Organs and Tissues of the Immune System

Organs and Tissues of the Immune System

Organs of the Immune System

Primary Lymphoid Organs

  • Bone Marrow

  • Thymus

  • Fetal Liver in the Embryo

  • Yolk Sac in the Embryo

Secondary Lymphoid Organs

  • Spleen

  • Tonsils

  • Lymph Nodes

  • Mucosa-Associated Lymphoid Tissues (MALT)

Functions of the Immune Organs

Primary Lymphoid Organs

  • Function: Lymphocyte development and maturation

Secondary Lymphoid Organs

  • Functions:

    • Support lymphocyte maturation, survival, and activation

    • Drain antigenic material from tissues and organs

Sites of Lymphocyte Development and Antigen Response

  • Lymphocytes develop in:

    • Thymus

    • Bone Marrow

    • Fetal Liver

    • Yolk Sac

  • Lymphocytes respond to antigens in:

    • Spleen

    • Lymph Nodes

    • Peyer's Patches (part of the MALT)

Yolk Sac

What is a Yolk Sac?

  • A structure that develops in the uterus during early pregnancy, providing nourishment to the embryo.

Functions of the Yolk Sac:

  1. Circulates gases between the pregnant individual and the embryo.

  2. Delivers nutrients to the embryo.

  3. Develops the earliest form of blood cells.

  4. Produces cells that develop into the umbilical cord, gastrointestinal system, and reproductive organs.

  5. Provides early immune function and metabolism.

Fetal Liver

  • Role: Serves as a primary immune organ during embryonic development in mammals.

  • Functions of the Fetal Liver:

    1. Hematopoiesis: Primary site of blood cell production from 6-8 weeks of gestation until the bone marrow takes over at 24-28 weeks. Produces hematopoietic stem cells that differentiate into all types of immune cells, including lymphocytes.

    2. Metabolism:

    • Nutrient Processing: Processes nutrients such as glucose, amino acids, and lipids from maternal circulation.

    • Glycogen Storage: Stores glycogen late in gestation for energy supply after birth.

    1. Protein Synthesis:

    • Produces key plasma proteins such as:

      • Albumin: Maintains osmotic pressure and transports substances.

      • Fibrinogen and Coagulation Factors: Essential for blood clotting.

      • Alpha-fetoprotein (AFP): A marker of fetal development and liver function.

    1. Detoxification:

    • Minimal detoxification capabilities compared to the adult liver as the placenta handles waste removal.

    1. Immune Function:

    • Contributes to developing certain immune cells, including macrophages and natural killer (NK) cells.

Bone Marrow

Main Functions of Bone Marrow:

  1. Production of Blood Cells (Hematopoiesis):

    • Produces immune cells such as neutrophils, monocytes, lymphocytes, eosinophils, and basophils.

  2. B-lymphocyte Maturation:

    • Crucial for B cells in mammals.

  3. Source of Hematopoietic Stem Cells:

    • Responsible for producing all blood cells.

  4. Storage of Iron:

    • Vital for various blood cell functions.

  5. Fat Storage:

    • Yellow marrow stores fat.

Thymus Gland

Structure:

  • A bilobed organ in the chest (mediastinum), large in young animals, peaks at puberty, and shrinks with age.

  • Surrounded by a connective tissue capsule; divided into lobules:

    • Cortex: Rich in immature T cells (thymocytes).

    • Medulla: Contains mature T cells and Hassall’s corpuscles; does not have lymphoid follicles or afferent lymphatic vessels.

Functions of the Thymus Gland:

  • Essential for T lymphocyte (T cell) development and maturation.

  • Produces hormones crucial for T-cell differentiation, such as:

    1. Thymopoietin: Fuels T-cell production and signals the pituitary gland.

    2. Thymosin: Aids in T-cell differentiation.

    3. Thymulin: Enhances T and NK cell activity.

Species Differences in the Thymus Gland:

  • Variability exists across species:

    • Dogs & Cats: Well-developed in newborns, reduces in size with age.

    • Ruminants: Large at birth, replaced by fat later.

    • Birds: Multiple thymic lobes along the neck.

    • Pigs: Remain large in young animals.

