Asplenia is the absence of normal spleen function. It predisposes to some septicemia infections. Therefore, vaccination and antibiotic measures are essential in such cases. There are multiple causes:

- Some people congenitally completely lack a spleen, although this is rare.

- Sickle-cell disease can cause a functional asplenia (or autosplenectomy) by causing infarctions of the spleen during repeated sickle-cell crises.

- It may be removed surgically (known as a splenectomy), but this is rarely performed, as it carries a high risk of infection and other adverse effects. Indications include following abdominal injuries with rupture and hemorrhage of the spleen, or in the treatment of certain blood diseases (Idiopathic thrombocytopenic purpura, hereditary spherocytosis, etc.), certain forms of lymphoma or for the removal of splenic tumors or cysts.

If the splenomegaly underlies hypersplenism, a splenectomy is indicated and will correct the hypersplenism. However, the underlying cause of the hypersplenism will most likely remain; consequently, a thorough diagnostic workup is still indicated, as, leukemia, lymphoma and other serious disorders can cause hypersplenism and splenomegaly. After splenectomy, however, patients have an increased risk for infectious diseases.

Patients undergoing splenectomy should be vaccinated against "Haemophilus influenzae", "Streptococcus pneumoniae", and "Meningococcus". They should also receive annual influenza vaccinations. Long-term prophylactic antibiotics may be given in certain cases.

In cases of infectious mononucleosis splenomegaly is a common symptom and health care providers may consider using abdominal ultrasonography to get insight into a person's condition. However, because spleen size varies greatly, ultrasonography is not a valid technique for assessing spleen enlargement and should not be used in typical circumstances or to make routine decisions about fitness for playing sports.

The most common causes of splenomegaly in developed countries are infectious mononucleosis, splenic infiltration with cancer cells from a hematological malignancy and portal hypertension (most commonly secondary to liver disease, and sarcoidosis). Splenomegaly may also come from bacterial infections, such as syphilis or an infection of the heart's inner lining (endocarditis).

The possible causes of moderate splenomegaly (spleen <1000 g) are many, and include:

The causes of massive splenomegaly (spleen >1000 g) are fewer, and include:

- visceral leishmaniasis (kala-azar)

- chronic myelogenous leukemia

- myelofibrosis

- malaria

- splenic marginal zone lymphoma

Pneumococcal septicemia, or whole-body infection caused by the "Streptococcus pneumoniae" bacteria, has been reported to cause autosplenectomy but is a very rare and poorly understood complication of the infection.

Absence of effective splenic function or absence of the whole spleen (asplenia) is associated with increased risks of overwhelming post splenectomy infection, especially from polysaccharide encapsulated bacteria and organisms that invade erythrocytes. People without a spleen have a weakened immune system, although other immune organs compensate for the missing spleen. Vaccination against encapsulated bacteria and prophylactic antibiotics can be used to counteract lowered immunity in asplenic patients. Specifically, people without a spleen are recommended to be vaccinated against pneumonia, influenza, Haemophilus influenza type b and meningococci.

Splenic diseases include splenomegaly, where the spleen is enlarged for various reasons. On the other hand, a lack of normal spleen function is called asplenia.

Enlargement of spleen, ascites, jaundice, and the result of destruction of various blood cells by spleen – anemia, leukopenia, thrombocytopenia, gastrointestinal bleeding – may constitute the presenting symptoms.

The basic pathology is some kind of obstructive pathology in the portal, hepatic or splenic vein that causes obstruction of venous blood flow from the spleen towards the heart. The cause of such obstruction may be abnormalities present at birth (congenital) of certain veins, blood clots, or various underlying disorders causing inflammation and obstruction of veins (vascular obstruction) of the liver.

Hepatosplenomegaly (commonly abbreviated HSM) is the simultaneous enlargement of both the liver (hepatomegaly) and the spleen (splenomegaly). Hepatosplenomegaly can occur as the result of acute viral hepatitis, infectious mononucleosis, and histoplasmosis or it can be the sign of a serious and life-threatening lysosomal storage disease. Systemic venous hypertension can also increase the risk for developing hepatosplenomegaly, which may be seen in those patients with right-sided heart failure.

Hemolytic anemia affects nonhuman species as well as humans. It has been found, in a number of animal species, to result from specific triggers.

Some notable cases include hemolytic anemia found in black rhinos kept in captivity, with the disease, in one instance, affecting 20% of captive rhinos at a specific facility. The disease is also found in wild rhinos.

