Lymphoid System and Immune Organs

12 Questions

What is the primary function of the thymus in the lymphoid system?

To provide a site for development and differentiation of T lymphocytes

What is the function of Hassall's corpuscles in the thymus?

Their function is still unknown

What is the purpose of the blood-thymic barrier in the thymus?

To prevent self-reactive T cells from entering the bloodstream

Which of the following is NOT a function of the spleen during fetal life?

Storage of antibodies

What occurs to the thymus after puberty?

It starts to diminish in size

What is the main function of the white pulp in the spleen?

Production of lymphocytes

What is the primary function of epithelial reticular cells in the thymus?

To present antigens to T cells and activate them

Which of the following zones is rich in T-lymphocytes and plasma cells?

Marginal zone

What is the primary function of the cortex in the thymus?

To provide a site for the development and differentiation of T lymphocytes

What is the function of the reticular stroma in the spleen?

Supports the organ

What is the relationship between the trabeculae and the capsule in the thymus?

The trabeculae are thin septae that descend from the capsule

What is the function of the trabeculae in the spleen?

Carries the blood vessels

Study Notes

Lymphoid (Immune) System

Responsible for protecting the body against invading microorganisms or foreign bodies
Distributed throughout the body, consisting of aggregations of lymphocytes (B and T lymphocytes) and epithelial cells

Lymphatic Tissues (Organs)

Classified into primary or central lymphoid organs and secondary or peripheral lymphoid organs
Primary lymphoid organs: thymus and bone marrow, where B and T lymphocytes develop
Secondary lymphoid organs: spleen, lymph nodes, tonsils, appendix, Peyer's patches, and solitary lymphatic nodules, where B and T lymphocytes settle

Thymus

Flattened bilobed organ lying behind the sternum
Large in infants, grows until puberty, then diminishes in size
Consists of stroma and parenchyma
Stroma:
- Capsule: thin connective tissue (C.T.)
- Trabeculae: thin septae of C.T. dividing the gland into 2 main lobes and incomplete lobules
Parenchyma:
- Cortex: outer dark part, contains thymocytes (T lymphocytes), epithelial reticular cells (APCs), and macrophages
- Medulla: inner pale part, contains thymocytes (T lymphocytes), epithelial reticular cells (APCs), macrophages, and Hassall's corpuscles
Hassall's corpuscles: large rounded structures composed of concentric layers of epithelial reticular cells around a central hyaline acidophilic mass
Functions:
- Site for development and differentiation of T lymphocytes
- Production of T lymphocytes
- Secretion of thymic hormones
- Formation of Blood-thymic barrier: protects newly formed T-lymphocytes from circulating antigen

Spleen

Single intra-abdominal organ covered by peritoneum
Consists of stroma and parenchyma
Stroma:
- Capsule: covered by peritoneum, formed of dense C.T. and smooth muscle fibers
- Trabeculae: radiate from the hilum, carrying blood vessels
- Reticular stroma: supports the organ
Parenchyma:
- White pulp (splenic nodules or Malpighian corpuscles): formed of 4 zones
  - Thymus-dependent zone: rich in T-lymphocytes
  - Germinal center zone: rich in B-lymphocytes
  - Follicular zone: rich in B-lymphocytes
  - Marginal zone: rich in T-lymphocytes and plasma cells
- Red pulp:
  - Blood sinusoids: wide blood spaces lined with flat cells and phagocytic cells
  - Splenic cords: lymphatic tissue containing blood cells, plasma cells, and macrophages
Functions:
- Hemopoiesis of blood elements during fetal life
- Production of lymphocytes (role in humoral and cell-mediated immunity)
- Destruction of abnormal, damaged, and aged blood cells
- Blood filtration from bacteria, foreign bodies, old RBCs, WBCs, and platelets
- Storage of blood