Lymph nodes or 'glands' or nodes or lymphoid tissue.are nodular bodies located throughout the body but clustering in certain areas such as the armpit, back of the neck and the groin. They are part of the lymphatic system.

The lymphatic system

Lymphatic system

The lymphoid system is the part of the immune system comprising a network of conduits called lymphatic vessels that carry a clear fluid called lymph unidirectionally toward the heart. Lymphoid tissue is found in many organs, particularly the lymph nodes, and in the lymphoid follicles associated...

 is part of the body's immune surveillance system. Blood

Blood

Blood is a specialized bodily fluid in animals that delivers necessary substances such as nutrients and oxygen to the cells and transports metabolic waste products away from those same cells....

 contains fluid

Fluid

In physics, a fluid is a substance that continually deforms under an applied shear stress. Fluids are a subset of the phases of matter and include liquids, gases, plasmas and, to some extent, plastic solids....

 and blood cell

Cell (biology)

The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....

s. The fluid, which may contain suspended foreign material such as bacteria

Bacteria

Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

 and virus

Virus

A virus is a small infectious agent that can replicate only inside the living cells of organisms. Viruses infect all types of organisms, from animals and plants to bacteria and archaea...

es, seeps through blood vessel

Blood vessel

The blood vessels are the part of the circulatory system that transports blood throughout the body. There are three major types of blood vessels: the arteries, which carry the blood away from the heart; the capillaries, which enable the actual exchange of water and chemicals between the blood and...

 walls into the tissues

Biological tissue

Tissue is a cellular organizational level intermediate between cells and a complete organism. A tissue is an ensemble of cells, not necessarily identical, but from the same origin, that together carry out a specific function. These are called tissues because of their identical functioning...

, where it bathes the body cells and exchanges substances with them. Some of this fluid is then taken up by lymphatic vessels and passed back to the heart, where it is again mixed with the blood. On its way the fluid passes through the lymph node

Lymph node

A lymph node is a small ball or an oval-shaped organ of the immune system, distributed widely throughout the body including the armpit and stomach/gut and linked by lymphatic vessels. Lymph nodes are garrisons of B, T, and other immune cells. Lymph nodes are found all through the body, and act as...

s. If nodes detect something foreign passing through them such as a bacterium or a cancer cell they will swell up. This is called lymphadenopathy

Lymphadenopathy

Lymphadenopathy is a term meaning "disease of the lymph nodes." It is, however, almost synonymously used with "swollen/enlarged lymph nodes". It could be due to infection, auto-immune disease, or malignancy....

or swollen glands. Usually this is localised (for example an infected spot on the scalp

Scalp

The scalp is the anatomical area bordered by the face anteriorly and the neck to the sides and posteriorly.-Layers:It is usually described as having five layers, which can conveniently be remembered as a mnemonic:...

 will cause lymph nodes in the neck on that same side to swell up), but when it is in two or more regions, it is called generalized lymphadenopathy.

Usually this is in response to a body-wide infectious disease such as influenza

Influenza

Influenza, commonly referred to as the flu, is an infectious disease caused by RNA viruses of the family Orthomyxoviridae , that affects birds and mammals...

 and will go away once the person has recovered, but sometimes it can persist long-term, even when there is no obvious cause of disease. This is then called persistent generalized lymphadenopathy

Persistent generalized lymphadenopathy

The lymphatic system is part of the immune surveillance system. Blood contains fluid and blood cells. The fluid, which may contain suspended foreign material such as bacteria and viruses, seeps through blood vessel walls into the tissues, where it bathes the body cells and exchanges substances with...

(PGL).

Causes of generalized lymphadenopathy

• Infection :
  • Viral : Infectious mononucleosis
    Infectious mononucleosis
    Infectious mononucleosis is an infectious, widespread viral...
    
    , Infective hepatitis
    Hepatitis
    Hepatitis is a medical condition defined by the inflammation of the liver and characterized by the presence of inflammatory cells in the tissue of the organ. The name is from the Greek hepar , the root being hepat- , meaning liver, and suffix -itis, meaning "inflammation"...
    
    , AIDS
    AIDS
    Acquired immune deficiency syndrome or acquired immunodeficiency syndrome is a disease of the human immune system caused by the human immunodeficiency virus...
    
