T cells originate in the bone marrow and then mature in the thymus.
Here's a more detailed breakdown:
-
Bone Marrow: The Starting Point: T cells begin their life cycle as hematopoietic stem cells located within the bone marrow. These stem cells are the precursors to all blood cells, including T cells.
-
Thymus: The Maturation Center: From the bone marrow, immature T cells, also called T-cell precursors or thymocytes, migrate to the thymus. The thymus is a specialized organ located in the upper chest. It is within the thymus that T cells undergo a rigorous selection and maturation process.
-
Selection and Education: During their time in the thymus, T cells are "educated" to distinguish between the body's own cells (self) and foreign invaders (non-self). This process involves:
- Positive Selection: T cells are tested for their ability to bind to major histocompatibility complex (MHC) molecules, which present antigens. T cells that cannot bind to MHC undergo apoptosis (programmed cell death).
- Negative Selection: T cells that bind too strongly to self-antigens presented on MHC molecules are also eliminated through apoptosis. This prevents the mature T cells from attacking the body's own tissues, a phenomenon known as autoimmunity.
-
Release into Circulation: Only T cells that successfully pass both positive and negative selection are allowed to mature and leave the thymus. These mature T cells then circulate in the bloodstream and lymphatic system, ready to respond to threats.
-
Lymphoid Tissue: After maturing, T cells reside in secondary lymphoid organs such as lymph nodes, the spleen, and mucosal-associated lymphoid tissue (MALT). This is where they encounter antigens and initiate immune responses.
In summary, T cells are born in the bone marrow, educated in the thymus, and then patrol the body, primarily residing in lymphoid tissues, to defend against infection and disease.
