Lymphoid Organs and Immune System

Study Notes

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.

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 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.

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.

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.