Spleen
Unlike the lymph node, the spleen is not connected to the lymphatic network, but only to the blood network thanks to an artery and a splenic vein. However, it drains more lymphocytes every day than all the lymph nodes. The spleen is therefore specialised in filtering and presenting antigens from the blood to lymphocytes.
Structure
The spleen is an organ located in the left hypochondrium. Ovoid in shape, it is encapsulated and the capsule extensions (also called trabeculae) structure different compartments. It is divided into three distinct tissues:
- red pulp (99%) with mainly macrophages phagocytosing the red blood cells at the end of their life.
- white pulp (1%) consisting of:
- PALS (PeriArteriolar Lymphoid Sheat): area located near vessels concentrating T cells and dendritic cells.
- lymphoid follicles, zone of B cells, which can transform into secondary follicles if germinal centres are formed (principle identical to that of lymph node).
- The marginal area, located between the white pulp and the red pulp, is composed mainly of macrophages and B cells. These B cells are specific enough to constitute a special population (B cells of the marginal zone) with specific properties (see B cell activation / B cell ontogeny).
Primary functions
The spleen being irrigated only by the blood circulation (and not lymphatic), the antigens, as well as the T and B cells, all arrive via the splenic artery, pass through the arteriolar network, then encounter the marginal zone.
In the marginal zone, antigens are captured:
- either by dendritic cells , which then migrate to PALS,
- or by B cells , which then migrate to the follicles.
At the same time, blood T and B cells pass through
- T cells transit to PALS. This is the site of their interaction with dendritic cells and their activation (see T cell activation).
- B cells pass to the follicles. This is the site of their activation and maturation into plasma cells (see B cell activation).
What should be remembered
Much like the lymph nodes, the spleen is organised to potentiate two primary functions:
- maximises the chances of encounter between the lymphocytes (T or B cells) and the antigen, which can be recognised by these lymphocytes.
- If this encounter takes place, allows the activation of lymphocytes (T or B cells), so that they can exercise their cytotoxic or auxiliary functions (for CD8+ and CD4+ T cells, respectively) or secretion of antibodies (B cells differentiated into plasma cells).
