In this talk I present a series of theoretical works
regarding the $\Lambda_b \to J/\psi K^- p$ reaction from where a recent experiment by the LHCb collaboration at CERN claimed the existence of
two hidden charm pentaquarks, $P_c(4380)^+$ and
$P_c(4450)^+$.
We discuss the possible explanations of the pentaquark states found within the picture of a dynamical meson-baryon molecule previously predicted made up mostly from $\bar D^* \Sigma_c$ and $\bar D^* \Sigma^*_c$ components.
We consider the total $K^- p$ and $J/\psi p$ data including all the relevant resonances contributing to the spectra, and discuss the possible nature of both $P_c(4380)^+$ and
$P_c(4450)^+$. We also discuss several important topics, like the role of the $\Lambda(1405)$ resonance, the effect of the contact term in the reaction, the viability of reproducing the data without the $P_c(4380)^+$ and the possible quantum numbers assignment to these pentaquarks.