White pulp

White pulp
White pulp
	trabecular vein
Identifiers
Latin	noduli lymphoidei splenici
TA98	A13.2.01.006
TA2	5176
FMA	62805
	Anatomical terminology [edit on Wikidata]

White pulp is a histological designation for regions of the

lymphoid tissue

.

Specifically, the white pulp encompasses several areas with distinct functions:

The
T lymphocytes
.
Lymph follicles with dividing
encapsulated bacteria
in particular.
The
dendritic cells and macrophages. Some of the white pulp's macrophages are of a specialized kind known as metallophilic macrophages.^[2]

Macrophages in the white pulp

The

secondary lymphoid organs

.

In the B cell follicles, the macrophages are important in clearing the apoptotic B cells that occur during the

isotype switching. B cells that cannot form their appropriate receptors will die of apoptosis and are subsequently cleared by the macrophages in the germinal centre. During intensive germinal centre reactions, this process is obvious due to the presence of the large macrophages in the germinal centre, known as tingible body macrophages. (They're named this because their 'tingible bodies' represent condensed apoptotic nuclei.) In order for the apoptotic cells to be taken up by macrophages, it is important that phosphatidylserine

is expressed on the outer surface of the apoptotic cells, which is recognized by multiple receptors. The tangible body macrophages express: tyrosine kinase Mer, the milk fat globule epidermal growth factor 8 and Tim-4, all of which supports the engulfment of the apoptotic cells into the macrophages.

Macrophages are also present in the T cell area of the white pulp but their role is less well understood. This population of macrophages can be found in all the other T cell zones of the secondary lymphoid organs. It is possible that these macrophages are descendants of patrolling monocytes that entered the white pulp from the blood. Due to them being positioned alongside T cells, it is suggested that these macrophages have a role in antigen presentation or the removal of dying lymphocytes.^[1]