Most commonly histiocytomas are found in young dogs and appear as a small, solitary, hairless lump, although Shar Peis may be predisposed to multiple histiocytomas. They are most commonly found on the head, neck, ears, and limbs, and are usually less than 2.5 cm in diameter. Ulceration of the mass is common. Diagnosis is made through cytology of the mass. Cytology reveals cells with clear to lightly basophilic cytoplasm and round or indented nuclei with fine chromatin and indistinct nucleoli.

A histiocytoma in the dog is a benign tumor. It is an abnormal growth in the skin of histiocytes (histiocytosis), a cell that is part of the immune system. A similar disease in humans, Hashimoto-Pritzker disease, is also a Langerhans cell histiocytosis. Dog breeds that may be more at risk for this tumor include Bulldogs, American Pit Bull Terriers, American Staffordshire Terriers, Scottish Terriers, Greyhounds, Boxers, and Boston Terriers. They also rarely occur in goats and cattle.

The disease in the lungs is characterized by enlargement of the tracheobronchial lymph nodes and infiltration of the lungs, sometimes leading to lung lobe consolidation and pleural effusion. Signs and symptoms include cough, loss of appetite, weight loss, anemia, and difficulty breathing. Seizures and rear limb weakness can be seen. Invasion of the bone marrow can cause pancytopenia. Diagnosis requires a biopsy.

Mast cell tumors are known among veterinary oncologists as 'the great pretenders' because their appearance can be varied, from a wart-like nodule to a soft subcutaneous lump (similar on palpation to a benign lipoma) to an ulcerated skin mass. Most mast cell tumors are small, raised lumps on the skin. They may be hairless, ulcerated, or itchy. They are usually solitary, but in about six percent of cases, there are multiple mast cell tumors (especially in Boxers and Pugs).

Manipulation of the tumor may result in redness and swelling from release of mast cell granules, also known as Darier's sign, and prolonged local hemorrhage. In rare cases, a highly malignant tumor is present, and signs may include loss of appetite, vomiting, diarrhea, and anemia. The presence of these signs usually indicates mastocytosis, which is the spread of mast cells throughout the body. Release of a large amount of histamine at one time can result in ulceration of the stomach and duodenum (present in up to 25 percent of cases) or disseminated intravascular coagulation. When metastasis does occur, it is usually to the liver, spleen, lymph nodes and bone marrow.

Malignant histiocytosis (also known as "Histiocytic medullary reticulosis") is a rare hereditary disease found in the Bernese Mountain Dog and humans, characterized by histiocytic infiltration of the lungs and lymph nodes. The liver, spleen, and central nervous system can also be affected. Histiocytes are a component of the immune system that proliferate abnormally in this disease. In addition to its importance in veterinary medicine, the condition is also important in human pathology.

It is one common source of appendicitis, as it may cause an obstruction of the appendiceal lumen, resulting in the subsequent filling of the appendix with mucus, causing it to distend and internal pressure to increase.

When mastocytomas affect humans, they are typically found in skin. They usually occur as a single lesion on the trunk or wrist. Although it is rare, mastocytomas are sometimes found in the lung. It can also affect children.

Histiocytic sarcoma is a tumor derived from histiocytes. The tumor is often positive for CD163 and can appear in the thyroid. However, in some cases it can also appear in the brain.

Lymphoid hyperplasia is the rapid growth proliferation of normal cells that resemble lymph tissue.

A histiocyte is an animal cell that is part of the mononuclear phagocyte system (also known as the reticuloendothelial system or lymphoreticular system). The mononuclear phagocytic system is part of the organism's immune system. The histiocyte is a tissue macrophage or a dendritic cell (histio, diminutive of histo, meaning "tissue", and cyte, meaning "cell").

