Periapical granuloma is mainly composed of:
Macrophages
Lymphocytes
Plasma cells
All of the above
The apical periodontal cyst is histologically identical with the periapical granuloma, from which it is actually derived, except for the presence of the epithelium-lined lumen. 
The lesion is predominantly composed of plasma cells that are mixed with macrophages and lymphocytes with endothelial cells and fibroblasts.
Periapical granuloma is not a true granuloma due to the fact that it does not contain granulomatous inflammation; however, periapical granuloma is a common term used.
Reference: Shafer’s Textbook of ORAL PATHOLOGY Eighth Edition page no 78