‘Abdu’l-Baha’s younger brother, Mirza Mihdi, known as the Purest Branch, was about 12 years old when he was reunited with the Holy Family in Baghdad in 1860
Mirza Mihdi was taken to Baghdad to join the Family in the year AH 1276 (circa AD 1860). It was in that city that this pure and holy youth, noted for his meekness, came in touch with the Divine Spirit and was magnetized by the energizing forces of Bahá'u'lláh's Revelation. From that time on, he devoted every moment of his life to the service of his heavenly Father. He was Bahá'u'lláh's companion in Baghdad, Adrianople and 'Akká, and served Him as an amanuensis [It must be noted that although Mirza Aqa Jan was Bahá'u'lláh's amanuensis, there were also others who were engaged in this task from time to time] towards the end of his life, leaving to posterity some Tablets in his handwriting. The last ten years of his life were filled with the hardship and suffering inflicted on Bahá'u'lláh and His companions in the course of the three successive banishments from Baghdad to 'Akká.
The Purest Branch resembled 'Abdu'l-Bahá, and throughout his short and eventful life he displayed the same spiritual qualities which distinguished his illustrious Brother. The believers loved and venerated him as they did 'Abdu'l-Bahá. (Adib Taherzadeh, 'The Revelation of Baha'u'llah')