Dogs and cats differ slightly from humans in some details of their RBC composition and have altered susceptibility to damage, notably, increased susceptibility to oxidative damage from consumption of onion. Garlic is less toxic to dogs than onion.

To minimise the risks associated with splenectomy, antibiotic and vaccination protocols have been established, but are often poorly adhered to by physicians and patients due to the complications resulting from antibiotic prophylaxis such as development of an overpopulation of Clostridium difficile in the intestinal tract.

Inherited mutations in three genes which all result in increased stability of hypoxia-inducible factors, leading to increased erythropoietin production, have been shown to cause erythrocytosis:

- Chuvash polycythemia is an autosomal recessive form of erythrocytosis which is endemic in patients from Chuvashia, an autonomous republic within the Russian Federation. Chuvash polycythemia is associated with homozygosity for a C598T mutation in the von Hippel-Lindau gene ("VHL"), which is needed for the destruction of hypoxia-inducible factors in the presence of oxygen. Clusters of patients with Chuvash polycythemia have been found in other populations, such as on the Italian island of Ischia, located in the Bay of Naples.

- PHD2 erythrocytosis: Heterozygosity for loss-of-function mutations of the "PHD2" gene are associated with autosomal dominant erythrocytosis and increased hypoxia-inducible factors activity.

- HIF2α erythrocytosis: Gain-of-function mutations in" HIF2α "are associated with autosomal dominant erythrocytosis and pulmonary hypertension.

Hereditary spherocytosis is the most common disorder of the red cell membrane and affects 1 in 2,000 people of Northern European ancestry. According to Harrison's Principles of Internal Medicine, the frequency is at least 1 in 5,000.

Increased platelet counts can be due to a number of disease processes:

- Essential (primary)

- Essential thrombocytosis (a form of myeloproliferative disease)

- Other myeloproliferative disorders such as chronic myelogenous leukemia, polycythemia vera, myelofibrosis

- Reactive (secondary)

- Inflammation

- Surgery (which leads to an inflammatory state)

- Hyposplenism (decreased breakdown due to decreased function of the spleen)

- Splenectomy

- Asplenia (absence of normal spleen function)

- Iron deficiency anemia or hemorrhage

Over-medication with drugs that treat thrombocytopenia, such as eltrombopag or romiplostim, may also result in thrombocytosis.

Other causes include the following

- Kawasaki disease

- Soft tissue sarcoma

- Osteosarcoma

- Dermatitis (rarely)

- Inflammatory bowel disease

- Rheumatoid arthritis

- Nephritis

- Nephrotic syndrome

- Bacterial diseases, including pneumonia, sepsis, meningitis, urinary tract infections, and septic arthritis.

The vast majority of causes of thrombocytosis are acquired disorders, but in a few cases, they may be congenital, such as thrombocytosis due to congenital asplenia.

Relative polycythemia is an apparent rise of the erythrocyte level in the blood; however, the underlying cause is reduced blood plasma (hypovolemia, cf. dehydration). Relative polycythemia is often caused by loss of body fluids, such as through burns, dehydration, and stress. A specific type of relative polycythemia is Gaisböck syndrome. In this syndrome, primarily occurring in obese men, hypertension causes a reduction in plasma volume, resulting in (amongst other changes) a relative increase in red blood cell count.

People with ascites due to cirrhosis are at risk of spontaneous bacterial peritonitis.

Because of the increased risk of infection, physicians administer oral antibiotics as a prophylaxis after a surgical splenectomy (or starting at birth, for congenital asplenia or functional asplenia).

Those with asplenia are also cautioned to start a full-dose course of antibiotics at the first onset of an upper or lower respiratory tract infection (for example, sore throat or cough), or at the onset of any fever.

This refers to changes in the mucosa of the stomach in people with portal hypertension, and is associated with cirrhosis severity.

Acquired hemolytic anemia may be caused by immune-mediated causes, drugs and other miscellaneous causes.

- Immune-mediated causes could include transient factors as in "Mycoplasma pneumoniae" infection (cold agglutinin disease) or permanent factors as in autoimmune diseases like autoimmune hemolytic anemia (itself more common in diseases such as systemic lupus erythematosus, rheumatoid arthritis, Hodgkin's lymphoma, and chronic lymphocytic leukemia).

- Spur cell hemolytic anemia

- Any of the causes of hypersplenism (increased activity of the spleen), such as portal hypertension.