Lymphadenopathy can occur in one or more groups of lymph nodes. Among 358 cases of Rosai–Dorfman disease that Rosai collected in a disease registry for which the location of lymphadenopathy was specified, 87.3% had cervical lymphadenopathy. Axillary, inguinal, and mediastinal lymphadenopathy are also found in Rosai–Dorfman disease.

Small image of an infected area of the body due to a reaction with an implant

The histiocytosis of Rosai–Dorfman disease can occur in lymph nodes, causing lymphadenopathy, or can occur outside lymph nodes in extranodal disease.

The typical patient with lymphocyte-variant hypereosinophilia presents with an extended history of hypereosinophilia and cutaneous allergy-like symptoms. Skin symptoms, which occur in >75% of patients, include erythroderma, pruritis, eczema, Poikiloderma, urticarial, and episodic angioedema. The symptom of episodic angioedema in lymphocyte-variant hypereosinophilia resembles that occurring in Gleich's syndrome, a rare disease that is accompanied by secondary hypereosinophilia plus a sub-population of CD3(-), CD4(+) T cells and therefore proposed, at least in many patients, a subtype of lymphocyte-variant hypereosiophilia. Biopsies of these erythroderma and eczema skin lesions find prominent accumulations of eosinophils. Other presentations include; a) lymphadenopathy occurring in ~60% of patients; b) eosinophil infiltrations in lung similar to, and often diagnosed as, eosinophilic pneumonia, occurring in ~20% of patients; c) episodic angioedema-related gastrointestinal symptoms that are sometimes similar to symptoms of the irritable bowel syndrome occurring in ~20% of patients; d) rheumatologic manifestations of inflammatory arthralgias in ~20% of patients; and e) splenomegaly occurring in ~10% of patients. Cardiovascular complications such as various types of heart damage (see above History section) and vascular injuries due to eosinophil infiltration and eosinophil-induced thrombosis are often critical components of persistent hypereosinohilia syndromes; These complications are not a prominent component of lymphocyte-variant hypereosionophilia, occurring in <10% of patients.

ATL is usually a highly aggressive non-Hodgkin's lymphoma with no characteristic histologic appearance except for a diffuse pattern and a mature T-cell phenotype. Circulating lymphocytes with an irregular nuclear contour (leukemic cells) are frequently seen. Several lines of evidence suggest that HTLV-1 causes ATL. This evidence includes the frequent isolation of HTLV-1 from patients with this disease and the detection of HTLV-1 proviral genome in ATL leukemic cells. ATL is frequently accompanied by visceral involvement, hypercalcemia, skin lesions, and lytic bone lesions. Bone invasion and osteolysis, features of bone metastases, commonly occur in the setting of advanced solid tumors, such as breast, prostate, and lung cancers, but are less common in hematologic malignancies. However, patients with HTLV-1–induced ATL and multiple myeloma are predisposed to the development of tumor-induced osteolysis and hypercalcemia. One of the striking features of ATL and multiple myeloma induced bone disease is that the bone lesions are predominantly osteolytic with little associated osteoblastic activity. In patients with ATL, elevated serum levels of IL-1, TGFβ, PTHrP, macrophage inflammatory protein (MIP-1α), and receptor activator of nuclear factor-κB ligand (RANKL) have been associated with hypercalcemia. Immunodeficient mice that received implants with leukemic cells from patients with ATL or with HTLV-1–infected lymphocytes developed hypercalcemia and elevated serum levels of PTHrP. Most patients die within one year of diagnosis.

Infection with HTLV-1, like infection with other retroviruses, probably occurs for life and can be inferred when antibody against HTLV-1 is detected in the serum.

A foreign-body giant cell is a collection of fused macrophages (giant cell) which are generated in response to the presence of a large foreign body. This is particularly evident with implants that cause the body chronic inflammation and foreign body response.

This reaction to the implant causes damages to the infected area, leaving the exterior surface with scars.

The nuclei are arranged in a disorganized manner. The nuclei in this cell are centrally placed and overlap each other. This is in contrast to a Langhans giant cell, where the nuclei are arranged on the border.

