Lymphatic and Immune System 

1. General Functions

• The lymphatic system is an accessory to the circulatory system that drains interstitial fluid (lymph), filters it, and returns it to the venous blood.

• The immune system protects the body from pathogens (viruses, bacteria, fungi, parasites) and abnormal cells (e.g., cancer), distinguishing “self” from “non-self.”

2. Components of the Lymphatic System

2.1 Lymph

• Clear fluid, similar to plasma, devoid of red blood cells but rich in lymphocytes.

• Derived from plasma filtrate that escapes the capillaries.

2.2 Lymphatic Capillaries

• Microscopic, blind-ended vessels with high permeability.

• Found in nearly all tissues, except the CNS, bone marrow, and avascular tissues (e.g., cornea).

2.3 Lymphatic Vessels

• Resemble veins but with thinner walls and more valves.

• Drain lymph into major lymphatic ducts:

• Thoracic duct: drains lymph from most of the body except the upper right quadrant.

• Right lymphatic duct: drains the right side of the head, neck, chest, and right upper limb.

3. Lymphatic Organs

3.1 Lymph Nodes

• Filter lymph and house lymphocytes and macrophages.

• Structure: fibrous capsule, outer cortex (with follicles), inner medulla.

• Found in clusters in strategic areas (axillary, inguinal, cervical regions).

3.2 Spleen

• Prenatal hematopoietic function; postnatal blood filter.

• White pulp: rich in lymphocytes; mounts immune responses.

• Red pulp: removes aged red blood cells; stores iron.

3.3 Thymus

• Site of T cell maturation (positive and negative selection).

• Most active in childhood; undergoes involution with age.

3.4 Tonsils

• Part of the Waldeyer’s ring (pharyngeal, palatine, lingual tonsils).

• Exposed to inhaled and ingested antigens.

3.5 Bone Marrow

• Origin of all immune cells.

• Site of B cell maturation.

4. Immune Cells

4.1 Lymphocytes

• B cells: produce antibodies; mature in bone marrow.

• T cells: mediate cellular immunity; mature in the thymus.

• T helper (CD4+)

• Cytotoxic T (CD8+)

• Regulatory T cells

4.2 Antigen-Presenting Cells (APCs)

• Macrophages, dendritic cells, Langerhans cells.

• Present antigens on MHC class II to T cells.

4.3 Natural Killer (NK) Cells

• Kill virus-infected or cancerous cells without prior sensitization.

5. Types of Immunity

5.1 Innate (Nonspecific) Immunity

• Present at birth.

• Includes physical (skin, mucosa), chemical (pH, enzymes), and cellular (phagocytes, NK cells) defenses.

5.2 Adaptive (Specific) Immunity

• Acquired after antigen exposure.

• Includes:

• Humoral immunity (B cells and antibodies)

• Cell-mediated immunity (T cells)

6. Lymphatic Circulation

• Lymph flows:

Lymph capillaries → collecting lymphatic vessels → lymph nodes → larger lymphatic trunks → lymphatic ducts → subclavian veins

7. Clinical Considerations

• Lymphedema: lymph accumulation in tissues (e.g., post-mastectomy).

• Lymphoma: malignant proliferation of lymphocytes (Hodgkin vs. Non-Hodgkin).

• Immunodeficiencies: congenital or acquired (e.g., HIV/AIDS).

• Autoimmune diseases: immune system attacks self-tissues (e.g., lupus, rheumatoid arthritis).