T-cells mature in the thymus gland or in the lymph nodes. Since the thymus is only 10-15% functional in the adult, the lymph nodes take on greater importance in the maturation process.
Thymus Gland: T Cells migrating to the Thymus gland from the bone marrow will undergo a process of selection to eliminate not only the weakest cells, but also those so strong that they may attack healthy tissue cells (autoimmunity). Cells educated in the Thymus generally are either Helper (CD4+) or Suppressor/Cytotoxic (CD8+) cells. Other types of T-helper cells include T-helper 17, T regulatory cells, and T follicular helper cells.
Lymph Node: Naïve T cells in the paracortex of the lymph node may be activated by dendritic cells that have internalized and processed pathogenic antigens that made their way to the lymph node via lymphatic drainage from the site of infection or inflammation; or, by dendritic cells that have migrated to the lymph node from the site of infection. Once activated, T cells undergo clonal expansion and differentiate into functional effector cells (short-lived) or memory effector cells (long-lived). Functional effector cells migrate to the site of infection or inflammation where they orchestrate T helper (CD4+) or T cytotoxic/suppressor (CD8+) functions to combat pathogens. Memory cells may enter the circulation or healthy tissue sites, or remain in the lymph node.
The B Cell: B cells mature in the bone marrow or in the lymph node.
Bone Marrow: Mature B cells express antibodies on their surface, which are specific for a particular antigen. The antibodies are expressed on the cell surface and are primarily IgM with some IgD. These cells circulate in the blood or home to sites of infection or inflammation. However, until they are activated by T-cells, they do not proliferate or differentiate to form antibody producing Plasma Cells.
Lymph Node: Antigen-dependent B cells in the cortex of the lymph node may be stimulated by Helper T cells to proliferate and differentiate into Plasma Cells and memory cells. Immunoglobulin (antibody) class switching of the B cell from IgM to IgG, IgA or IgE may also take place as a result of the T cell interaction.
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