The ability to interact with cell membranes suggests possible biological roles for free light chain.

During antibody synthesis, immunoglobulin light chains are produced in excess of heavy chains and, as a consequence, can be secreted by plasma cells as free light chains (FLC). Thus, FLC were considered to be a by-product of immunoglobulin synthesis, lacking any biological function or relevance. However, mounting evidence suggests that FLC are bioactive molecules. For example, FLC can induce antigen specific type I hypersensitivity and inhibit viral replication in encephalomyocarditis infected mice. We have recently shown that FLC can associate with the outer membrane of certain plasma cells via interaction with saturated phosphocholine lipids such as sphingomyelin. As these lipids are highly abundant in mammalian cell membranes, we set out to determine whether FLCs can bind to membranes from a variety of cell types. We found that FLCs bind to the plasma membrane of cells from a wide range of lineages. Interestingly, the highest level of binding was to monocytes. As these cells are professional antigen presenting cells, we postulate that membrane-associated FLCs may provide a novel mechanism of antigen uptake by these cells.