EFFECT OF 50% ETHANOLIC EXTRACT OF PEGAGAN HERB (Centella asiatica (L.) Urban) ON CELL PROLIFERATION OF LYMPHOCYTES IN Balb/c MALE MICE INDUCED BY HEPATITIS B VACCINE PENGARUH EKSTRAK ETANOLIK 50% HERBA PEGAGAN (Centella asiatica (L.) Urban) TERHADAP PENINGKATAN PROLIFERASI SEL LIMFOSIT MENCIT JANTAN GALUR Balb/c YANG DIINDUKSI VAKSIN HEPATITIS B