MHC2 mounts peptide on endosome (breaks antigen in cytosol and combines with premade MHCII complex inside endosome and shoots it out).
MHC1 mounts peptide on ER (breaks antigen in cytosol and uptake inside the ER where forming MHC1 and antigen particle assemble together and shoots out via ER-Golgi).
Simplified version: invariant chain serves as a placeholder that sits in the antigen binding site of MHC-II before it reaches the APC endosomes,
Important because it stops MHC-II from activating against endogenous proteins prior to its arrival to the endosome.
Inside the endosome the Invariant chain is degraded by acid-activated proteases, and only then the exogenous antigens are loaded on MHC II.
His calcium is in the reference range due to bone resorption. We also see a decrease in phosphorus which hints at an increase in PTH.