Cytology of inflammation and infectious diseases (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.

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).


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.

Classification of inflammation

Inflammation is classified by the type of cells that predominate. Sometimes inflammatory processes are classified as mixed if there are roughly equivalent numbers of different types of inflammatory cells. The degree of inflammation (mild, moderate or marked) should be noted.

Suppurative (or purulent or neutrophilic) inflammation

     • Usually > 90% neutrophils.

     • Septic, suppurative inflammation is the term used if bacteria are seen.

     • You can't assume that something is non-septic just because bacteria are not seen cytologically. Culture is used to make the determination.

Mixed inflammation

     • More than one type of inflammatory cell is present.

     • Usually comprised predominantly of neutrophils with 10-50% macrophages and/or lymphocytes.

     • Commonly seen with foreign body reaction or fungal infection.

     • May be an indication of chronicity.

Mononuclear or granulomatous inflammation

     • The majority of cells are macrophages and giant cells.

     • Typical inflammatory process associated with mycobacterial agents and steatitis.

Eosinophilic inflammation

     • If >15% of the cells are eosinophils an inflammatory reaction is usually considered to have a significant eosinophilic component.

     • Commonly associated with allergic reactions or parasites.

In differentiating between a neoplastic and an inflammatory lesion, remember that neoplasms are often associated with some inflammation either because they have necrotic areas, or they impinge on surrounding tissues or they cause ulceration and surface inflammation (e.g. squamous cell carcinoma). Differentiation can also be made difficult by the fact that inflammation causes reactivity in surrounding cells (also known as dysplasia). Reactive epithelial cells can be mistaken for carcinoma and reactive mesenchymal cells can be mistaken for sarcoma.

Identification of organisms


When most bacteria are stained with the common stains such as DiffkQuik or Wright's stain, they are purple or dark blue. An exception are the Mycobacteria spp. which exclude stain, so they appear to be negatively-stained (clear).

Bacterial shapes include cocci, rods (small and large) and filamentous bacteria that have the appearance of beads-on-a-string. Large rods that have spores are most likely Clostridium spp.

When pathogenic bacteria are causing a lesion, they should be seen within neutrophils (or occasionally macrophages). If there are only extracellular bacteria, especially if they are a mixed population and/or typical of normal flora, they are most likely contaminants of the sample.

Fungi and yeast

Fungal infections usually cause a pyogranulomatous inflammatory reaction, with eosinophils sometimes also present. There are a number of fungi and yeast that might be seen in cytologic samples. Most yeast forms have a typical morphologic appearance that make them identifiable by cytology. When fungal hyphae form in tissues, the different fungi cannot be distinguished cytologically and culture or immunohistochemical staining is needed for a definitive diagnosis


The algal organism Prototheca spp may be found in cytologic samples and is associated with a pyogranulomatous or granulomatous inflammatory response.


The protozoan pathogens that infect animals are most often identifiable in cytologic samples. These include Toxoplasma gondii, Cytauxaoon felis, and Leishmania spp. Neospora spp. and Sarcocystis spp. may also be seen, but are probably more often identified using other techniques.

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