- Acquired hemolytic anemia is also encountered in burns and as a result of certain infections (e.g. malaria).

- Lead poisoning resulting from the environment causes non-immune hemolytic anemia.

- Runners can suffer hemolytic anemia due to "footstrike hemolysis", owing to the destruction of red blood cells in feet at foot impact.

- Low-grade hemolytic anemia occurs in 70% of prosthetic heart valve recipients, and severe hemolytic anemia occurs in 3%.

Experimental gene therapy exists to treat hereditary spherocytosis in lab mice; however, this treatment has not yet been tried on humans due to all of the risks involved in human gene therapy.

Splenomegaly is a condition of the spleen causing it to be enlarged. The splenic condition involving Felty syndrome is more specifically noted as "inflammatory" splenomegaly. The spleen is an important lymphatic organ that is involved in filtration of the blood by discarding old and damaged red blood cells as well as maintaining platelet levels. The spleen is a lymphatic organ, which means it is largely involved in the immune system and immune responses. When the spleen becomes enlarged, it is a strong sign of infection somewhere in the body, and can be caused by inflammatory conditions such as rheumatoid arthritis. The increased need for production assistance of white blood cells to affected areas causes hyperfunction of the spleen. This increase in defense activities ultimately causes hypertrophy of the spleen, leading to splenomegaly. The spleen is found in the left upper quadrant (LUQ) of the peritoneal cavity and due to its enlargement, can cause stress on neighboring organs.

The symptoms of Felty's syndrome are similar to those of rheumatoid arthritis. Patients suffer from painful, stiff, and swollen joints, most commonly in the joints of the hands, feet, and arms. In some affected individuals, Felty's syndrome may develop during a period when the symptoms and physical findings associated with rheumatoid arthritis have subsided or are not present; in this case, Felty's syndrome may remain undiagnosed. In more rare instances, the development of Felty's syndrome may precede the development of the symptoms and physical findings associated with rheumatoid arthritis.

Felty's syndrome is also characterized by an abnormally enlarged spleen (splenomegaly) and abnormally low levels of certain white blood cells (neutropenia). As a result of neutropenia, affected individuals are increasingly susceptible to certain infections. Keratoconjunctivitis sicca may occur due to secondary Sjorgen's syndrome. Individuals with Felty's syndrome may also experience fever, weight loss, and/or fatigue. In some cases, affected individuals may have discoloration (abnormal brown pigmentation) of the skin, particularly of the leg, sores (ulcers) on the lower leg, and/or an abnormally large liver (hepatomegaly). In addition, affected individuals may have abnormally low levels of circulating red blood cells (anemia), a decrease in circulating blood platelets that assist in blood clotting functions (thrombocytopenia), abnormal liver function tests and/or inflammation of the blood vessels (vasculitis).

High platelet levels do not necessarily signal any clinical problems, and are picked up on a routine full blood count. However, it is important that a full medical history be elicited to ensure that the increased platelet count is not due to a secondary process. Often, it occurs in tandem with an inflammatory disease, as the principal stimulants of platelet production (e.g. thrombopoietin) are elevated in these clinical states as part of the acute phase reaction.

High platelet counts can occur in patients with polycythemia vera (high red blood cell counts), and is an additional risk factor for complications.

A very small segment of patients report symptoms of erythromelalgia, a burning sensation and redness of the extremities that resolves with cooling and/or aspirin use.

Scientific literature sometimes excludes thrombocytosis from the scope of thrombophilia by definition, but practically, by the definition of thrombophilia as an increased predisposition to thrombosis, thrombocytosis (especially primary thrombocytosis) is a potential cause of thrombophilia. Conversely, secondary thrombocytosis very rarely causes thrombotic complications.

An accessory spleen ("supernumerary spleen", "splenule", or "splenunculus") is a small nodule of splenic tissue found apart from the main body of the spleen. Accessory spleens are found in approximately 10 percent of the population and are typically around 1 centimeter in diameter. They may resemble a lymph node or a small spleen. They form either by the result of developmental anomalies or trauma. They are medically significant in that they may result in interpretation errors in diagnostic imaging or continued symptoms after therapeutic splenectomy.

If splenectomy is performed for conditions in which blood cells are sequestered in the spleen, failure to remove accessory spleens may result in the failure of the condition to resolve. During medical imaging, accessory spleens may be confused for enlarged lymph nodes or neoplastic growth in the tail of the pancreas, gastrointestinal tract, adrenal glands or gonads.