The IMMSIM simulator was conceived in late 80’s in the collaboration between the immunologist F.Celada and the astrophysicist P.Seiden; the latter wrote the first code using the IBM’s APL language. The APL’s handicap was its limited diffusion among modellers. Thus the physicist R. Puzone, starting in 1999 developed a new version of IMMSIM in C language (IMMSIM-C), and aligned it with Seiden’s. When Seiden died prematurely in 2001, IMMSIM-C became the only official simulator and is maintained at Puzone’s workplace IST. The code, all the interfaces and also the former IMMSIM3 APL and its manuals are available at www.immsim.org.