Natural resistance to bacterial infection is a protective mechanism to all animals and man. Availability of the element iron is key to successful functioning of our antibacterial systems, according to the authors. It depends entirely upon an extremely low level of free iron in normal tissue fluids and well-oxygenated tissues where electrochemical oxidation-reduction occurs and tissue acidity (pH) controls binding of iron to transferrin and lactoferrin.

Bacterial virulence, it turns out, is greatly increased by freely available iron, such as that in fully-saturated transferrin or free haemoglobin. In tissue injury, lack of blood flow causes tissues to be more acidic, making iron in transferrin freely available and abolish the bactericidal properties of tissue fluids. This allows us to become more prone to infections.

Reference:
FEMS Immunol Med Microbiol. 2005 Mar 1;43(3):325-30,