The axillary lymph node group does not include the sentinel lymph node and Wikipedia says it is a “hypothetical ” lymph node which is the first to be drained by a tumour. It also has a significant role in diagnosis of breast cancer. [Thieme atlas of anatomy][2]

My questions

1) Is sentinel lymph node actually present? If yes why is it not included in axillary lymph node group?

2)How do they affect metastasise?

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    You seem to not understand what the word "hypothetical" means. It means the node doesn't physically exist. A sentinel lymph node can be any lymph node in the body. It's a term used to refer to the first lymph node that receives drainage from the area of a tumor, wherever that tumor might be. – Carey Gregory Dec 29 '19 at 16:25