An abscess is a large collection of immune cells inside tissue. Most abscesses are made up of specialized immune cells called neutrophils although other types of immune cells including lymphocytes can also be seen in smaller abscesses. Pus is a term used to describe the soft material found on the inside of an abscess. Abscesses can be caused by infection (in particular bacterial infections), trauma, inflammatory conditions, or obstruction of an organ or tissue. An abscess can also be seen in the tissue surrounding a tumour. In this situation, the abscess may represent the immune system’s attempt to contain the growing tumour or may result from the spillage of material from the tumour into the surrounding tissue.