based on the reported findings of three randomized clinical trials (RCTs) that were carried out more than a decade ago in Africa. Boyle & Hill (2011) have shown these RCTs to be have significant methodological flaws and statistical errors that render their claims invalid.<ref name="boyle-hill2011">{{REFjournal