The immune response of males and females is not identical but instead has been shown to be dimorphic in its nature, with females generally demonstrating a greater overall response than males. This dimorphism extends to both the humoral and cell mediated systems and appears to be mechanistically based on the differences in type and concentration of sex steroids in males vs females. Furthermore, growth hormone and prolactin secretions which are different in males and females may also be partly responsible for the observed dimorphism. Because autoimmune disease results from a pathological perturbation of normal immune function, it follows that expression of these disease will also demonstrate a dimorphic pattern. Examples of this autoimmune dimorphism include (but are not limited to) lupus, rheumatoid arthritis and multiple sclerosis with the two former more prevalent in females than males and the latter more severe during pregnancy. To explain autoimmune dimorphism it therefore becomes necessary firstly to describe the cellular and hormonal interactions found in normal immune regulation and thereafter extrapolate these to autoimmune phenomena.