def add_hp_label(merged_annotations_column, label_type):
    """Adds prefix to annotation labels that identify the annotation as
    belonging to the provided label_type (e.g. 'h@' for host proteins).

    Parameters
    ----------
    merged_annotations_column : array-like (pandas Series))
        An array containing sets of annotations that need to be labeled.
        e.g.
            0       {GO:0010008, GO:0070062, IPR036865, GO:0048471...
            1       {GO:0006351, GO:0070062, GO:0007623, GO:004851...
            2       {GO:0019888, GO:0006470, GO:0001754, GO:009024...
    label_type : str
        The prefix to be appended (without the "@" separator).

    Returns
    -------
    labeled_annotations : array-like (pandas Series)
        A new pandas Series where all annotations have received a prefix.
    """
    labeled_annotations = merged_annotations_column.map(
        lambda x: set([label_type + '@' + i for i in x]))
    return labeled_annotations