Foreign body cells can detect and eliminate

bacteria caught within the body, by sensing the unique sugar coating that are

on the invading prokaryotes. These macrophage cells are one of a few

phagocytic cells, but not the first to come to an injury site, and tend to

linger from anytime between days to weeks. There has been some research done on other variations of

giant calls with different functions.

Lymphocyte-variant hypereosinophila, also termed lymphocyte variant eosinophilia, is a rare disorder in which eosinophilia or hypereosinophilia (i.e. a large or extremely large increase in the number of eosinophils in the blood circulation) is caused by aberrant population of lymphocytes. These aberrant lymphocytes function abnormally by stimulating the proliferation and maturation of bone marrow eosinophil-precursor cells termed colony forming unit-Eosinophils or CFU-Eos.

The overly stimulated CFU-Eos cells mature to apparently normal eosinophils, enter the circulation, and may accumulate in, and severely damage, various tissues. The disorder is usually indolent or slowly progressive but may proceed to a leukemic phase and at this phases is sometimes classified as acute eosinophilic leukemia. Hence, lymphocyte-variant hypereosinophilia can be regarded as a precancerous disease.

The order merits therapeutic intervention to avoid or reduce eosinophil-induced tissue injury and to treat its leukemic phase. The latter phase of the disease is aggressive and typically responds relatively poorly to anti-leukemia chemotherapeutic drug regimens.

As of 2011 five cases had been reported, involving rib, tibial epiphysis, ulna, distal tibia and femur. Young individuals are prevalently affected but one case involved a 50-year-old woman. Pain, swelling of possibly long duration, fever and increased ESR are some of the main clinical findings. X-ray examination shows lytic foci with sclerotic margins. A neoplastic process can be suspected.

Epithelioid sarcoma is a rare soft tissue sarcoma arising from Mesenchyme tissue and characterized by epithelioid-like features. It accounts for less than 1% of all soft tissue sarcomas. It was first clearly characterized by F.M. Enzinger in 1970. It commonly presents itself in the distal limbs (fingers, hands, forearms, or feet) of young adults as a small, soft mass or a series of bumps. A proximal version has also been described, frequently occurring in the upper extremities. Rare cases have been reported in the pelvis, vulva, penis, and spine.

Histologically, epithelioid sarcoma forms nodules with central necrosis surrounded by bland, polygonal cells with eosinophilic cytoplasm and peripheral spindling. Epithelioid sarcomas typically express vimentin, cytokeratins, epithelial membrane antigen, and CD34, whereas they are usually negative for S100, desmin, and FLI-1. They typically stain positive for CA125.

Epithelioid sarcoma most commonly strikes young adults, yet no age group is immune. The disease has a tendency to develop local recurrences and metastasis thereafter to regional lymph nodes, lung, bone, brain, and other locations, including the scalp. Generally speaking, epithelioid sarcoma has a high rate of relapse after initial treatment and tends to recur locally (at or near the original tumor site). Epithelioid sarcoma also demonstrates lymphatic spread (in 22-48% of cases), and metastasis (in 21-63% of cases). These events, as well as advanced stage (progression) and grade (aggressiveness), are predictive of an overall worse outcome. The overall five-year survival rate for epithelioid sarcoma is anywhere from 25 to 78%. Importantly, the 10-year and 15-year survival rate drops off significantly. Associated with a more positive outcome are younger age, female vs. male sex, distal vs. proximal location, smaller tumor size, and negative margins upon tumor resection.

Natural killer T (NKT) cells are a heterogeneous group of T cells that share properties of both T cells and natural killer cells. Many of these cells recognize the non-polymorphic CD1d molecule, an antigen-presenting molecule that binds self and foreign lipids and glycolipids. They constitute only approximately 0.1% of all blood T cells. Natural killer T cells should not be confused with natural killer cells.

