•  Neutrophils are involved in the innate cell-mediated immunity. One of their key functions is to engulf and neutralize infectious agents (mostly bacteria).

•  Neutrophilic inflammation is diagnosed when the sample contains a vast predominance of neutrophils. Purulent inflammation is sometimes used as a synonym.

•  Neutrophils may present in different forms:

•  Non-degenerate neutrophils
Similar to those observed in the circulatory stream. They have segmented, densely stained nuclei with clumped chromatin. Their presence is primarily suggestive of a sterile inflammatory process; however, infection may still be present.

•  Degenerate neutrophils

•  Karyolytic neutrophils: characterized by a pale, swollen nucleus with coarse and pale chromatin; typical nuclear segmentation and cell borders may be partially lost. These changes are the result of endotoxins released by the infectious agents that damage the cellular and nuclear membranes, allowing water influx and swelling of cellular components. They are often associated with bacterial and fungal infections. Degenerate neutrophils may also be found in association with severe necrosis.

•  Pyknotic neutrophils: the nucleus loses the typical segmentations and becomes rounder, smaller and denser (apoptosis). This is usually a feature of age-related cell death.

•  Karyorrhectic neutrophils: pyknotic neutrophils following the fragmentation of the nucleus into small fragments.

•  Neutrophils may be associated with other cell types, in particular macrophages. In those cases, the inflammation is often referred to as mixed or pyogranulomatous.

•  Infectious agents (mostly bacteria).

•  Trauma/irritation.

•  Tissue necrosis (often associated with underlying, rapidly growing neoplasia).

•  Immune-mediated process.

•  Macrophages are large mononuclear cells involved in the innate cell-mediated immunity. They originate from circulating monocytes, which migrate into tissues through the endothelium.

•  Macrophages are commonly seen in established or chronic inflammatory processes.

•  Macrophagic inflammation is diagnosed when the sample contains a vast predominance of macrophages and is also known as granulomatous.

•  Macrophagic inflammation may be associated with other inflammatory cells, including neutrophils, lymphocytes and plasma cells. When admixed with neutrophils, the inflammation is referred to as pyogranulomatous.

•  Other cell types, including epithelioid macrophages, multinucleated giant cells and reactive fibroblasts, may also be present.

•  Epithelioid macrophages: mononuclear cells, often arranged in cohesive groups, with large amounts of uniformly basophilic cytoplasm, overall resembling epithelial cells.

•  Multinucleated giant cells: result of the fusion of multiple epithelioid macrophages. They have a large cytoplasm and contain multiple small nuclei arranged either haphazardly (foreign-body-type giant cells) or peripherally (Langhans-type giant cells).

•  Macrophages often display phagocytosis. In haemorrhagic events, they may contain red blood cells (erythrophagocytosis), haemosiderin (haemosiderophages) and/or haematoidin crystals. They can also engulf cellular debris, leucocytes (leucophagia) and/or infectious agents.

•  Specific types of macrophagic inflammation (e.g. panniculitis, foreign body/injection reaction) will be discussed in specific chapters.

•  Selected infectious agents (e.g. Mycobacterium spp., Leishmania spp., fungi, feline infectious peritonitis (FIP) infection).

•  Endogenous or exogenous foreign body reaction.

•  Chronic irritation.

•  Eosinophils regulate acute hypersensitivity reactions (type I hypersensitivity) and are typically present in allergen- and parasite-mediated inflammatory reactions.

•  Eosinophilic inflammation is diagnosed when the sample contains significant numbers of eosinophils (by convention > 10%).

•  Eosinophils are polymorphonuclear leucocytes with lobulated nuclei.

•  Canine eosinophils contain round orange to pink granules, which may significantly vary in size and colour.

•  Feline eosinophils contain numerous small rod-shaped granules with more delicate staining properties than in dogs.

•  In eosinophilic plaque/granuloma lesions, there is a prevalence of eosinophils, especially in the feline species. Small numbers of granulated mast cells, small lymphocytes, macrophages and neutrophils are also commonly observed. Reactive fibroblasts and amorphous basophilic material may be present as a result of collagenolysis.

•  In most of the eosinophilic inflammations caused by hypersensitivity, such as insect/flea-bite reaction, inflammatory cells are often mixed. Eosinophils, small lymphocytes, granulated mast cells, neutrophils and macrophages are present in variable proportions.

•  Insect-bite reaction.

•  Eosinophilic granuloma complex (EGC).

•  Immune-mediated process.

•  Parasitic or fungal infection.

•  Paraneoplastic inflammation (e.g. mast cell tumour, T-cell lymphoma).

•  Lymphocytes participate in both the humoral and cell-mediated adaptive immune response. They are also involved in type IV (delayed) hypersensitivity reactions.

•  They are commonly seen in established or chronic inflammatory processes.

•  Lymphocytic inflammation is diagnosed when the sample contains a vast majority of lymphocytes.

•  This type of inflammation is often characterized by a mixed population of lymphoid cells, predominantly small lymphocytes. Low numbers of intermediate and/or large lymphoid cells may also be noted.

•  Macrophages and plasma cells can also accompany small lymphocytes in chronic inflammatory processes. When significant numbers of plasma cells are present, the term lymphoplasmacytic inflammation can be used instead.