The typical patient with angioimmunoblastic T-cell lymphoma (AITL) is either middle-aged or elderly, and no gender preference for this disease has been observed. AITL comprises 15–20% of peripheral T-cell lymphomas and 1–2% of all non-Hodgkin lymphomas.

Mast cell tumors mainly occur in older adult dogs, but have been known to occur on rare occasions in puppies. The following breeds are commonly affected by mast cell tumors:

- Boxer

- Staffordshire bull terrier

- Bulldog

- Basset hound

- Weimaraner

- Boston terrier

- Great Dane

- Golden retriever

- Labrador retriever

- Beagle

- German shorthaired pointer

- Scottish terrier

- Pug

- Shar pei

- Rhodesian ridgeback

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.

Interdigitating dendritic cell sarcoma is a form of malignant histiocytosis affecting dendritic cells.

It can present in the spleen. It can also present in the duodenum.

Two types of mast cell tumors have been identified in cats, a mast cell type similar to dogs and a histiocytic type that appears as subcutaneous nodules and may resolve spontaneously. Young Siamese cats are at an increased risk for the histiocytic type, although the mast cell type is the most common in all cats and is considered to be benign when confined to the skin.

Mast cell tumors of the skin are usually located on the head or trunk. Gastrointestinal and splenic involvement is more common in cats than in dogs; 50 percent of cases in dogs primarily involved the spleen or intestines. Gastrointestinal mast cell tumors are most commonly found in the muscularis layer of the small intestine, but can also be found in the large intestine. It is the third most common intestinal tumor in cats, after lymphoma and adenocarcinoma.

Diagnosis and treatment are similar to that of the dog. Cases involving difficult to remove or multiple tumors have responded well to strontium-90 radiotherapy as an alternative to surgery. The prognosis for solitary skin tumors is good, but guarded for tumors in other organs. Histological grading of tumors has little bearing on prognosis.

Most histiocytomas will regress within two or three months. Surgical removal may be necessary if the tumor does not regress or if it is growing rapidly to a large size. Histiocytomas should never be treated with an intralesional injection of a corticosteroid, as remission relies on recognition of the tumour by the body's immune system which is suppressed by steroids.

Up to 25 percent of Bernese Mountain Dogs may develop malignant histiocytosis in their lifetime. Other breeds with a possible genetic tendency toward malignant histiocytosis include Rottweilers, Flat-Coated Retrievers, and Golden Retrievers.

Treatment with chemotherapy has been used with some success, particularly using lomustine, prednisone, doxorubicin, and cyclophosphamide. Because of the rapid progression of this aggressive disease, the prognosis is very poor.

Langhans cells are often found in transbronchial lung biopsies or lymph node biopsies in patients suffering from sarcoidosis.

Angioimmunoblastic T-cell lymphoma (AITL, sometimes misspelled AILT) (formerly known as "angioimmunoblastic lymphadenopathy with dysproteinemia") is a mature T-cell lymphoma of blood or lymph vessel immunoblasts characterized by a polymorphous lymph node infiltrate showing a marked increase in follicular dendritic cells (FDCs) and high endothelial venules (HEVs) and systemic involvement.

Langhans giant cells (also known as Pirogov-Langhans cells) are large cells found in granulomatous conditions.

They are formed by the fusion of epithelioid cells (macrophages), and contain nuclei arranged in a horseshoe-shaped pattern in the cell periphery.

Although traditionally their presence was associated with tuberculosis, they are not specific for tuberculosis or even for mycobacterial disease. In fact, they are found in nearly every form of granulomatous disease, regardless of etiology.

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.

Follicular dendritic cell sarcoma (FDCS) is an extremely rare neoplasm. While the existence of FDC tumors was predicted by Lennert in 1978, the tumor wasn’t fully recognized as its own cancer until 1986 after characterization by Monda et al. It accounts for only 0.4% of soft tissue sarcomas, but has significant recurrent and metastatic potential and is considered an intermediate grade malignancy. The major hurdle in treating FDCS has been misdiagnosis. It is a newly characterized cancer, and because of its similarities in presentation and markers to lymphoma, both Hodgkin and Non-Hodgkin subtypes, diagnosis of FDCS can be difficult. With recent advancements in cancer biology better diagnostic assays and chemotherapeutic agents have been made to more accurately diagnose and treat FDCS.

