The spleen

Published on 03/04/2015 by admin

Filed under Hematology, Oncology and Palliative Medicine

Last modified 22/04/2025

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The spleen

Although the spleen has been known of since ancient times, its function has remained obscure until relatively recently. Hippocrates thought it was the source of ‘black bile’. Galen suggested it might be a filter, in view of its spongy consistency. Our current understanding of the spleen is dependent on a detailed appreciation of its vascular supply and the organisation of its main component parts: the lymphoid white pulp, the blood-containing red pulp and the intervening marginal zone.

Structure

The spleen is derived from condensation of the mesoderm in the dorsal mesogastrium of the embryo. It plays a modest haematopoietic role in the middle part of fetal life, but in the adult haematopoiesis is usually only seen in pathological states. An average adult spleen weighs about 150 g and it has to become enlarged to at least three times its normal size before becoming palpable on clinical examination (p. 17).

The splenic artery penetrates the thick capsule which invests the organ (Fig 5.1). Branches of the splenic artery are surrounded by a highly organised aggregate of lymphoid tissue which is termed the ‘white pulp’ (Fig 5.2). Intimate to the central arteriole is the ‘periarteriolar lymphatic sheath’ – an area mainly populated by T-lymphocytes. Among these T-lymphocytes are non-phagocytic, antigen-presenting cells known as ‘interdigitating cells’. Spaced at intervals in the periarteriolar lymphatic sheath are lymphoid follicles (‘Malpighian bodies’). In an inactive state these follicles are composed of recirculating B-lymphocytes intertwined with cytoplasmic processes of follicular dendritic cells. The latter cells may play a role in long-term antibody production. When contact with antigen stimulates B-cell activation, a germinal centre of rapidly dividing cells forms in the follicle. This is a key area in the normal B-lymphocyte proliferative response and development of B-cell memory (see p. 8 for discussion of lymphocytes).

Fig 5.1 Structure of the spleen.
The white pulp is composed of the periarteriolar lymphatic sheath and lymphatic nodules. The red pulp contains the splenic cords and sinuses and is separated from the white pulp by the marginal zone. See text for full discussion.

Fig 5.2 Light microscopy of the spleen clearly showing the distribution of red and white pulp.

The periarteriolar lymphatic sheath and B-lymphocyte follicles are separated from the red pulp by a ‘marginal zone’ constituted mainly of non-circulating B-cells. The marginal zone also contains specialised macrophages able to take up carbohydrate antigens. The red pulp is composed of two alternating structures: the splenic sinuses and the splenic cords (the ‘cords of Billroth’). The cords are a reticular meshwork packed with macrophages and antibody-secreting plasma cells. The sinuses are broad channels lined with fusiform endothelial cells.

Most of the central arterioles open into the marginal zone. As alluded to already, circulating T-lymphocytes move into the periarteriolar lymphatic sheath and B-lymphocytes migrate to the follicles. Other blood cells move slowly through the complex meshwork of the red pulp, and cells which are sufficiently deformable and compliant squeeze between the endothelial cells in the sinus wall into the lumen of the sinus and back into the circulation. The organisation of the spleen into the different compartments is under the control of various cytokines and adhesion molecules.

Function

The spleen has two key functions. It removes older red cells, blood-borne microorganisms and cellular debris from the blood. It also plays a vital role in the body’s response to bacterial and fungal infections.

It clears unwanted red cells and particles from the blood in three ways. Firstly, they can be removed by phagocytes. Bacteria, particularly encapsulated organisms that are not opsonised by antibodies and complement, are cleared from the circulation. The spleen is probably the site of the initial immune response to these organisms. Phagocytic cells in the spleen also remove red cells coated with IgG antibody.

The second mechanism at work is the removal of red cells which are not sufficiently deformable to pass through the sinus wall. Pathological states where red cells lose deformability and are destroyed prematurely in the spleen include sickle cell anaemia, hereditary spherocytosis and malaria.

Finally, the spleen can remove debris or organisms from within cells. Howell–Jolly bodies (fragments of nucleus) and malarial parasites are removed when most of the cell passes through the inter-endothelial slit with the intracellular particle abandoned on the cord side.

The spleen has the capacity to mount complex innate and adaptive immune responses. Both types of response occur in the marginal zone, rich in macrophages and marginal zone B-cells, while the white pulp is limited to adaptive immunity.

Abnormal splenic states

Asplenism and hyposplenism

Surgical removal of the spleen (splenectomy) may be indicated in a variety of haematological disorders and following trauma. The spleen may also be absent as a congenital anomaly, often associated with transpositions or malformations of the great vessels and viscera (‘asplenia syndrome’). Reduced splenic function can result from splenic atrophy in disorders such as sickle cell anaemia, adult coeliac disease and essential thrombocythaemia (Table 5.1).

Hyposplenism leads to characteristic changes in the blood film (Fig 5.3). Changes in red cell appearance include the presence of Howell–Jolly bodies, Pappenheimer (siderotic) granules and target cells. Other less regular red cell features are lipid-rich acanthocytes and circulating nucleated cells. There is often a moderate rise in the lymphocyte, monocyte and platelet count. Approximately one-third of circulating platelets are pooled in the normal spleen. The increase in platelets post-splenectomy is frequently impressive (greater than 1000 × 10⁹/L) but the count usually falls to a lower level in the longer term. Quantitation of splenic function is not straightforward. Methods include the measurement of the percentage of pitted erythrocytes using interference phase microscopy, various immunological parameters and scintigraphy.

Fig 5.3 The blood film in hyposplenism.
A Howell–Jolly body is seen within a red cell. There are target cells and acanthocytes.

The clinical significance of an absent spleen is the associated increased risk of life-threatening infection. The risk is greatest in children under 5 years of age and where there is a serious underlying medical disorder such as Hodgkin’s lymphoma or thalassaemia. Most infections occur within 2 years of splenectomy but fulminating infection can strike at any stage. In most cases infection is with encapsulated bacteria, notably Streptococcus pneumoniae, Haemophilus influenzae and Neisseria meningitidis. In temperate regions more than half of serious infections are caused by the pneumococcus, with high mortality. Splenectomised patients have an increased susceptibility to severe malaria. Prophylaxis against such infections is the best approach and recommendations for the management of asplenic patients are shown in Table 5.2.

Table 5.2

Management recommendations in the asplenic patient

Immunisation¹	Pneumococcus, Haemophilus influenzae type B, group C meningococcus, influenza
Antibiotic prophylaxis²	Oral phenoxymethylpenicillin or erythromycin
Prompt treatment of infection	Patients need systemic antibiotics and urgent admission to hospital
Medicalert disc or card	Detailing asplenic state and medical contacts
Avoid travel to high-risk malarial areas