Can someone please explain what does 'fixing' of an antigen presenting cell mean?
According to this paper, fixation of an antigen presenting cell refers to the exposure of antigen presenting cells to paraformaldehyde prior to immunostaining. This treatment is used to aggregate membrane proteins and strengthen the intercellular transient protein-protein interaction.
Barisas BG, Wade WF, Jovin TM, Arndt-Jovin D, Roess DA. Dynamics of molecules involved in antigen presentation: effects of fixation. Mol Immunol. 1999 Aug;36(11-12):701-8. doi: 10.1016/s0161-5890(99)00091-7. PMID: 10593509.