The Lymphoid System and Immunity
Dr. Jim Aston

Bacteria, viruses, parasites, allergens, foreign chemicals, and even cancer cells are agents called pathogens that can cause disease. The body has cells capable of defending itself in a nonspecific and very specific way if called upon to do so. The location, housing, development, and circulation pathways for these cells are found in organs and vessels that comprise the lymphoid system.
I. The Lymphoid /Immune System
A. Lymphatic vessels
Lymphatic capillaries are in all tissues except CNS and bone marrow. They are
Closed-ended capillaries as distinguished from the blood capillaries and have one-way valves in their lining. They generally follow the course of veins and empty into the blood circulation at the:
1. Thoracic Duct
Begins in the abdomen, passes alongside the aorta and joins the left subclavian vein. It drains lymph from the entire body and left side of the chest, face, and head.
2. Right Lymphatic Duct
It drains the rest of the body (RUE and right chest) and right side of head and face.
B. Lymphoid tissues
Lymphoid tissues are collection (nodules) of lymphocytes in many areas of connective tissue. The tonsils are good examples:
Palatine (2)
Pharyngeal (adenoid) (1)
Lingual (2)
Peyers patches (intestine)
(Note: they are positioned close to possible body entry points)
C. Lymphoid organs
These too are collections of lymphocytes but they also have a capsule separating them from the surrounding cells. They are designed to slow down the circulation and filter it.
1. Lymph node
Lymph nodes are filters.
 They receive afferent lymph and circulate this contaminated interstitial fluid past lymphocytes and macrophages that remove pathogens and/or initiate an immune response.
 In addition to the filter function, the nodes also are sites of some lymphocyte production.
 They are located in chains in the regions they serve (inguinal, cervical, axillary, etc.) A swollen mode is a clue to the location of the battlefield. ds/adjuncts/Dr.AstonFlashFall10/The Lymphoid System and Immunity Sept 2010

2. Thymus Gland
 Located in the mediastinum and gets smaller with age (one-tenth the size by age 70).
 Secretes thymosin, a hormone that promotes the maturation of lymphocytes into t cells. T cells stands for thymus derived.

Hemocytoblasts
Lymphoid
Stem cells
80%

Myeloid
Stem cells

Thymus
20%

RBC
WBC
megararyocyte
NK Lymphocytes
B Lymphocytes

t Lymphocytes

3. Spleen
The largest lymph organ. The lobules are white (containing many lymphocytes) and red (containing blood sinuses). Arterial blood enters the hilum and red blood cells squeeze through the endothelium of the sinus walls into venules. Old or damaged RBCs and debris are thus filtered out as they pass through the endothelial wall---so just as lymph nodes filter lymph, the spleen filters blood.
II. Non Specific Resistance
These defense mechanisms do not distinguish between one pathogen or another but seek to stop invasion by any foreign invader. Once the barrier is crossed an inflammatory response always occurs.
A. Physical barriers
To cause disease the pathogen has to cross some epithelial surface. Intercellular connections, sweat, mucous, keratin etc. provide some resistance to invasion
Therefore tonsils and other lymph tissues are placed close to an epithelial surface.
B. Phagocytes
1. Neutrophils
2. Eosinophils
3. Macrophages (monocytes that slip through capillary wall diapedesis and change into macrophages) ds/adjuncts/Dr.AstonFlashFall10/The Lymphoid System and Immunity Sept 2010

4. NK Lymphocytes
Natural killer (NK) cells are lymphocytes that attack any cell they recognize as foreign. They respond faster than other lymphocytes and are always looking for bad guys. (immunologic surveillance).

C. Proteins that Contribute to the Efficiency of Phagocytosis
1. Interferons—these are cytokine proteins that the infected phagocytes