The human immune system (and those of most mammals) is composed of physical barriers (skin and mucus), innate and adaptive cells, chemical messengers (cytokines), and primary and secondary lymphoid tissue.
Primary lymphoid tissue includes the bone marrow and the thymus. Immune cell progenitors arise in the bone marrow, along with red blood cells and platelets (Figure 1). Bone marrow is divided into red marrow and yellow marrow. The red marrow is the site of white blood cell progenitor development, and B cell differentiation.
Precursors to T cells travel to the other primary lymphoid organ, the thymus. The thymus is a small organ sitting on the anterior side of the heart. It is the site of T cell education, primary tolerance induction, and differentiation. The thymus is most active post-birth up to preadolescence, then becomes progressively smaller and less active (thymic involution).
In the thymus, T cells undergo differentiation, and are then exported to the rest of the body to mature and respond to pathogenic insult.
Secondary lymphoid organs include the spleen, Peyer’s patches, lymph nodes, tonsils, and adenoids. Transit between primary and secondary organs occurs via the circulatory and lymphatic systems. The lymphatic system is a motion-driven set of vessels which move a clear protein rich fluid, lymph, and white blood cells toward the heart, where they re-enter the circulatory system.
The spleen is a critical site of B cell-T cell interaction, B cell maturation and the amplification of adaptive immune reactions. The spleen has multiple lymphocyte zones, each with a different function.
Peyer’s patches are specialized lymphoid tissue found throughout the digestive system. They serve a critical function in sampling and monitoring intestinal bacterial populations.
Lymph nodes are small, encapsulated organs found distributed throughout the body. Lymph drains from areas of the body through the local lymph node. Within the lymph node are resident antigen-presenting cells and immune cells which can recognize and amplify signals generated by infection. In the course of infection, autoimmune disease, cancer and other pathogenic states, lymph nodes can swell, becoming hard and tender.
Tonsils and adenoids are found in the throat and represent the first line of defense against ingested or respiratory pathogens. M cells, special antigen-capturing cells on the tonsillar surface, “grab” antigen from pathogens and present it to B cell and T cells resident in the tonsil. B cells, in response to antigen, can then mature and proliferate to fight infection.
Featured Image: Bobjgalindo, Wikimedia Commons CC BY-SA 4.0