What happens if the "D" gene of chromosome #14 is missing? How does it
affect the immune system?

IgD is normally found on the surface of Immature B Cells and helps
promote B-cell growth

A complete knockout of the "d" section of Chromosome 14 may or may not
happen as a disease of humans, however, in labratory testing "IgD
knockout" mice have been produced (they have no IgD gene).  According
to the abstract referenced below, these mice are able to mount a
normal immune response to antigens and their B-cells have more surface
IgM. However they have 30-50% fewer b-cells in the Thymus and Spleen
than normal controls -- this might affect overall immune function.

see:
Nitschke L, Kosco MH, Kohler G, Lamers MC:
Immunoglobulin D-deficient mice can mount normal immune responses to
thymus-independent and -dependent antigens.
Proceedings of the National Academy of Sciences of the United States
of America 1993 Mar 1;90(5):1887-91

Reference
http://www.bioscience.org/knockout/ref/nitschk.htm