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Bactericidal Activity of Human Milk Complement System

Contact Person: Michael O. Ogundele (mogundel@yahoo.com)


There is a conclusive evidence that breastfeeding protects the infant against a wide range of infectious and other diseases. It also favourably affects the morbidity of children, particularly in the developing countries (1-3).

Efforts have been directed in the past few years to identify various immune-active substances in the human breast-milk (HBM) which account for the observed protective effects of breastfeeding. These include specific anti-viral antibodies, anti-bacterial antibodies, non-specific IgG, IgA, IgM, lactoferrin, lysozyme, glycoconjugates, oligosaccarides, products of milk lipids, different cytokines, lymphocytes, polymorphonuclear leucocytes, macrophages, and complement (C) proteins (4-11,16).

The complement system in the blood was first discovered by Jules Bordet in 1899 as a heat labile serum component that augments the ability of antibodies to eliminate pathogenic bacteria from the site of infection. The complement is a complex system in the normal serum consisting of at least 20 different proteins mostly in an inactive state,which can be sequentially activated in a multi-step cascade reaction either by the classical or alternative pathways.

The classical pathway is activated mainly by antigen-antibody complexes (IgG or IgM mostly) starting with C1q, C1r, C1s, C4 and C2, and eventually leading to the activation of C3 by cleavage into C3a and C3b (12). The alternativw pathway (APC) utilizes active sites (such as are present on zymosan, yeast, cobra venom, many gram-negative bacteria, sheep erythrocytes and human cells deficient in the expression of membrane regulatory molecules) in the presence of properdin, serum factors B and D, to activate C3. This step unifies the two pathways and proceeds uniformly thereafter to the formation of C5b-9, the membrane attack complexes (MAC), capable of inserting into biological membranes and producing cell lysis and death. APC activating surfaces are characterized by possession of structures which restrict access to inhibitory factor H to deposited C3b, thereby amplifying the formation of further C3b in the presence of factor B (13).

The potential role of HBM C in eliminating common gastrointestinal pathogens in humans have not been well studied. The C system might constitute the most important bactericidal HBM component. Others components are mainly bacteriostatic, apart from the phagocytic leucocytes, which in turn depend on the C system for efficient bacteriolysis. The bacteriocidal effect of bovine milk colostrum and specific antibodies have been suggested to account for the favourable influence of breastfeeding on gut colonization (14). The opsonic effect of HBM cells for pathogens was found to be mainly due to its IgG content. The HBM C was found to be required only for inducing oxidative burst in the phagocyting cells, which is probably necessary for effective intracellular killing of the pathogens (15,17). Another study found neither antibacterial nor antiviral activity in various fresh human milk samples after one hour of incubation at 37C, until partial digestion in the stomach led to formation of anti-microbial free fatty acids and mono-glycerides (16). The present studies have however confirmed, using appropriate laboratory techniques, the presence of C-mediated bactericidal activities in fresh HBM identified the ability of bacteria to activate the C components of the HBM. It probably points to the physiological significance of the HBM C.

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Ogundele, M.O.; (1998). Bactericidal Activity of Human Milk Complement System. Presented at INABIS '98 - 5th Internet World Congress on Biomedical Sciences at McMaster University, Canada, Dec 7-16th. Available at URL http://www.mcmaster.ca/inabis98/immunology/ogundele0172/index.html
© 1998 Author(s) Hold Copyright