About Lymphocytes

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Belajarbiologi  Fellow reader, Know we will learn more about lymphocytes. In this post we will explain it more further.

LYMPHOCYTES

How do lymphocytes look?

Morphologically, both T and B lymphocytes are small cells whose diameter (8–10 mm unless activated) is comparable with that of large bacteria.

They are spherical in shape and have a relatively large nucleus surrounded by a thin rim of cytoplasm. Organelles in resting lymphocytes are developed rather poorly.

Electron microscopy reveals a number of microvilli on their surfaces. This ‘‘boring,’’ bland morphological pattern says virtually nothing about the complexity of the function that lymphocytes perform.

Methods based on immunochemistry and molecular biology show that the surface of a lymphocyte contains thousands of different kinds of molecules, many of which serve as important intermediaries during antigen recognition, cellular activation, differentiation, and attack against antigens.

Lymphocyte colored blue

 Where are lymphocytes located?

Lymphocytes mature from their precursors in the so-called primary,orgenerative, lymphoid organs. These are the bone marrow, the bursa of Fabricius (in birds), and the thymus.

After maturing, lymphocytes circulate in the peripheral blood, although they do not function there. The recognition of antigens, as well as the complex events that follow this recognition, happen mostly in the so-called secondary lymphoid organs.

These are anatomically de?ned organs like the spleen, lymph nodes, tonsils, appendix, and Peyer’s patches (accumulations of lymphocytes in the small intestine). In addition, lymphocytes accumulate as diffuse conglomerates in all tissues except the central nervous system. 

Why are the primary lymphoid organs called generative? 

In these organs, lymphocytes are ‘generated’; they mature from their precursors. In the 1950s to 1960s, it was firmly established that all ‘blood cells’, including lymphocytes, originate from a common precursor cell called a ‘‘stem cell’.

The process of B lymphocyte maturation from stem cells takes place in the bone marrow. In birds, B lymphocyte precursors develop in the bone marrow and then in a special organ called the ‘‘bursa of Fabricius.’’.

Only those avian lymphocytes that pass through the bursa during embryogenesis become cells that produce antibodies. There are no known homologues of the bursa in mammals.

It is thought that the entire process of B lymphocyte maturation in mammals can take place in the bone marrow. Therefore, the abbreviation ‘‘B’’ may point to the bursa of Fabricius or to the bone marrow. 

T lymphocyte precursors must undergo a stage of their maturation that takes  place entirely in the organ called the thymus. The latter is an encapsulated glandular organ located in the anterior mediastinum (behind the chest bone).

The thymus is relatively large in late fetuses and newborn infants and then undergoes an involution. In a newborn or in a young infant it occupies a sizeable space behind the chest bone; in an adult human, the thymus is invisible, hidden inside the layers of fat tissue.

This organ is classiffied as ‘‘generative’’ because, at least in early life, it is entirely responsible for the Finalization of T-lymphocyte maturation. The above process occurs through rigorous selection of the T-cell precursors in the thymus.

Are lymphocytes, as cell populations, homogenous? 

No. Both T and B lymphocytes are functionally heterogeneous and consist of more than one subpopulation (or cell subset). T lymphocytes can be subdivided into two major functionally distinct subsets: helper T cells (T ) and cytolytic T cells.

The former population consists of cells that do not attack antigens directly but, rather, differentiate into producers of biologically active substances called cytokines. Through these cytokines, as well as through direct contact, T promote or enhance the function of other cells (notably, B lymphocytes and macrophages).

T attack and destroy foreign cells directly by attaching to them and releasing cytotoxic granules that break the integrity of the target cell’s membrane and also destroy its DNA. T c h and T express TCRs that have the same structure. Other cell-surface molecules are different in these two subsets, however.

In particular, most Th c express a molecule called CD4, while most T express a somewhat similar but structurally different accessory molecule called CD8. All T cells that express CD8 recognize antigenic peptides in conjunction with MHC molecules that belong to the so-called ‘‘Class I’’ of the MHC molecules. The T cells that express CD4 recognize the antigenic peptides in conjunction with the so-called ‘‘MHC Class II’’ molecules. 

B lymphocytes also consist of two functionally distinct subpopulations or subsets. 

A subset called B-1 lymphocytes produces polyreactive antibodies, i.e., antibodies that can bind a more or less wide variety of antigens. B-2 lymphocytes produce antibodies that are usually monoreactive.

The paramount feature of B-1 lymphocytes is that they transcribe the gene that codes for a protein called CD5. This transcription may or may not result in the translation and the surface expression of CD5. B-1 lymphocytes are very abundant in fetal life, and in the adult they tend to accumulate in certain body compartments, e.g., the peritoneal cavity and the omentum.

The percentage of B-1 cells is very small in adult mice, and in adult humans may vary from a few percent to 25–30% of all B lymphocytes. It is thought that B-1 cells develop from precursors that are distinct from those of B-2 lymphocytes and constitute a self-sustaining (or self-replenishing) population of cells. B-1 and B-2 cells profoundly differ in their responses to stimulation of certain enzymatic (signal transduction) pathways. The exact function of B-1 lymphocytes remains unknown.