DTH skin tests in MM receiving Id vaccines. Two representative patients with a positive (A) and a negative (B) reaction are shown. Unconjugated autologous Id alone (solid arrow) or an equivalent amount of polyclonal human Ig as a control (open arrow) were injected intradermally into the opposite forearms. Histologic assessments (original magnification × 40) of skin biopsies taken at sites of autologous Id challenge are shown in (C) and (D).