Clinical Relevance:

  • Critical for normal cell-mediated immunity; disorders can lead to immunodeficiency or autoimmune diseases.

Secondary Lymphoid Organs

Spleen

  • The spleen is the largest organ of the lymphatic system, located in the left rib cage above the stomach.

Functions of the Spleen:

  1. Immune Functions:

    • Filters blood-borne antigens.

    • Activates B and T lymphocytes.

    • Produces antibodies (mainly IgM).

    • Removes opsonized bacteria and defends against encapsulated bacteria.

  2. Hematological Functions:

    • Destroys aging RBCs.

    • Recycles iron.

    • Stores platelets and blood.

    • Involved in extramedullary hematopoiesis during fetal life and disease states.

Pathological Conditions of the Spleen:

  • Splenic Hemangioma: Common tumor in dogs, rare in cats.

  • Hemangiosarcoma: A malignant tumor occurring mostly in older dogs (8-10 years) especially in breeds like German Shepherds, Golden Retrievers, and Labradors.

    • Signs may include weakness, collapse, internal bleeding leading to sudden death, pallor of mucous membranes, and increased heart and respiratory rates.

  • Splenomegaly: Enlargement of the spleen due to conditions such as portal hypertension, lymphomas, infections, congestion, inflammation, and splenic vein thrombosis.

Tonsils

General Functions:

  • Part of the mucosal immune system, located at the entry of the respiratory and digestive tracts.

  • Trap antigens, activate B and T lymphocytes, and produce IgA for mucosal immunity.

Types of Tonsils in Animals:

  • Palatine Tonsils (located in the oropharynx): Well-developed in dogs; commonly involved in tonsillitis.

  • Pharyngeal Tonsils (located in the nasopharynx): Related to adenoid hypertrophy.

  • Lingual Tonsils (at the base of the tongue): Rare complications due to airway issues.

  • Tubal Tonsils (near the Eustachian tube): Protect the middle ear.

Species Specifics:

  • Dogs: Palatine tonsils well developed.

  • Cats: Smaller palatine tonsils.

  • Horses: Lingual and palatine tonsils.

  • Cattle: Lingual, pharyngeal, and soft palate tonsils.

  • Pigs: Soft palate and pharyngeal tonsils.

  • Birds: No true tonsils.

Lymph Nodes

General Functions:

  • Secondary lymphoid organs filtering lymphatic fluid from tissues, trapping pathogens and toxins, preventing systemic infection.

  • Activation of T and B lymphocytes and antigen presentation.

  • Memory cell generation for quicker immune response on re-exposure to antigens.

Mucosa-Associated Lymphoid Tissue (MALT)

Description:

  • MALT is a network of lymphoid tissue in mucosal sites, including the gastrointestinal tract, lungs, and other areas, containing various immune cells.

  • Types of MALT:

    • GALT: Gut-associated lymphoid tissue including Peyer's patches.

    • BALT: Bronchial-associated lymphoid tissue.

    • SALT: Skin-associated lymphoid tissue.

    • NALT: Nose-associated lymphoid tissue.

    • LALT: Larynx-associated lymphoid tissue.

    • CALT: Conjunctiva-associated lymphoid tissue in the eye.

Peyer's Patches:

  • Located in the wall of the small intestine, they capture and destroy bacteria and prevent foreign pathogens from penetrating the intestinal wall.

  • Two types based on size and location in pigs:

    • Jejunal Peyer's Patches: Small, distributed along the jejunum with both B and T lymphocytes.

    • Ileocecal Peyer's Patches: Larger patches in the terminal ileum, mainly B lymphocytes.

Study Guide Questions:

  1. Describe the primary and secondary lymphoid organs.

  2. List the functions of the spleen, thymus gland, bone marrow, and lymph nodes.

  3. Compare the avian immune system with the mammalian immune system.

  4. What are the causative agents and differential diagnoses of necrotic tonsillitis in pigs?

  5. List the functions of Peyer's patches.

  6. Describe hemangiosarcoma and its significance.

  7. Define the functions of thymopoietin, thymosin, and thymulin.

  8. What are the common diseases associated with the spleen?

  9. What are the functions of the yolk sac?