Infiltration

In pathology, the term “infiltration” refers to the spread of one type of cell or substance into an area of tissue where it is not normally found. This can happen with normal immune system cells that move into tissues during inflammation, or with abnormal cells, such as cancer cells, that grow into surrounding structures. Pathologists use the term “infiltration” to describe what they observe under the microscope and to explain how far a disease process has progressed into surrounding tissue.

What types of infiltration can occur?

• Inflammatory cell infiltration: When the body responds to injury, infection, or irritation, white blood cells (such as lymphocytes, neutrophils, or plasma cells) can move into tissues. This is called an inflammatory infiltrate. It is a normal part of the immune response, but can also be seen in chronic inflammatory diseases and autoimmune conditions.

• Tumor or cancer cell infiltration: Cancer cells can grow beyond their original site and infiltrate into nearby tissues. This can also be described as invasion. For example, cancer that starts in the lining of the colon may infiltrate into deeper layers of the bowel wall. Describing infiltration helps doctors understand how aggressive the tumor is and whether it has spread.

• Other types of infiltration: In some diseases, other substances can infiltrate tissues. For example, fatty infiltration describes the spread of fat cells into organs such as the liver or heart, and ainfiltration refers to deposits of abnormal proteins seen in a disease called amyloidosis.

Why is infiltration important?

The presence and extent of infiltration can provide important clues about your diagnosis and prognosis. For inflammatory conditions, infiltration helps identify what type of immune response is taking place. For cancer, infiltration is often used to determine the stage of the disease. The more deeply a tumor infiltrates into nearby tissue, the more advanced the stage may be, which can impact treatment decisions.

How will infiltration be described in my report?

Pathologists often include details about infiltration in their microscopic description or diagnosis.

Some examples include:

• “Dense lymphocytic infiltration” describes many lymphocytes present in a tissue sample.

• “Tumor infiltration into muscle” describes cancer cells spreading into nearby muscle tissue.

• “Fatty infiltration of the liver” describes fat cells moving into liver tissue.

The exact wording depends on the type of cells or substances involved and the organ being examined.

Questions to ask your doctor

• What type of infiltration is mentioned in my report?

• Does this infiltration represent inflammation, cancer, or another condition?

• How does this finding affect the stage or severity of my disease?

• Does the degree of infiltration change my treatment options or follow-up plan?