Dr. Nélissen is a Clinical Head of Plastic Surgery Department (Pr Nizet)  at the University Hospital of Liège (CHU Liège). He was head of department for more than 15 years at the CHC Liège. He owns a private clinic in the center of Liège (Clinique du Mont Saint Martin). 

He is vice president of the Royal Belgian Society for Plastic Surgery (RBSPS) and vice president of the Belgian Society for Private Clinics (BSPC).

He is a member of the French-speaking accreditation committee for plastic surgery.

He is one of the inventors of a fully automated lipofilling technique (ADIMATE).

He is a consultant for Microaire.

His areas of specialization are breast surgery, both reconstructive and cosmetic, microsurgery, and lipofilling.