White pulp is a histological designation for regions of the

spleen The spleen is an organ found in almost all vertebrates. Similar in structure to a large lymph node, it acts primarily as a blood filter. The word spleen comes .

(named because it appears whiter than the surrounding

red pulp The red pulp of the spleen is composed of connective tissue known also as the cords of Billroth and many splenic sinusoids that are engorged with blood, giving it a red color. Its primary function is to filter the blood of antigens, microorgani ...

on gross section), that encompasses approximately 25% of splenic tissue. White pulp consists entirely of

lymphoid tissue The lymphatic system, or lymphoid system, is an organ system in vertebrates that is part of the immune system, and complementary to the circulatory system. It consists of a large network of lymphatic vessels, lymph nodes, lymphatic or lymphoid o ...

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

periarteriolar lymphoid sheaths Periarteriolar lymphoid sheaths (or periarterial lymphatic sheaths, or PALS) are a portion of the white pulp of the spleen. They are populated largely by T cells and surround central arteries within the spleen; the PALS T-cells are presented with bl ...

(PALS) are typically associated with the arteriole supply of the spleen; they contain

T lymphocytes A T cell is a type of lymphocyte. T cells are one of the important white blood cells of the immune system and play a central role in the adaptive immune response. T cells can be distinguished from other lymphocytes by the presence of a T-cell rec ...

. * Lymph follicles with dividing

B lymphocytes B cells, also known as B lymphocytes, are a type of white blood cell of the lymphocyte subtype. They function in the humoral immunity component of the adaptive immune system. B cells produce antibody molecules which may be either secreted or ...

are located between the PALS and the

marginal zone The marginal zone is the region at the interface between the non-lymphoid red pulp and the lymphoid white-pulp of the spleen. (Some sources consider it to be the part of red pulp which borders on the white pulp, while other sources consider it to ...

bordering on the red pulp.

IgM Immunoglobulin M (IgM) is one of several isotypes of antibody (also known as immunoglobulin) that are produced by vertebrates. IgM is the largest antibody, and it is the first antibody to appear in the response to initial exposure to an antig ...

and

IgG2 Immunoglobulin G (Ig G) is a type of antibody. Representing approximately 75% of serum antibodies in humans, IgG is the most common type of antibody found in blood circulation. IgG molecules are created and released by plasma B cells. Each IgG ...

are produced in this zone. These molecules play a role in opsonization of extracellular organisms, encapsulated bacteria in particular. * The

exists between the white pulp and red pulp. It is located farther away from the central arteriole, in proximity to the

. It contains antigen-presenting cells (APCs), such as

dendritic cells Dendritic cells (DCs) are antigen-presenting cells (also known as ''accessory cells'') of the mammalian immune system. Their main function is to process antigen material and present it on the cell surface to the T cells of the immune system. The ...

and

macrophage Macrophages (abbreviated as M φ, MΦ or MP) ( el, large eaters, from Greek ''μακρός'' (') = large, ''φαγεῖν'' (') = to eat) are a type of white blood cell of the immune system that engulfs and digests pathogens, such as cancer cel ...

s. Some of the white pulp's macrophages are of a specialized kind known as metallophilic macrophages.

Macrophages in the white pulp

The

T cell A T cell is a type of lymphocyte. T cells are one of the important white blood cells of the immune system and play a central role in the adaptive immune response. T cells can be distinguished from other lymphocytes by the presence of a T-cell r ...

zone (periarteriolar sheath) and

B cell B cells, also known as B lymphocytes, are a type of white blood cell of the lymphocyte subtype. They function in the humoral immunity component of the adaptive immune system. B cells produce antibody molecules which may be either secreted or ...

follicles contain discrete

populations; however, not much is known about these macrophage populations in terms of their origin and lifespan. These macrophages are not unique to the spleen but instead make up an integral part of the lymphoid parts of all

secondary lymphoid organ The lymphatic system, or lymphoid system, is an organ system in vertebrates that is part of the immune system, and complementary to the circulatory system. It consists of a large network of lymphatic vessels, lymph nodes, lymphatic or lymphoid ...

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

germinal centre Germinal centers or germinal centres (GCs) are transiently formed structures within B cell zone (follicles) in secondary lymphoid organs – lymph nodes, ileal Peyer's patches, and the spleen – where mature B cells are activated, prolifera ...

reaction in the process of

somatic hypermutation Somatic hypermutation (or SHM) is a cellular mechanism by which the immune system adapts to the new foreign elements that confront it (e.g. microbes), as seen during class switching. A major component of the process of affinity maturation, SHM div ...

and

isotype switching Immunoglobulin class switching, also known as isotype switching, isotypic commutation or class-switch recombination (CSR), is a biological mechanism that changes a B cell's production of immunoglobulin from one type to another, such as from the ...

. B cells that cannot form their appropriate receptors will die of

apoptosis Apoptosis (from grc, ἀπόπτωσις, apóptōsis, 'falling off') is a form of programmed cell death that occurs in multicellular organisms. Biochemical events lead to characteristic cell changes (morphology) and death. These changes incl ...

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 macrophage A tingible body macrophage (TBMs) is a type of macrophage predominantly found in germinal centers, containing many phagocytosis, phagocytized, apoptosis, apoptotic cells in various states of degradation, referred to as tingible bodies (tingible me ...

s. (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 Phosphatidylserine (abbreviated Ptd-L-Ser or PS) is a phospholipid and is a component of the cell membrane. It plays a key role in cell cycle signaling, specifically in relation to apoptosis. It is a key pathway for viruses to enter cells via ap ...

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

monocyte Monocytes are a type of leukocyte or white blood cell. They are the largest type of leukocyte in blood and can differentiate into macrophages and conventional dendritic cells. As a part of the vertebrate innate immune system monocytes also inf ...

s 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 Antigen presentation is a vital immune process that is essential for T cell immune response triggering. Because T cells recognize only fragmented antigens displayed on cell surfaces, antigen processing must occur before the antigen fragment, n ...

or the removal of dying

lymphocyte A lymphocyte is a type of white blood cell (leukocyte) in the immune system of most vertebrates. Lymphocytes include natural killer cells (which function in cell-mediated, cytotoxic innate immunity), T cells (for cell-mediated, cytotoxic ad ...

References

External links

* * * - "Spleen: White Pulp splenic nodule" Spleen (anatomy) {{lymphatic-stub

Macrophages in the white pulp

See also

References

External links