EMC immunity is the ability for an equipment to properly operate in its electromagnetic environment by limiting the reception of electromagnetic energy that may cause physical damages. Electrostatic Discharge called ESD fall into this category. ESD can be either conducted under the form of a Transient Voltage, but it can be also radiated due to its fast-raising time, generating harmonics, powerful enough to generate erratic behavior of your application by coupling with other signals. It means that ESD is a threat against which your printed Circuit Boards must be protected.