Lymphocytes are the smallest and the second most common type of white blood cell. They are known as the cells of immunity. Lymphocytes have large round/oval nuclei that occupies most of the cell with little cytoplasm. The nucleus will stain dark purple or blue when it is exposed to a certain stain called Wright’s stain (please see figure 1 ).

Lymphocytes function to provide a specific response to invading organisms.

There are two types of lymphocytes T lymphocytes and B lymphocytes which the immune response depends upon. They are derived from the hematopoietic stem cell (HSC).

 Lymphopoiesis

The development of lymphocytes begins as the earliest branch from HSC.

As we already know, HSC can develop into common lymphoid progenitor (CLP) or common myeloid progenitor (CMP). CLP then will generate T and B lymphocytes while CMP generates myeloid elements. So, lymphocytes originate from CLP in the bone marrow.

One should note that during postnatal life, the bone marrow and thymus are the primary lymphoid organs. The secondary lymphoid organs in which specific immune response take place are the lymph nodes, spleen and lymphoid tissue.

 B-Lymphocyte Development

B-cells originate in the bone marrow and circulate the peripheral blood until they recognize an antigen. There are various stages in the development of B lymphocytes:

1. Stem Cell (HSC)
2. CLP
3. Pro-B cell
4. Pre-B cell
5. Immature B cell
6. Mature B cell

Their development is altered by cytokines. Interleukin-7 interacts with stem cell factor to begin the process. In order for the cells to function, they will express on their surface immunoglobulin (Ig).

Ig production begins at the Pro-B cell stage of B cell development.  Ig is needed for the development and maturation of the B cell from CLP to pre-B cell.

First there will be an expression of IgM, then IgD and finally IgG or IgA.  The termination of B cell will take place in the peripheral lymph organs such as the spleen and lymph nodes.  In these organs B cells change into plasma cells and memory cells.

The plasma cells are capable of producing and releasing antibodies, while the memory cells remember antigens that they were once exposed to in the past.

T-Lymphocyte Development

T- Lymphocyte development begins with CLP cells that migrate to the thymus where they will differentiate into mature T cells. It is associated with the movement of the cells through the cortex and medulla of the thymus. Maturing begins in the cortex, and as the cells develop more they move towards the medulla.

During the developmental stages of T cells, the cells contain specific surface proteins. As the cell progresses through maturity, they are identified by antibodies that recognize the surface proteins. In other words, there are cell surface markers which are used by antibodies to identify the T cell.

T cell eventually differentiates into two types of T lymphocytes: cytotoxic T-cell and helper T-cell. Cytotoxic T-cell is important because they produce a substance called lymphokines which help B cells destroy foreign substances. Cytotoxic T-cells also have CD8 antigen.

Helper T-cell on the other hand produces CD4 antigen and functions to assists killer T cells with the protection of the body against invading organisms.

Like B-cells, T-cells depend on interleukin-7 and other interleukins. One should note that most of T-cell development occurs in the thymus; however the final steps in which cytotoxic and helper T-cell are produced occur in the peripheral blood.