Susceptivity is defined as the state of being easily affected by something external. The word itself has no direct antonyms. However, we can consider different word combinations that denote the opposite of susceptivity. Words like resistance, insusceptibility, immune, and immunity can convey the idea of being unresponsive or unreactive to external influences. They imply a state of being difficult to influence or affect. Similarly, words like imperviousness, impermeability, and impenetrability convey the idea of being impenetrable or non-penetrable. These words connote a state of being impervious or invulnerable to external influences. While there may not be a single-word antonym for "susceptivity," the idea of resistance or invulnerability can provide an interesting contrast.