Final answer:
The antonym of 'susceptible' is 'immune' or 'resistant', referring to someone or something that isn't easily affected or can withstand certain influences.
Step-by-step explanation:
The antonym of susceptible, which means easily influenced or likely to be affected by something, would be immune or resistant.
These terms describe someone or something that is not easily affected or is able to withstand particular influences or conditions. For instance, if someone is susceptible to colds, they catch them easily. Conversely, a person who is resistant to colds would not catch them as easily, showing a degree of immunity to the virus causing the cold.