Langerhans cells are dendritic cells (antigen-presenting immune cells) of the skin and mucosa, and contain organelles called Birbeck granules. They are present in all layers of the epidermis and are most prominent in the stratum spinosum. They also occur in the papillary dermis, particularly around blood vessels, as well as in the mucosa of the mouth, foreskin, and vagina. They can be found in other tissues, such as lymph nodes, particularly in association with the condition Langerhans cell histiocytosis (LCH).

Langerhans cell histiocytosis (LCH) is a rare disease involving clonal proliferation of Langerhans cells, abnormal cells deriving from bone marrow and capable of migrating from skin to lymph nodes. Clinically, its manifestations range from isolated bone lesions to multisystem disease. LCH is part of a group of clinical syndromes called histiocytoses, which are characterized by an abnormal proliferation of histiocytes (an archaic term for activated dendritic cells and macrophages). These diseases are related to other forms of abnormal proliferation of white blood cells, such as leukemias and lymphomas.

The disease has gone by several names, including Hand–Schüller–Christian disease, Abt-Letterer-Siwe disease, Hashimoto-Pritzker disease(a very rare self-limiting variant seen at birth) and histiocytosis X, until it was renamed in 1985 by the Histiocyte Society.

Most patients with "ETV6-ACSL6"-related disease present with findings similar to eosinophilia, hypereosinophila, or chronic eosinophilic leukemia; at least 4 cases presented with eosinophilia plus findings of the red blood cell neoplasm, polycythemia vera; three cases resembled acute myelogenous leukemia; and one case presented with findings of a combined Myelodysplastic syndrome/myeloproliferative neoplasm. Best treatments for "ETV6-ACSL6"-related disease are unclear. Patients with the polycythemia vera form of the disease have been treated by reducing the circulating red blood cell load by phlebotomy or suppressing red blood cell formation using hydroxyurea. Individual case studies report that "ETV6-ACSL6"-associated disease is insensitive to tyrosine kinase inhibitors. Best treatment currently available, therefore, may involve chemotherapy and bone marrow transplantion.

Langerhans cells may be initial cellular targets in the sexual transmission of HIV, and may be a target, reservoir, and vector of dissemination.

Langerhans cells have been observed in foreskin, vaginal, and oral mucosa of humans; the lower concentrations in oral mucosa suggest that it is not a likely source of HIV infection relative to foreskin and vaginal mucosa.

On March 4, 2007 the online Nature Medicine magazine published the research letter "Langerin is a natural barrier to HIV-1 transmission by Langerhans cells." One of the authors of the study, Teunis Geijtenbeek, said that "Langerin is able to scavenge viruses from the surrounding environment, thereby preventing infection" and "since generally all tissues on the outside of our bodies have Langerhans cells, we think that the human body is equipped with an antiviral defense mechanism, destroying incoming viruses."

Plasmacytoid dendritic cells (pDCs) are innate immune cells that circulate in the blood and are found in peripheral lymphoid organs. They develop from bone marrow hematopoietic stem cells and constitute < 0.4% of peripheral blood mononuclear cells (PBMC).

In humans they exhibit plasma cell morphology and express CD4, HLA-DR, CD123, blood-derived dendritic cell antigen-2 (BDCA-2), Toll-like receptor (TLR) 7 and TLR9 within endosomal compartments, but do not express high levels of CD11c or CD14, which distinguishes them from conventional dendritic cells or monocytes, respectively. Mouse pDC express CD11c, B220, BST-2/Tetherin (mPDCA) and Siglec-H and are negative for CD11b.

As components of the innate immune system, these cells express intracellular Toll-like receptors 7 and 9 which detect ssRNA and unmethylated CpG DNA sequences, respectively. Upon stimulation and subsequent activation, these cells produce large amounts (up to 1,000 times more than other cell type) of type I interferon (mainly IFN-α (alpha) and IFN-β (beta)), which are critical pleiotropic anti-viral compounds mediating a wide range of effects.

