Mammalian lectins in activation and clearance mechanisms involving the complement system.

CRP and the mammalian lectins containing collagen-like structure (collectins) can clearly participate in a variety of antibody-independent recognition and clearance mechanisms which result in the neutralisation and elimination of pathogenic organisms (Fig. 2). Only CRP and MBP appear to have the capacity to activate complement, and while all the collectins (MBP, SP-A, SP-D and conglutinin) can utilise the C1q receptor, only conglutinin shows specificity for iC3b. The C-type lectin domains in the globular heads of the collectins have the capacity to recognise a range of specific carbohydrate structures which are found on the surfaces of pathogens commonly associated with infections in blood, lung and amniotic fluids. The proposed presentation of the carbohydrate-collectin complex to C1q receptors, via the collagen-like regions in the collectins is an attractive hypothesis for the triggering of protective mechanisms and there are already a number of publications which support this view. However, the precise manner by which these collagen 'stalks' interact with the C1q receptor and the complete characterisation of the receptor on a variety of different cell types remains to be elucidated. Similarly, whether or not there is a role for any of these lectins in autoimmunity/immunopathology has not, as yet, been addressed.