The definition of interacting higher spin theories is notoriously challenging, especially in the case of massless fields in flat spacetime where well-known no-go theorems strongly restrict possible constructions. Following the results of a series of papers[1-3], we discuss the effective actions obtained by integrating out a scalar and a fermion field coupled to external higher spin source fields.The theories defined according to this procedure turn out to be endowed with an L_infinity structure which is known to be present in a large class of consistent theories. The corresponding higher spin symmetry also allows for a geometrical intepretation which generalizes well-known structures of Yang-Mills and gravity theories. [1] L.Bonora, M.Cvitan, P.Dominis Prester, S.Giaccari, M.Paulišić and T.Štemberga, Worldline quantization of field theory, effective actions and $L_\infty$ structure, JHEP 1804, 095 (2018) doi:10.1007/JHEP04(2018)095 [arXiv:1802.02968 [hep-th]]. [2] L.Bonora, M.Cvitan, P.Dominis Prester, S.Giaccari and T.Štemberga, One-loop effective actions and higher spins. Part II, JHEP 1801, 080 (2018) doi:10.1007/JHEP01(2018)080 [arXiv:1709.01738 [hep-th]]. [3] L.Bonora, M.Cvitan, P.Dominis Prester, S.Giaccari, B.Lima de Souza and T.Štemberga, One-loop effective actions and higher spins, JHEP 612, 084 (2016) doi:10.1007/JHEP12(2016)084 [arXiv:1609.02088 [hep-th]].