    
  • Bacterial : Tuberculosis
    Tuberculosis
    Tuberculosis, MTB, or TB is a common, and in many cases lethal, infectious disease caused by various strains of mycobacteria, usually Mycobacterium tuberculosis. Tuberculosis usually attacks the lungs but can also affect other parts of the body...
    
    , Brucellosis
    Brucellosis
    Brucellosis, also called Bang's disease, Crimean fever, Gibraltar fever, Malta fever, Maltese fever, Mediterranean fever, rock fever, or undulant fever, is a highly contagious zoonosis caused by ingestion of unsterilized milk or meat from infected animals or close contact with their secretions...
    
    , 2ry syphilis
  • Protozoal : Toxoplasmosis
    Toxoplasmosis
    Toxoplasmosis is a parasitic disease caused by the protozoan Toxoplasma gondii. The parasite infects most genera of warm-blooded animals, including humans, but the primary host is the felid family. Animals are infected by eating infected meat, by ingestion of feces of a cat that has itself...
    
    
  • Fungal : Histoplasmosis
    Histoplasmosis
    Histoplasmosis is a disease caused by the fungus Histoplasma capsulatum. Symptoms of this infection vary greatly, but the disease primarily affects the lungs...
    
    
• Malignant :
  • Leukaemia
  • Lymphoma
    Lymphoma
    Lymphoma is a cancer in the lymphatic cells of the immune system. Typically, lymphomas present as a solid tumor of lymphoid cells. Treatment might involve chemotherapy and in some cases radiotherapy and/or bone marrow transplantation, and can be curable depending on the histology, type, and stage...
    
    
  • Metastatic carcinoma
    Metastatic carcinoma
    Metastatic carcinoma is able to grow at sites distant from the primary site of origin; thus, dissemination to the skin may occur with any malignant neoplasm, and these infiltrates may result from direct invasion of the skin from underlying tumors, may extend by lymphatic or hematogenous spread, or...
    
    
• Immunological :
  • Systemic lupus erythematosus
    Systemic lupus erythematosus
    Systemic lupus erythematosus , often abbreviated to SLE or lupus, is a systemic autoimmune disease that can affect any part of the body. As occurs in other autoimmune diseases, the immune system attacks the body's cells and tissue, resulting in inflammation and tissue damage...
    
    
  • Felty's syndrome
    Felty's syndrome
    Felty's syndrome, also called Felty syndrome, is characterized by the combination of rheumatoid arthritis, splenomegaly and neutropenia.-Symptoms:The symptoms of Felty's syndrome are similar to those of rheumatoid arthritis....
    
    
  • Still's disease
    Still's disease
    Still's disease can refer to:* Juvenile idiopathic arthritis* Adult-onset Still's disease...
    
    
  • Drug hypersensitivity as Hydantoin
    Hydantoin
    Hydantoin, which is also known as glycolylurea, is a heterocyclic organic compound that can be thought of as a cyclic "double-condensation reaction" product of glycolic acid and urea...
    
    , Hydralazine
    Hydralazine
    Hydralazine is a direct-acting smooth muscle relaxant used to treat hypertension by acting as a vasodilator primarily in arteries and arterioles...
    
    , Allopurinol
    Allopurinol
    Allopurinol is a drug used primarily to treat hyperuricemia and its complications, including chronic gout.- Mechanism of action :...
    
    
• Misc. :
  • Sarcoidosis
    Sarcoidosis
    Sarcoidosis , also called sarcoid, Besnier-Boeck disease or Besnier-Boeck-Schaumann disease, is a disease in which abnormal collections of chronic inflammatory cells form as nodules in multiple organs. The cause of sarcoidosis is unknown...
    
    
  • Amyloidosis
    Amyloidosis
    In medicine, amyloidosis refers to a variety of conditions whereby the body produces "bad proteins", denoted as amyloid proteins, which are abnormally deposited in organs and/or tissues and cause harm. A protein is described as being amyloid if, due to an alteration in its secondary structure, it...
    
    
  • Lipid storage disease
  • Hyperthyroidism
    Hyperthyroidism
    Hyperthyroidism is the term for overactive tissue within the thyroid gland causing an overproduction of thyroid hormones . Hyperthyroidism is thus a cause of thyrotoxicosis, the clinical condition of increased thyroid hormones in the blood. Hyperthyroidism and thyrotoxicosis are not synonymous...