Page 358 - Textbook of Pathology, 6th Edition

342

Disorders of Leucocytes and
Chapter 14
Chapter 14
Lymphoreticular Tissues

As illustrated in Fig. 12.3, pluripotent stem cells in the bone the zone between the peripheral cortex and the inner medulla.
marrow divide to form two types of stem cells: myeloid The medulla is predominantly composed of cords of plasma
trilineage stem cells and lymphoid stem cells. Myeloid cells and some lymphocytes. The capsule and the structure
trilineage stem cells further divide to form three types of cell within the lymph node are connected by supportive delicate
lines: erythroid progenitor cells (discussed in Chapter 12), reticulin framework (Fig. 14.1,A).
megakaryocytic progenitor cells (discussed in Chapter 13), Functionally, the lymph node is divided into T and B
and granulocyte-monocyte progenitor cells. In this chapter, lymphocyte zones:
we will discuss the abnormalities pertaining to the remaining B-cell zone lies in the follicles in the cortex, the mantle
two cell lines—granulocyte-monocyte cell line and the zone and the interfollicular space, while plasma cells are also
lymphoid cell lines, besides discussion on all haematopoietic present in the interfollicular zone.
SECTION II
neoplasms. Both these cell lines constitute leucocyte pool in T-cell zone is predominantly present in the medulla.
the body at two distinct locations: in circulating blood and There are two main functions of the lymph node—to
in the tissues. This concept holds more true for lymphoid mount immune response in the body, and to perform the
cells in particular, which are present in circulation as well as function of active phagocytosis for particulate material.
are distributed in the lymphoid tissues of the body (lymph Besides T and B-cells, the follicular centre has dendritic
nodes, spleen, mucosa-associated lymphoid tissue—MALT, histiocytes and antigen-presenting Langerhans’ histiocytes
pharyngeal lymphoid tissue). B, T and NK lymphoid cells (formerly together called tingible body macrophages due to
are formed after differentiation from lymphopoietic engulfment of particulate material by them) and endothelial
precursor cells in the bone marrow undergo further cells. The follicular centre is a very active zone where
maturation in peripheral lymphoid organs and thymus lymphocytes from peripheral blood continuously enter and
(Chapter 4). Thus, relationship of haematopoietic tissues and leave, interact with macrophage-histiocytes and endothelial
lymphoreticluar tissues is quite close. In fact, current cells and undergo maturation and transformation.
lymphoma-leukaemia classification system does not consider Lymphocytes and endothelial cells have surface molecules
diseases of lymphocytes in the blood and in the lymphoid which interact and serve as ‘addresses’ so that endothelial
tissues as separate disorders but are seen to represent cells can direct the lymphocytes; these molecules are
different stages of the same biologic process. Hence, in appropriately termed as addressins or homing receptors.
contemporary learning of these diseases, it is desirable to Peripheral blood B and T lymphocytes on entering the lymph
study diseases of leucocytes together with diseases of node are stimulated immunologically which transforms them
lymphoreticular tissues of the body. to undergo cytoplasmic and nuclear maturation which may
be in the follicular centre or paracortex as per following
LYMPH NODES: NORMAL AND REACTIVE sequence and schematically depicted in Fig. 14.1,B:
Haematology and Lymphoreticular Tissues
i) Follicular centre, small non-cleaved cells or centroblasts
NORMAL STRUCTURE
ii) Follicular centre, small cleaved cells or centrocytes
The lymph nodes are bean-shaped or oval structures varying iii) Follicular centre, large cleaved cells
in length from 1 to 2 cm and form the part of lymphatic iv) Follicular centre, large non-cleaved cells
network distributed throughout the body. Each lymph node v) Immunoblasts (in paracortex)
is covered by a connective tissue capsule. At the convex vi) Convoluted cells or lymphoblasts (in paracortex)
surface of the capsule several afferent lymphatics enter which vii) Plasma cells.
drain into the peripheral subcapsular sinus, branch into the Lymph nodes are secondarily involved in a variety of
lymph node and terminate at the concavity (hilum) as a single systemic diseases, local injuries and infections, and are also
efferent lymphatic vessel. These lymphatic vessels are lined the site for some important primary neoplasms. Many of
by mononuclear phagocytic cells. these diseases such as tuberculosis, sarcoidosis,
The inner structure of the lymph node is divided into a histoplasmosis, typhoid fever, viral infections etc. have been
peripheral cortex and central medulla. The cortex consists of considered elsewhere in the textbook along with description
several rounded aggregates of lymphocytes called lymphoid of these primary diseases. Reactive lymphadenitis is
follicles. The follicle has a pale-staining germinal centre discussed below while the subject of lymphoid neoplasms
surrounded by small dark-staining lymphocytes called the including plasma cell disorders and Langerhans’ cell
mantle zone. The deeper region of the cortex or paracortex is histiocytosis is discussed under haematologic neoplasms.