Lymphatic System and Immune Function

Lymphatic System Overview

• Functions:
  • Transports excess interstitial fluid back to the blood.
  • Transports absorbed fats from the intestines to the blood via lacteals.
  • Provides immunological defenses against pathogens.
  • Returns leaked proteins to the bloodstream, maintaining osmotic pressure.

Lymph Fluid Characteristics

• Formation:
  • Lymph is formed as a filtrate of plasma through blood capillaries.
• Composition:
  • Similar to blood plasma but with a lower concentration of proteins.
• Pressure:
  • Flows under very low pressure.

Lymphatic System Structure

• One-Way System:
  • Lymph travels only toward the heart.
  • Functions include:
  • Return of collected excess tissue fluid.
  • Return of leaked proteins.
  • Lymph is the fluid within this system.
  • Consequences of Blockage:
  • Edema can occur if the lymphatic system is blocked or surgically removed.

Elephantiasis

• Description:
  • A chronic condition characterized by lymphatic edema, leading to fluid retention and swelling of tissues.
• Symptoms:
  • Thickening of the skin and underlying tissues, most notably in the legs and male genitals.
• Cause:
  • Often caused by parasitic worms (nematodes) entering through mosquito bites.
• Geographic Distribution:
  • Common in tropical and subtropical regions.

Lymphatic Capillaries

• Structure:
  • Closed-ended tubes located near blood capillaries.
  • Highly permeable endothelium with one-way minivalves to allow excess fluid in.
• Function:
  • Collects bacteria, viruses, and proteins.
  • Lymph nodes function to destroy most pathogens.
• Distribution:
  • Absent in bone, bone marrow, teeth, and the central nervous system.

Lymphatic Collecting Vessels

• Structure:
  • Similar to blood vessels (3 layers) but thin and delicate.
  • Superficial vessels travel with superficial veins; deep vessels travel with deep arteries.
• Pressure:
  • Flow under very low pressure, draining into lymph nodes.

Lymph Nodes

• Description:
  • Bean-shaped organs located along lymphatic collecting vessels.
  • Surrounded by a connective tissue capsule.
• Cells:
  • Contains phagocytic cells that help filter lymph.
• Size:
  • Can be up to 1 inch in size.

Lymphatic Trunks

• Types:
  • Paired Trunks:
    1. Lumbar
    2. Broncho-mediastinal
    3. Subclavian
    4. Jugular
  • Single Trunk:
    • Intestinal (receives fatty lymph, or chyle, from intestines)

Small Intestine and Absorption

• Structure:
  • Adapted for absorption with a large surface area.
• Features:
  • Circular folds (plicae circulares) and fingerlike villi enhance absorptive ability.
  • Microvilli increase surface area for absorption.
• Lacteals:
  • Special capillaries in the intestine that absorb lipids before they enter the bloodstream.

Immune System Components

• Function:
  • Recognizes specific foreign molecules.
  • Response increases in effectiveness with repeated exposure to pathogens.
• Lymphoid Organs:
  • Include lymph nodes, spleen, thymus, tonsils, and aggregated lymphoid nodules in the small intestine and appendix.

Lymphocytes

• Types of Immune Cells:
  • T cells (thymus-dependent) - account for about 80% of lymphocytes.
  • B cells (bone marrow-derived).
  • NK cells (natural killer cells).
• Responses:
  • React to invading organisms, abnormal or cancerous body cells, and foreign proteins (toxins released by bacteria).

Development of Lymphocytes

• Origin:
  • Both B cells and T cells originate from lymphoid stem cells in bone marrow.
  • B cells mature in bone marrow; T cells mature in the thymus.

Thymus

• Description:
  • Bilobed organ crucial for T lymphocyte maturation and immunocompetence.
  • Prominent in newborns but diminishes with age.

Spleen

• Function:
  • Largest lymphoid organ resembling lymph nodes, responsible for:
  • Removing blood-borne antigens.
  • Destroying aged or defective blood cells.
  • Storing platelets and synthesizing antibodies.
  • In fetuses, it is a site of hematopoiesis.
  • Risk:
  • Susceptible to injury; removal (splenectomy) increases the risk of bacterial infections.

Tonsils

• Types:
  • Palatine (commonly involved in tonsillitis)
  • Lingual (located at the tongue)
  • Pharyngeal (or adenoids)
  • Tubal Tonsils
• Function:
  • Simplest form of lymphoid tissue, involved in immune response.

Aggregated Lymphoid Nodules

• Location:
  • Found in the distal small intestine (ileum) and the appendix.
• Significance:
  • These areas feature dense MALT (mucosa-associated lymphoid tissue), qualifying them as lymphoid organs.