The ornamental design for a head and neck immobilizer, as shown and described.
FIG. 1 is a top plan view showing my new design;
FIG. 2 is a bottom plan view of my new design;
FIG. 3 is a left, top perspective view of my new design; and,
FIG. 4 is a right, bottom perspective view of my new design.
The broken lines shown in FIGS. 1-4 represent unclaimed subject matter. The left side, right side, front and rear views do not form part of the claimed design.