A plasma protein which specifically binds retinol (the alcohol form of vitamin A) and prevents it from being excreted. In vitamin A sufficiency, all the RBP is bound to retinol, in a 1:1 complex, but in vitamin A deficiency the protein may become partially desaturated, so that the ratio of protein to retinol then exceeds 1:1 and this can be measured as an index of vitamin A status.