Complex sentence: Diagnosis of TA-GvHD can be suspected from a biopsy of the affected skin, and established from HLA analysis of the circulating lymphocytes, which shows that they have different HLA phenotypes from the tissue cells of the host.
Split into simpler sentences: Diagnosis of TA-GvHD can be suspected from a biopsy of the affected skin, and established by HLA analysis of the circulating lymphocytes. This testing can show that the circulating lymphocytes have different HLA phenotypes from the tissue cells of the host.