This article is from the Childhood Vaccinations FAQ, by Lynn Gazis-Sax firstname.lastname@example.org with numerous contributions by others.
Most of the risk in the DTP vaccine comes from its pertussis
component; the diptheria and tetanus vaccines are quite safe.
Reactions to the diptheria vaccine are quite rare. Most reactions are
local, limited to swelling at the injection site. The same is true of
the tetanus shot. "Severe local reactions can occur if too many shots
are received; this phenomenon was frequently seen in military recruits
who received unneeded immunizations." (Pantell, Fries, and Vickery)