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I am sorry to hear you are worried about Papa. My name is Rebecca; I am a veterinarian and will do my best to help.
Do you have a copy of the report? A "reactive" lymph node can be normal. Lymph nodes are supposed to react to things like infection. However, if the node appears to have really abnormal cells, and may be be cancerous, that is more of a worry. Seeing your report might help me to know why your vet is recommending the additional "immunohistochemistry staining.
This extra staining is done on a sample that is suspectd to be cancerous, and helps to tell what kind of cancer we are dealing with, and how best to treat. If this does turn out to be cancer of the lymph nodes, or lymphosarcoma, that is serious and possibly deadly. This type of cancer often responds to chemotherapy, which can give a remission of many months.
Let me know what else I can answer. Kindly consider rating my answer (always appreciated!); you can still get back to me with questions and updates after rating. Rebecca
How is Papa? Can I help with or answer anything else?