Mannan-binding lectin (MBL) was first discovered as a plasma opsonin for baker's yeast and was independently characterized biochemically. It belongs to the small subfamily of collectins: C-type lectins possessing a collagen-like domain. MBL is synthesized by the liver and secreted into the bloodstream. It is believed to be an important component of innate immunity, acting as an ante-antibody and/or as a disease modifier. It is thought to influence disorders as diverse as meningococcal disease, rheumatoid arthritis, cystic fibrosis and recurrent miscarriage. Lack of MBL may be most relevant in the context of a co-existing secondary immune deficiency. Replacement therapy, first carried out 30 years ago with unfractionated plasma, appears promising. The development of a recombinant product should permit the extension of MBL therapy to randomized clinical trials of sufficient size to provide clear evidence about the physiological significance of this intriguing glycoprotein.