Immune response more complicated than thought

By Piriya Mahendra, medwireNews Reporter

Researchers have found that the orientation and concentration of antibodies could determine the control of bacterial infection.

It has been known since the 1960s that antibodies bind to the bacterial surface via the fragment crystallizable (Fc) region, explain Pontus Nordenfelt (Harvard Medical School, Boston, Massachusetts, USA) and team in the Journal of Experimental Medicine.

However, they have, for the first time, shown that antibodies bind to the streptococcal bacterial surface via the fragment antigen-binding (Fab) region in serious and life-threatening infections such as sepsis and necrotizing fasciitis.

"This information is important and fundamental to improving our understanding of streptococcal infections, but our results also show that the principle described could apply to many different types of bacteria," commented Nordenfelt in a press statement.

The team found that the concentration of antibodies in the local environment of the body controls how the antibodies bind to bacterial surfaces.

In the throat, where the concentration of antibodies is low, the antibodies bind via Fc and are protected against phagocytic killing .

However, in the blood, where immunoglobulin (Ig)G concentration is high, the antibodies bind to the bacteria via Fab, allowing opsonization and bacterial killing, the researchers explain.

"IgG-poor environments represent the natural habitat for IgGFc-binding bacteria. And IgGFc-binding proteins may have evolved to execute their function in such environments."

They say the lack of bacterial protection against phagocytosis in plasma explains why severe infections are rare in comparison to more common and often milder cases of streptococcal throat and skin infections.

The findings come from an analysis of samples from a patient with invasive Streptococcus pyogenes infection.

Co-author Lars Björck (Lund University, Sweden) remarked: "It is fantastic to have been involved in moving a major step closer to understanding the biological and medical importance of these proteins together with talented young colleagues."

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