The number of circulating pDCs are found to be decreased during chronic HIV infection as well as HCV infection.

SCTC exhibits a highly aggressive phenotype, thus prognosis of that malignancy is extremely poor. The overall survival is less than 1 year in most of cases.

Lymphocyte-variant hypereosinophilia is a rare disease in which eosinophilia is caused by aberrant T cell lymphocytes which secrete cytokines (e.g. interleukin-5) that stimulate the proliferation of eosinophil precursor cells. The disease, which occasionally proceeds to a malignant lymphocytic phase, clearly reflects a clonal disturbance in lymphocytes, not eosinophils, and therefore is not a clonal hypereosinophilia. Similar non-clonal eosinophilia due to eosinophil precursor cell stimulation by clonal malignant cells is sometimes seen in cases of Hodgkin disease, B-cell lymphoma, T-cell lymphomas, T cell leukemias, and Langerhans cell histiocytosis. Other hematological diseases are associated with eosinophilia but regarded as clonal eosinophilia associated with a more important clonal malignancy in another cell type. For example, eosinophilia occurs in 20% to 30% of patients with systemic mastocytosis. Also referred to as SM-eo (systemic mastocytosis with eosinophilia) or SM-SEL (systemic mastocytosis with chronic eosinophilic leukemia), this disease's clonal eosinophils bear the same driving mutation, D816V in the"KIT" gene, as the clonal mast cells.

Follicular dendritic cells are localized in germinal centers of lymphoid follicles and have an integral role in regulation of the germinal center reaction and present antigens to B cells. Most cases of FDCS develop in the lymph nodes, but about 30% develop in extranodal sites. In 1998 the largest study on the disease was a retrospective review with fifty-one patients. Of these fifty-one patients, no conclusive pattern was found in regard to age, sex, race or presentation. The median patient age was 41 (range 14–76), and while most cases presented with cervical and axillary lymphadenopathy, 17 presented in extranodal sites including the liver, spleen, bowel and pancrease. With such a range of patient histories no definitive cause has been linked to FDCS. There has, however, been some evidence that previous exposure to the Epstein Barr Virus (EBV) or diagnosis of Castleman's disease can increase the risk of developing FDCS—medical literature in 2000 reported approximately 12% of all cases of FDC tumors are associated with EBV, with variance in different organs, but the role of EBV remains unclear in FDC tumor pathogenesis; and EBV does not appear to play a role in the transformation process of Castleman's disease to FDC sarcoma because all cases the report found associated with Castleman's disease were EBV negative.

Symptoms of FDCS vary, and are largely dependent on the part of the body the tumor develops. The most common symptom is painless swelling in lymph nodes. This symptom alone, however, is nonconclusive as it is associated with many other diseases including the common cold. Other symptoms include cough, sore throat, difficulty swallowing, weight loss and tiredness. In cases that present in extranodal sites outside of the head and neck region, organ specific symptoms are observed.

The disease spectrum results from clonal accumulation and proliferation of cells resembling the epidermal dendritic cells called Langerhans cells, sometimes called Dendritic Cell Histiocytosis. These cells in combination with lymphocytes, eosinophils, and normal histiocytes form typical LCH lesions that can be found in almost any organ. A similar set of diseases has been described in canine histiocytic diseases.

LCH is clinically divided into three groups: unifocal, multifocal unisystem, and multifocal multisystem.

In most series, LCLC's comprise between 5% and 10% of all lung cancers.

According to the Nurses' Health Study, the risk of large cell lung carcinoma increases with a previous history of tobacco smoking, with a previous smoking duration of 30 to 40 years giving a relative risk of approximately 2.3 compared to never-smokers, and a duration of more than 40 years giving a relative risk of approximately 3.6.

Another study concluded that cigarette smoking is the predominant cause of large cell lung cancer. It estimated that the odds ratio associated with smoking two or more packs/day for current smokers is 37.0 in men and 72.9 in women.

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.