In general, epithelioid sarcoma is a slow-growing and relatively painless tumor, often resulting in a lengthy period of time between presentation and diagnosis. Due to its ambiguity, it is often misdiagnosed, mistaken as a persistent wart or cyst. It most commonly presents itself in the distal limbs (fingers, hands, forearms, or feet) as a small, soft mass or a series of bumps. It is most often described as a firm-to-hard palpable mass, either in the deep soft tissue or in the dermis. Often, superficial lesions will ulcerate causing a mistaken diagnosis of a poorly healing traumatic wound or wart. About 13% of patients will present with multifocal tumors, and about 13% of patients will present with metastatic disease.

Monocytes are a type of "leukocyte", or white blood cell. They are the largest type of leukocyte and can differentiate into macrophages and myeloid lineage dendritic cells. As a part of the vertebrate innate immune system monocytes also influence the process of adaptive immunity. There are at least three subclasses of monocytes in human blood based on their phenotypic receptors.

Dendritic cells (DCs) are antigen-presenting cells (also known as "accessory cells") of the mammalian immune system. Their main function is to process antigen material and present it on the cell surface to the T cells of the immune system. They act as messengers between the innate and the adaptive immune systems.

Dendritic cells are present in those tissues that are in contact with the external environment, such as the skin (where there is a specialized dendritic cell type called the Langerhans cell) and the inner lining of the nose, lungs, stomach and intestines. They can also be found in an immature state in the blood. Once activated, they migrate to the lymph nodes where they interact with T cells and B cells to initiate and shape the adaptive immune response. At certain development stages they grow branched projections, the "dendrites" that give the cell its name (δένδρον or déndron being Greek for "tree"). While similar in appearance, these are structures distinct from the dendrites of neurons. Immature dendritic cells are also called veiled cells, as they possess large cytoplasmic 'veils' rather than dendrites.

Tumor-associated macrophages (TAMs) are the multifarious group of cells that originate mainly from the peritumoral tissue or bone marrow and can be divided into two main types: M1 and M2. Among them are the infiltrating M1 tumor-associated macrophages present in the early stages of tumorigenesis, which can secrete proinflammatory cytokines and turn in inhibit tumor growth. On the contrary, M2 tumor-associated macrophages are predominant in the late stage of tumor formation. Type II cytokines, which are secreted by them, can promote anti-inflammatory reaction and thus promote tumor growth. However, it remains unclear when M1 tumor-associated macrophages are transformed to M2 tumor-associated macrophages, but tumor hypoxia is currently thought to be associated with such a shift. M2 tumor-associated macrophages secrete many proteases such as cathepsin, cytokines, and an epidermal growth factor. The presence of M2 TAMs make the tumor prone to growth and angiogenesis, which in turn damages other tissues.

Clinically, in 128 patients in the first or second stage of breast cancer it was found that more patients with M2 tumor-associated macrophages had a higher degree of histology, more angiogenesis, and worse overall survival. Patients with more M1 tumor-associated macrophages displayed the opposite effect. .

Macrophages are the most common cells in breast tumors. They have a profound effect on the immune status of tumor tissues. Macrophages act as professional phagocytes in the body, specifically killing and eliminating cells or microbes that are perceived to be a threat. They represent the first line of defence and the bridge of connecting the innate and adaptive arms of the immune system. However, the large number of tumor cells and the derived factors present within the tumour microenvironment (TME) act to attenuate the tumoricidal activity function of macrophages. The TME consists of hypoxic conditions, growth factors and immunosuppressive cytokines that convert trophic macrophages to tumor-associated macrophages (TAM). These features promote tissue growth and repair and are an integral part of development so that macrophages within breast tumors are inadvertently authorized to promote tumor growth and metastasis.

Xanthogranulomatous osteomyelitis (XO) is a peculiar aspect of osteomyelitis characterized by prevalent histiocytic infiltrate and foamy macrophage clustering.