Immune cells that 'remember' past encounters with foreign molecules have been found in human umbilical cord blood, even though the fetal environment is presumed to be sterile.

Human T cells begin to form when the fetus is about ten weeks old, but the cells were thought to remain unexposed to foreign antigens until after birth. However, Richard Lo-Man of the Pasteur Institute in Paris and his team found a subset of fetal T cells known as memory cells, which remember foreign molecules and respond more rapidly to them during a second exposure. These memory cells make up 1–6% of CD4, or helper, T cells in healthy neonatal cord blood. The identities of the antigens they react with are unknown.

Sci. Transl. Med. 6, 238ra72 (2014)