You asked for “step by step instructions” on summoning and I told you where to find them. The spirit you summon with those instructions doesn’t really matter because the method is adaptable.
Here are some notable all-around healing entities:
The archangel Raphael (he’s kind of number 1 in this area)
The Shem angels Chavual and Rehael
The demon Buer from the Goetia
The angel Rantka from Kingdoms of Flame
The Mercurial spirits of the House of Kel
The Greek gods Asclepius and Apollo
These spirits do not specialize in immune disorders but they are great healers for all ailments.
As for mantras, here is one for physical healing channeled from Raphael:
And here’s another channeled from the spirit Fastos:
And: