Lactotransferrin (Lf), an iron-binding glycoprotein present as a major component in the specific granules of human neutrophilic granulocytes is released in the blood during the acute phase of infection and participates in the regulation of the host-defence mechanisms. Our previous observations (Mazurier et al., 1989) showing i) that the activation by PHA of T-lymphocytes induces the appearance at the cell surface of Lf-receptors which are absent from the membrane of resting lymphocytes and ii) that Lf becomes a growth factor for the activated lymphocytes, led us to undertake a series of researches on the presence of Lf receptors at the surface of different blood cells.

Characterization of Lf receptors was performed by flow cytofluorimetry using either Lf labelled on its glycan moiety with fluorescein or purified anti-lymphocyte Lf receptor antibodies.

High affinity receptors for Lf were characterized only at the surface of human activated lymphocytes and of non-activated platelets. These two receptors possess common physicochemical properties and antigenic epitopes.

Low affinity receptors for Lf were characterized on monocytes, eosinophils and neutrophils. These receptors are immunologically different from those found on activated lymphocytes and on non-activated platelets.

Cell-lines of human lymphocyte T and megakaryocyte possess lactotransferrin receptors whose properties are similar to those found on peripheral blood cells.

The soluble form of the receptor identified in the lymphocytes T culture medium possesses a molecular mass close to that of the membrane receptor suggesting that the cytoplasmic tail of the receptor should be very short.