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).