Estradiol (E2) occurs in high concentrations in non-pregnant as well as pregnant females and is responsible for a majority of the `classic' estrogenic effects in reproductive and non-reproductive tissues. Estrogen receptors (ERs) are expressed in various lymphoid tissue cells as well as in lymphocytes, macrophages, and dendritic cells (DCs) (Kovats et al., 2010). There are two subtypes of the receptor for estrogens, ERα and ERβ, that exhibit differential expression in subsets of immune cells, with ERα being highly expressed in T cells and ERβ being upregulated in B cells (Phiel et al., 2005). The differential effects of estrogens on parameters of immune function may reflect not only the concentration of estrogen (i.e., whether physiological or pregnancy doses are used), but the density, distribution, and type of ERs in immune cells.