Lymphoid Organs and Immune System Overview

Lymphoid Organs and System

• Lymphoid tissues are categorized as primary, secondary, and tertiary
• Primary lymphoid organs (Thymus, bone marrow, yolk sac, liver, and spleen) are where lymphocytes develop and mature.
• Secondary lymphoid organs (lymph nodes, spleen, tonsils, payer's patches, appendix) are where immune responses occur.
• Tertiary lymphoid organs refer to sites where immune responses occur anywhere. 
• Lymphocytes include T cells (80% of circulating lymphocytes), B cells (10-15% of circulating lymphocytes), and natural killer (NK) cells (5-10% of circulating lymphocytes)
• T cells are further categorized, including cytotoxic, regulatory, and memory cells, each with distinct functions in the immune system. T cells, in general, attack foreign and faulty cells.
• B cells differentiate into plasma cells to produce antibodies to target pathogens.
• NK cells are part of the immune system's surveillance mechanism, attacking and destroying infected or cancerous cells.
• Different cell types express distinct cell surface receptors (e.g., CD3+, CD4+, CD8+, CD19+, CD20+, CD16+, CD56+, CD14+, CD11c+)
• The process of producing naïve lymphocytes (in mice, 1-3 months) is under equilibration, involving lymphocyte death via apoptosis for homeostasis.
• Lymphoid tissues include lymph nodes, spleen, thymus, payer's patches, tonsils, and bone marrow and appendix.
• Lymphatic capillaries are similar to blood capillaries but are very permeable and have valves preventing backflow.
• Lymph nodes filter lymph, removing antigens, bacteria, and other debris; B and some T cells mature in these nodes.

Lymph

• Lymph is the fluid that moves via the lymphatic vessels
• Lymph transport relies on skeletal muscle activity and pulsating arteries.
• About 3 liters of lymph enter the bloodstream per 24 hours.

Lymphatic System

• The lymphatic system is a one-way system from the tissues to the heart
• It collects excess tissue fluid and leaked proteins in the body.
• Edema occurs if the system is blocked or surgically removed
• Lymphatic vessels are present in most areas of the body where blood capillaries occur.
• The lymphatic system is part of the immune system, helping to return interstitial fluid, protein, and other fluid elements from the tissues to the bloodstream.

Lymph Node Anatomy

• Lymph nodes contain follicles with germinal centers
• Lymph nodes are not fully encapsulated.
• Epithelial tissue invaginates to form tonsillar crypts, trapping and destroying bacteria and particulate matter.
• Lymphocytes congregate in the lymph node.

What is in the lymphatic system?

• Lymph nodes are part of the lymphatic system, filtering lymph fluid for antigens, bacteria, and other foreign substances.
• Lymph nodes are a common site for certain cancers, such as Hodgkin's lymphoma (lymph node malignancy) and Non-Hodgkin's lymphoma.
• The multiplication and metastasis of lymphocytes, part of the lymphatic system, are involved in immune responses.

Lymphatic Capillaries

• Lymphatic capillaries are very permeable, taking up cell debris, pathogens, and cancer cells.
• Endothelial cells overlap to form one-way minivalves, preventing collapse.

Lymphocyte Recirculation

• Lymphocyte homing is a selective process for specific tissues, not general.
• It is antigen-dependent.
• Naive T cell migration differs from effector and memory T cell migration.
• Cell migration is controlled by adhesion molecules (selectins, integrins, super Igs).

Immune System in Skin

• The immune system in skin involves Langerhans cells, intraepidermal lymphocytes (IEL), and perivascular macrophages.

MALT

• Mucosa-associated lymphatic tissue (MALT) protects digestive and respiratory systems from foreign matter.
• MALT includes Peyer's patches, tonsils, and appendix.

Aggregates of Lymphoid Follicles

• Peyer's patches and appendix are aggregates of lymphoid follicles in the small intestine.
• They destroy bacteria, preventing them from breaching the intestinal wall, and generate memory lymphocytes.

Tonsils

• Tonsils are simplistic lymphoid organs that form a ring in the pharynx.
• Tonsils include palatine, lingual, pharyngeal, and tubal tonsils.
• Tonsils prevent pathogens from penetrating the mucous membrane in the respiratory and gastrointestinal tracts by having a high antibody response and tolerance in T cells.

Spleen

• The spleen is the largest lymphoid organ.
• It's served by splenic artery and vein, entering and exiting at the hilus.
• The spleen is the site of lymphocyte proliferation and response.
• It cleanses the blood of aged cells, platelets, foreign matter, bacteria, viruses, and debris.
• It stores breakdown products of red blood cells (e.g., iron).
• Erythropoiesis occurs in the fetus.

Structure of the Spleen

• The spleen is divided into white pulp (lymphocytes around central arteries) and red pulp (venous sinuses and cords).
• White pulp is involved in immune functions, while red pulp disposes of worn-out red blood cells and pathogens.

Thymus

• Thymus is located on top of the heart.
• It secretes thymosin and thymopoietin, causing T-lymphocytes to function against pathogens.
• It is most active during childhood and often replaced by fibrous and fatty tissue in adulthood.
• It is important for T cell maturation.
• The thymus has distinct cortex and medulla regions containing lymphocytes and macrophages. Thymic Hassal's corpuscles are in the medulla and involved in regulatory T cell development.

Other

• High endothelial venules (HEV) are specific to lymph nodes and Peyer's patches.
• Naive T-cells express L-selectin, and activated T cells express other ligands allowing for entry into the lymph nodes.

Explore the essential components of the lymphoid system, including the differentiation between primary, secondary, and tertiary lymphoid organs. Understand the roles of various lymphocytes, including T cells, B cells, and NK cells, in the immune response. This quiz will deepen your knowledge of how the immune system operates and the function of each lymphoid organ.