Function of The Spleen

White Pulp

The white pulp is lymphoid tissue involved in normal immune responses to blood-borne pathogens. It is arranged around a central arteriole and contains specialised regions of immune cells, including:

• Periarteriolar lymphoid sheaths (PALS) – rich in T-lymphocytes and macrophages.
• Lymphoid follicles – rich in naive B-lymphocytes.
• A marginal zone – contains macrophages and specialised B-cells involved in antigen capture and presentation.

Antigens entering the spleen are taken up by antigen-presenting cells (APCs). If they enter the PALS they can be presented to T-lymphocytes which become activated and stimulate B-lymphocytes in the follicles. This causes the B-lymphocytes to differentiate into antibody producing plasma cells. The response initially produces IgM, followed later by IgG.

Pathogens that enter the lymphoid follicles are detected by B-lymphocytes which present the antigen to T-lymphocytes. This leads to a process known as co-stimulation, in which the two cell types activate each other. When the T-lymphotcytes activates the B-lymphocyte they can differentiate into plasma cells.

The white pulp is particularly important for protection against encapsulated bacteria. Common examples can be remembered using “SHiN” and include:

• Streptococcus pneumoniae.
• Haemophilus influenzae. 
• Neisseria meningitidis. 

Encapsulated bacteria are difficult for phagocytes to engulf due to their smooth, negatively charged surface, so effective antibody production and opsonisation are essential for clearance.

The white pulp is surrounded by red pulp, which is responsible for filtration and removal of abnormal blood cells.

Red Pulp

The red pulp makes up around 80% of the spleen and is primarily responsible for blood filtration. It is composed of:

• Splenic cords (cords of Billroth) – contain macrophages.
• Venous sinuses – allow blood cells to re-enter the circulation.

Blood passing through the red pulp must move through narrow gaps in the venous sinuses. Healthy red blood cells are flexible enough to pass through, whereas older or abnormal red blood cells are less deformable and become trapped in the cords. These cells are then removed by macrophages.

The main functions of the red pulp include:

• Removal of old or abnormal red blood cells
• Removal of red cell inclusions (e.g. Howell-Jolly bodies)
• Phagocytosis of opsonised bacteria
• Sequestration of platelets (approximately 1/3 of the body’s platelets are stored in the spleen)
• Storage of red blood cells in case of hypovolaemia
• Acts as a reservoir for monocytes – may be mobilised during inflammation and tissue injury
• Haematopoiesis – during fetal development, before the bone marrow becomes the dominant site during the 5th month of gestation

By CNX OpenStax [CC BY 4.0 (https://creativecommons.org/licenses/by/4.0)], via Wikimedia Commons

Fig 1
Diagram showing the location and structure of the spleen.

Clinical Relevance

Splenomegaly

Splenomegaly refers to enlargement of the spleen. It may occur due to infection, portal hypertension, or haematological disease (e.g. leukaemia, lymphoma, or haemolytic anaemic).

The spleen is not normally palpable on abdominal examination. In splenomegaly, however, the spleen enlarges diagonally towards the right iliac fossa (RIF). Therefore, palpation should begin in the RIF and progress towards the the LUQ.

Splenomegaly can lead to hypersplenism, where increased sequestration and destruction of blood cells results in:

• Thrombocytopenia
• Anaemia
• Leukopenia

Management depends on the underlying cause. In some cases, splenectomy (removing the spleen) may be required.

Hellerhoff. Labeling by Mikael Häggström, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons

Fig 2
CT scan showing splenomegaly.

Clinical Relevance

Asplenia

Asplenia refers to absence of a functional spleen. This may be congenital, or acquired following splenectomy or diseases such as sickle cell anaemia (autosplenectomy).

Patients are left immunocompromised and therefore are at higher risk of severe infection, particularly from encapsulated bacteria.

Blood films in asplenic patients may show Howell-Jolly bodies (red blood cell inclusions), due to reduced splenic filtration of red blood cells.

To reduce infection risk, patients are offered vaccination against encapsulated bacteria and are often given prophylactic antibiotics (particularly children and high-risk patients). Annual influenza vaccination is also recommended. Patients should seek urgent medical advice if they develop signs of infection.

Many patients also carry an asplenia alert card, pendant or bracelet to inform healthcare professionals in emergencies.

Paulo Henrique Orlandi Mourao, [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons

Fig 3
Blood film showing Howell-Jolly bodies (small, round inclusions in red blood cells stained purple).