Lymphoid Organs and Immune System

Lymphoid Organs

• Secondary lymphoid organs, such as the spleen and lymph nodes, are sites where antigen-driven proliferation and maturation of lymphocytes occur.
• Primary lymphoid organs, such as the liver (in the fetus) and bone marrow (after birth), are responsible for generating lymphoid cells through lymphopoiesis.

Thymus

• The thymus is a pyramid-shaped gland located beneath the breastbone, responsible for educating T lymphocytes to distinguish between self and non-self.
• Immature lymphocytes leave the bone marrow and circulate through the thymus, where they are committed to a specific pathway of activity and are ready to perform their effector functions.

Lymph Nodes

• Lymph nodes are focal nodules connected via the draining lymphatic highway, placed throughout the body, with groupings found in the groin, armpits, and abdomen.
• Lymph nodes are local nodes for antigen and cellular drainage, allowing lymphocytes to interact and communicate with APCs, enabling local presentation of antigenic particulates.
• Germinal centers are areas of response within lymph nodes, where antigen-driven proliferation and differentiation occur.

Spleen

• The spleen is a filtering organ for circulating blood, located in the upper portion of the abdomen, where resting lymphocytes reside to readily mobilize in response to the detection of foreign materials.
• The spleen acts as a holding facility where both innate and resting adaptive cells reside in specialized compartments.

Immune System

• The immune system is a network of protective barriers, organs, cells, and molecules that provide a coordinated response to foreign agents.
• The immune system consists of central lymphoid organs (e.g., bone marrow, thymus, spleen, and lymph nodes) containing high levels of lymphocytes, which mature, differentiate, and multiply.
• Leukocytes of myeloid and lymphoid origin can be found within every major organ and tissue, available to be called into action at short notice.