The developmental routes for these cells are very complex and unique but there are similarities. B-cells produce antibodies to bind onto foreign bodies that have invaded the host organism; this allows cells such as cytotoxic T-cells will then destroy the infected cell/structure. On the other hand, T-cells such as T-helper cells that secrete cytokines to control immune responses and cytotoxic T-cells that destroy pathogenic cells and structures.
B-cells develop from pluripotent hematopoietic stem cells, which give rise to lymphoid progenitor cells in the bone marrow. These stem cells have become Pro-B cells when they have begun to express B-cell marker proteins such as CD34 and rearrange the genes that code for the heavy chains in its B-Cell Receptor (BCR). In the next stage of development, Pro-B cells will become Pre-B cells. The transition to Pre-B cells involves the expression of the μ heavy chain on the cell surface. “The RNA transcript is spliced to produce the mRNA” to synthesize the μ chain and express it on the surface of the cell in the Pro-B cell to Pre-B cell stage. Now the Pre-B cell can develop into an immature B-cell. Light chains of the BCR need to be expressed before the cell can be called an immature B-cell. Once this is complete, an immature B-cell has been produced.
Immature B-cells play a vital role in the prevention of autoimmunity. If immature B-cells happen to be self-reactive, autoimmunity can arise. These cells would be killed as soon as they come into contact with a self-antigen, as they are so self-reactive. Cells will abnormally high self-reactivity are killed via apoptosis before they can enter the general cell population, this is where ~90% of produced cells are lost before being exported out of the bone marrow. Immature B-cells now migrate from the primary to the secondary lymphoid organs such as the spleen to mature. 
T-cells develop from a lymphoid progenitor that has been developed in the bone marrow. T-cell development can be separated into early and late stages by the lack of CD4+ and CD8+ on the cell surface. A lymphoid progenitor will begin to develop into a T-cell if the Notch1 protein is overexpressed in the cell. During the double negative 1 (DN) phase of development, the cells are migrating into the thymic cortex as pro-thymocytes from the bone marrow. They are yet to commit to a lineage so can still mature into other types of T-cell such as natural killer T-cells, intraepithelial lymphocytes and regulatory T-cells. Also, the cells are yet to express CD4+ or CD8+ and only express c-kit (CD117) and CD25 on their cell membrane.
DN2 is a vital phase, it occurs in the subcapsular cortex of the thymus. It is here that the “genes for the gamma, delta and beta T-cell receptor (TCR) chains”  start their rearrangement process. The alpha chain does not commence its rearrangement, as the recombinase machinery cannot reach its gene. In this phase, cells fully commit to the...