Histology of Lymphoid Organs Study Notes

Dr. Elizabeth Moffett - Associate Professor Anatomical Sciences

Roles of Lymphatic Organs
- Distinction between primary and secondary lymphatic organs.
- Comparison of shared features among lymphoid organs.
- Unique histological features for differentiating lymphoid organs.
Thymus
- Functions and morphology of the thymus.
- Explanation of the blood-thymus barrier.
Lymphocyte Travel
- Mechanisms by which lymphocytes travel.
- Differences between lymphatic vessels and blood vessels.
Lymph Nodes
- Morphology and function of lymph nodes.
- Entry of lymphocytes into nodes through lymph and blood circulation.
- Understanding lymph node metastasis.
Lymphatic Tissue
- Similarities and differences between diffuse lymphatic tissue and lymphatic nodules.
- Locations of these lymph tissues.
Spleen
- Functions and morphology of the spleen.
- Ability to differentiate between white and red pulp in histological sections.
- Location of splenic sinuses.

Lymphocytes: B-cells, T-cells, NK cells.
Common Lymphoid Progenitors (CLPs).
Reticular Cells and Epithelioreticular Cells.
Primary & Secondary Lymphatic Organs.
Thymus: capsule, parenchyma, cortex, medulla, thymic corpuscles.
Blood-Thymus Barrier.
Lymph & Lymphatic Vessels (efferent/afferent).
Lymph Nodes.
Diffuse Lymphatic Tissue (MALT, BALT, GALT, VALT, CALT).
Lymphatic Nodules (tonsils, Peyer patches).
Spleen: red pulp, white pulp, Periarterial Lymphatic Sheaths (PALs), splenic sinuses.

Immune System: Protects against transformed cells and foreign invaders.
Lymphatic System: A component of the immune system comprising cells, tissues, and organs involved in immune responses.
- Embedded Cells: Lymphocytes, including B-cells, T-cells, and NK cells, are essential components of the lymphatic system.

Most immune system cells derive from hemopoietic stem cells in the bone marrow:
- Common Myeloid Progenitor (CMP): Produces monocytes, macrophages, granulocytes, dendritic cells, as well as red blood cells (RBCs) and platelets.
- Common Lymphoid Progenitor (CLP): Specific for lymphocytes (T, B, NK cells).

Other White Blood Cells: Include monocytes, macrophages, eosinophils, basophils, and dendritic cells.
- Langerhans Cells: Found primarily in epidermis, lungs, liver, and lymphoid tissue.
Structural Support Cells:
- Reticular Cells: Secrete reticular fibers to form structural elements in lymphoid organs.
- Epithelioreticular Cells: Specialized cells in the thymus that form a mesh-like structure, playing a role in regulating immune responses.

Types of Immunity:
- Nonspecific (Innate) Immunity: First line of defense, responding uniformly to invaders.
- Specific (Adaptive) Immunity: Acquired over time, based on lymphocyte interaction with antigens.
- Humoral Response: Involves antibody production via B lymphocytes.
- Cellular Immune Response: Involves T lymphocytes targeting transformed or infected cells.

Antigen: Any substance inducing a specific immune response, can also include transformed cells with foreign characteristics.
Immune response triggered by specific antigen recognition.

Inflammation: Initial response to antigen, may sequester or digest it.
Specific Immune Responses are classified into:
- Primary Immune Response: First exposure, characterized by a lag phase in antibody detection.
- Secondary Immune Response: Faster and stronger due to memory B cells from prior exposures.

Primary Lymphatic Organs: Bone marrow and thymus; sites for lymphocyte development and education.
Secondary Lymphatic Organs: Areas for lymphocyte activation upon antigen exposure.

Contains Hematopoietic Stem Cells (HSCs) and Common Lymphoid Progenitors (CLPs).
CLPs destined for T Cells migrate to the thymus; those destined to become B Cells start maturation in the bone marrow.

Located in the mediastinum; critical for T cell development and education.
Thymic Education: Involves positive and negative selection of thymocytes.
- Positive Selection: Occurs in the cortex, selecting functional T cells.
- Negative Selection: Occurs in the medulla, eliminating self-reactive T cells.
Blood-Thymus Barrier: Ensures developing T cells are shielded from antigens, composed of continuous endothelium and occluding junctions.

Surrounded by dense connective tissue capsule and trabeculae forming lobules: cortex (more basophilic) and medulla (less basophilic).
- Cortex: Densely packed developing T lymphocytes.
- Medulla: Contains fewer, larger T cells and Thymic/Hassall corpuscles.

Highly vascularized via branches from multiple arteries, facilitating CLP entry and maturation.

Reticular Tissue: Composed of reticular cells and fibers; provides structural support for lymphatic organs.
Lymph Nodes: Filter lymph, harbor germinal centers for B cell proliferation.
Spleen: Largest lymphatic organ; filters blood and contains red and white pulp differentiating functions.

Lymph nodes trap cancer cells leading to metastasis, overwhelmed by excessive cancer cells, forming a new metastatic site.

Diffuse Lymphatic Tissue: Non-discrete lymphocyte collections in mucosal layers, essential for fluid balance and immune initiation.
Lymphatic Nodules: Discrete lymphocyte concentrations, crucial for filtering lymph and presenting antigens.
- Peyer Patches and Tonsils: Specialized structures for pathogen monitoring and immune response activation.

Functions primarily in filtering blood, immune response initiation, with high blood supply filtering ~5% of cardiac output per minute.
Microstructure includes a dense capsule, trabeculae, and two forms of pulp: white (lymphocytic) and red (RBC-centric).

Blood flows from the splenic artery, passes through trabecular arteries into splenic sinuses where filtration occurs, monitoring for foreign antigens.

Comparison chart summarizing the features and functions of lymphatic structures including diffuse lymphatic tissue, lymphatic nodules, lymph nodes, thymus, and spleen.