  The structure of the electromagnetic vertex function of spin-3/2 particles is
analyzed in a general way, for the diagonal and off-diagonal couplings, of
charged as well as neutral particles including the case of self-conjugate
(Majorana) particles. The restrictions imposed by common principles such as
electromagnetic gauge invariance and hermiticity are studied, and the
implications due to the discrete space-time symmetries or the Majorana
condition are deduced when they are applicable. In some cases certain features
of the vertex function are analogous to the known ones for the spin-1/2 or
spin-1 particles, but we find and discuss other particular properties which are
related to the spin-3/2 Rarita-Schwinger representation. For example, in the
diagonal Majorana case, the vertex function can contain a term of the form
$\gamma_\mu\gamma_5$, which resembles the axial charge radius term for Majorana
neutrinos, plus another one that resembles the vertex function for
self-conjugate spin-1 particles, but with the particularity that the two terms
may not appear independently of each other, but instead with a specific
relative coefficient. In essence this is due to the requirement that the vertex
function does not mix the genuine spin-3/2 degrees of freedom with the spurious
spin-1/2 components of the Rarita-Schwinger representation. The analogous
results for the other cases (off-diagonal and charged couplings) are discussed
as well.
