by Jason Wasserman MD PhD FRCPC
March 24, 2022
Lymphoid aggregate is a general term used to describe a group of lymphoid (immune) cells such as lymphocytes, plasma cells, and histiocytes. A lymphoid aggregate may be found anywhere in the body but it is more commonly found in the skin, throat, and digestive tract.
A lymph node is a special type of organ that is made up almost entirely of lymphoid cells. However, the cells found in a lymph node may be described as a lymphoid aggregate if only a small sample is removed and the normal organization of the lymph node cannot be seen.
A lymphoid aggregate seen under the microscope
A lymphoid aggregate may be described as being prominent if the size of the group of cells is larger than normal for that area of the body. Most prominent lymphoid aggregates are made up of non-cancerous cells. However, in some instances, your pathologist may perform additional tests such as immunohistochemistry and flow cytometry to confirm that the cells are not a type of cancer called lymphoma.
A reactive lymphoid aggregate is a group of non-cancerous lymphoid cells (immune cells) that are reacting or responding to conditions and signals in their environment. These signals and conditions may include inflammation, infection, physical stress (such as trauma), radiation, or medications. Reactive lymphoid aggregates are very common and they may be seen anywhere in the body.
Lymphoid aggregates are a normal finding in some areas of the body, such as the stomach, small bowel, and colon. The immune cells in these normal lymphoid aggregates help protect the body from micro-organisms, such as bacteria that may enter the tissue from the external environment.
Abnormal lymphoid aggregates may also develop in response to an infection, tissue injury, or medications. For example, an infection of the stomach by the bacteria helicobacter pylori often results in large numbers of lymphoid aggregates in the tissue that lines the inside of the stomach. In this instance, lymphoid aggregates provide pathologists with a clue that prompts them to look for bacteria or to perform additional tests such as immunohistochemistry and special stains.
Lymphoma is a type of cancer made up of immune cells. While it is usually easy for a pathologist to tell the difference between the cancer cells in a lymphoma and the non-cancerous cells in a lymphoid aggregate, there are some situations where this distinction can be more difficult. For example, it can be very difficult to tell the difference between some types of lymphoma and a non-cancerous lymphoid aggregate when a very small tissue sample is removed and examined under a microscope by a pathologist. In this instance, the pathologist may perform tests, such as immunohistochemistry and flow cytometry, to look for additional clues such as markers that may be expressed by cancer cells but are not expressed by normal, healthy cells. If the amount of tissue is too small to make a diagnosis, the pathologist may recommend performing another biopsy and removing a larger tissue sample.