Immunology Lecture 3: Major Organs of the Immune System

This flashcard set covers the primary and secondary lymphoid organs, their histological features, functions in the immune response, and the mechanics of the lymphatic circulatory system.

Primary Lymphoid Organs

Sites where immune cells develop from stem cells to maturation and where positive and negative T and B cell selection occurs; specifically the bone marrow and thymus.

Secondary Lymphoid Organs

Highly structured organs, such as the spleen, lymph nodes, and MALT, where naïve T and B cells encounter antigens and become activated to initiate adaptive immune responses.

Hematopoiesis

The production of new blood cells, including erythrocytes $(RBCs)$, leukocytes $(WBCs)$, and platelets, occurring in the hollow interior of bones.

Bone Marrow Composition

A highly cellular tissue consisting of approximately $50\%$ stem cells and $50\%$ fat cells, with the fat percentage increasing with age.

DiGeorge Syndrome

An immune deficiency resulting from the absence of thymic tissue during development, leading to a loss of T cells.

Thymic Cortex

The outer region of the thymus that is darker in appearance and contains many immature T cells undergoing development.

Thymic Medulla

The central region of the thymus containing more mature T cells and Hassall’s corpuscles.

Positive Selection

The process in the thymus where a T cell receptor $(TCR)$ receives a signal to live if it can engage MHC with appropriate strength.

Negative Selection

The process where T cells are signaled for apoptosis if their $(TCR)$ binds too strongly to MHC.

Lymphadenopathy

The enlargement of lymph nodes during an active immune response due to the proliferation of lymphocytes.

High Endothelial Venules (HEVs)

Specialized vessels through which naïve T and B cells can directly enter the lymph nodes from the blood circulation.

Paracortex

The T cell zone within a lymph node where dendritic cells present antigens to T cells.

Red Pulp

Splenic tissue that filters blood, removes pathogens, recycles old erythrocytes, and holds a reserve of macrophages and dendritic cells.

White Pulp

The region of the spleen containing Periarteriolar lymphoid sheaths $(PALS)$ for T cells and lymphoid follicles for B cells.

Marginal Zone

The area between splenic red pulp and white pulp where antigen is trapped and presented to lymphocytes.

MALT

Mucosa-Associated Lymphoid Tissue, which includes system-specific subdivisions like GALT, BALT, and NALT.

M cells

Specialized cells in the Gut-Associated Lymphoid Tissue $(GALT)$ that perform luminal antigen transport of intact microbes to dendritic cells.

Tonsils

Secondary lymphoid tissue consisting of palatine, pharyngeal, and lingual sets that contain follicles and crypts to trap inhaled and ingested antigens.

Interstitial Fluid (ISF)

Plasma that has leaked out of capillaries to bathe tissues, with approximately $2-8\,L$ formed daily per person.

Lymphatic Fluid Composition

A fluid consisting of water, immune proteins, electrolytes, and immune cells, but notably containing NO $RBCs$.

Lymphedema

The ongoing buildup of interstitial fluid in tissues caused by the disruption of lymph vessels due to trauma, surgery, or infection.

Wuchereria bancrofti

The parasite responsible for causing filariasis or elephantiasis, which impairs lymphatic fluid flow through chronic inflammation and scarring.

Thoracic Duct

The large lymphatic vessel that drains lymph from most of the body into the left subclavian vein at the jugular-subclavian junction.

Right Lymphatic Duct

The vessel that drains lymph from the upper right quadrant of the body into the right subclavian vein.