31. The immune system. Central and peripheral lymphoid organs.

Origin of Lymphocytes

• All lymphocytes begin their development from pluripotent hematopoietic stem cells located in the bone marrow
• These stem cells give rise to multipotent progenitor cells which further differentiate into:
• Common myeloid progenitors that lead to the myeloid lineage
• Multi-lymphoid progenitors that give rise to lymphoid cells
• Some lymphoid progenitors remain in the bone marrow to become B cells or natural killer cells
• Others leave the bone marrow and migrate to the thymus where they begin their journey to become T cells

Central or Primary Lymphoid Organs

• The bone marrow and thymus are known as the central lymphoid organs
• These organs are responsible for the generation and maturation of lymphocytes but do not participate in immune responses
• The key function of these organs is to help immature lymphocytes develop antigen receptors, making them immunocompetent
• This differentiation is known as antigen-independent because it occurs before the lymphocytes encounter any external antigen

B Cell Differentiation in the Bone Marrow

Structure and Environment

• Bone marrow is located in the spongy cavities of bones and provides a nurturing environment for hematopoiesis
• Differentiation of B cells occurs in specialized regions called niches which provide necessary growth signals
• These niches are formed by interactions between developing cells and supportive stromal cells that create a three-dimensional network
• Stromal cells regulate access to nutrients, oxygen, and survival signals and are essential for proper B cell development

Stages of B Cell Development

• Common lymphoid progenitors commit to the B cell lineage and first become pro-B cells
• Pro-B cells undergo rearrangement of the immunoglobulin heavy chain genes through VDJ recombination
• This process is tightly regulated through allelic exclusion to ensure only one heavy chain is produced
• If the first attempt fails, rearrangement is attempted on the second chromosome
• If both attempts fail, the cell undergoes programmed cell death
• Successfully rearranged pro-B cells become pre-B cells and begin rearranging the light chain genes (kappa first, followed by lambda if needed)
• Once a functional light chain is produced, the B cell expresses surface IgM and becomes an immature B cell

Antigen Receptor Construction

• The B cell receptor or BCR is not pre-encoded but is assembled from multiple gene fragments using VDJ recombination
• Terminal deoxynucleotidyl transferase introduces random nucleotides at the junctions, contributing to antibody diversity
• Successful assembly of the BCR is the hallmark of immunocompetence in B cells

Central Tolerance and Negative Selection

• Immature B cells undergo a tolerance check to ensure they do not strongly react to self-antigens