Role and significance of the complement system in mucosal immunity: particular reference to the human breast milk complement.

The complement system plays an important role in a host's defence mechanisms, such as in immune bacteriolysis, neutralization of viruses, immune adherence, immunoconglutination and in enhancement of phagocytosis. The possible role of this important biological system in biological fluids on the mucosal surfaces, including breast milk, has however been largely neglected. Its contribution to the 'common' mucosal immunity is still enigmatic and largely speculative. Assessment of the complement system in human breast milk, which has so far largely been limited to different assays of the individual component proteins, is reviewed. A brief review of the classical and the alternative pathways of complement activation is presented. The potential physiological roles of various complement components and their activation fragments in human milk in particular, and other mucosal surfaces in general, are also presented. It was concluded that the complement system might play a complementary role to other immunological and non-immunological protective mechanisms on the mucosal surfaces.