Immunohistochemistry uses the principle of combining antigens and antibodies to determine if a tissue is carrying an antigen or not. Think of it like a key and a lock, which can only be expressed when they match and show a positive expression underneath the immunohistochemistry. Positive expression can usually be seen as brownish-yellow positivity, where cells stained brownish-yellow indicate that the immunohistochemistry is positive. Positive has many meanings: 1. Simple positive: for example, cytokeratin, an immunohistochemical reagent that marks epithelial origin. If the epithelial keratin is positive, it means that the pathological specimen of the lesion to be tested originates from the epithelium; 2, the site of positive cells: cells include cell membrane, cytoplasm and nucleus, whether the positivity occurs in the cell membrane or in the cytoplasm or in the nucleus has a completely different meaning, so it also depends on the specific site of positivity; 3, positive cells: for example, CD34 is an immunohistochemistry expressed on blood vessels, if the tissue above If the tissue above the blood vessel expresses CD34, it is normal because the blood vessel will be positive. However, if it occurs on top of the tissue that is not vascular and also occurs brownish-yellow positive expression, it may suggest the possibility of having vascular tumor. So it depends on the specific parts where the positive immunohistochemistry occurs; 4. Look at the percentage: for example, with ER and PR, the positivity will be expressed with + to ++++. + indicates that the degree of positivity is relatively small, ++ indicates moderate positivity, and if ++++ indicates that the degree of positivity is relatively strong. In addition, there is the expression of Ki67 proliferation index for immunohistochemistry. A higher percentage, such as 90%, indicates a very high proliferative activity of the lesion; if it is only 10% or less, it indicates a relatively low proliferative activity.