Microscopically, metastatic adenocarcinoma (composed of malignant gland-like structures) is seen in a lymph node here. It is common for carcinomas to metastasize to lymph nodes, traveling via lymphatic channels from the site of origin. The first nodes involved are usually those receiving lymphatic drainage from the site of the primary neoplasm. Hence, if this is a left axillary lymph node, then the primary site of the cancer is likely in the left breast.