Infectious disease and inflammation cytology (Proceedings)


Infectious disease and inflammation cytology (Proceedings)

Inflammatory lesions may present as visible or palpable lumps, bumps, plaques, ulcers, accumulations of excessive fluid, or as abnormalities in organs that are visualized using imaging techniques. Cytologic examination of these types of lesions may be definitively diagnostic in many cases, or contribute to a diagnosis in other cases. When certain types of infectious agents are present, cytologic examination may be particularly rewarding.

When examining a cytologic sample, an initial determination as to whether the sample consists entirely of inflammatory cells should then lead to an assessment of what type(s) of inflammation, and subsequently to a search for organisms, if appropriate. Relative percentages of the different types of inflammatory cells should be assessed and the process classified in order to figure out what the underlying etiology might be.

Cell types encountered in inflammatory lesions


Commonly present due to hemorrhage at the time of collection.

The presence of erythrophagocytosis (i.e. macrophages that have eaten erythrocytes), hemosiderin or hematoidin (golden crystalline structures) suggests there has been hemorrhage prior to the time you stuck a needle into the lesion.


Commonly seen in low numbers, often due to blood contamination. Increased numbers indicates inflammation. The morphology of neutrophils should be examined. In non-septic inflammation (i.e. no bacteria present), neutrophils are usually well preserved and non-degenerate. Degeneration of neutrophils is indicated by swelling of the nucleus, with the nucleus appearing lighter staining and smudged-kind of like it is swollen. The cells may also lyse. When degenerate neutrophils are seen, you should look carefully for microorganisms – especially bacteria. When neutrophils are in fluid for awhile, they can undergo aging change, which appears as hypersegmentation, with just a thin strand of nucleus connecting the lobes of the nucleus. Eventually these cells become pyknotic and appear as shrunken cells with small blobs of purple chromatin.


Normal lymphocytes look like they do in blood, i.e. small cells (smaller than a neutrophil) with a condensed chromatin pattern and small amounts of blue cytoplasm. Lymphoblasts are large cells with visible nucleoli. With antigenic stimulation, mature cells may transform to larger cells with more abundant, very blue cytoplasm. Antibody-producing B cells often differentiate into plasma cells.

Plasma cells

Plasma cells are present in chronic inflammatory lesions and are an indication of antigenic stimulation. They have an eccentric nucleus that is the same size as a lymphocyte's, but the chromatin is typically denser. The cytoplasm is usually more abundant, deeply basophilic, and contains a perinuclear clear area (Golgi apparatus).


These cells are the activated, tissue form of blood monocytes. These are the cleaning-up cells. Increased numbers seen with chronic inflammation and certain types of infections, especially some types of fungi, protozoa, and rickettsia. It is unusual to see bacteria other than mycobacteria within phagocytic vacuoles of macrophages. Macrophages may contain erythrocytes, red blood cell pigments, neutrophils, cellular debris, lipid, and foreign debris.

Macrophages have round, oval, to indented nuclei that sometimes contain nucleoli. The abundant lightly basophilic cytoplasm is frequently vacuolated. Macrophages can become multinucleated, sometimes referred to as "giant cells."


Eosinophils in tissues have a similar appearance to blood eosinophils. The normal number varies according to site and species. Increased numbers typically associated with allergy or parasitism. A high number of eosinophils is frequently seen in mast cell tumors.

Mast cells

Mast cells may be present in certain types of inflammation, particularly when it has a hypersensitivity component. These are round cells with round nuclei and abundant cytoplasm filled with numerous dark purple granules.