The Immune System

To date, we have looked at a number of the body’s systems and their functions.  We’ve seen the complexity of the various systems and how so many of them are interconnected with each other.  Today we will look at a system that is specifically designed to protect the entire body and all of its systems from invading bacteria, viruses, parasites, toxins and any other foreign substance—the immune system.

The immune system is composed of a number of specialized cells, tissues and various organs.

I. Specialized Immune System Cells – The specialized cells of the immune system are divided into two categories: innate and adaptive. 

A.        The Innate Immune System – The innate immune system is a more general system that protects the body in non-specific manner. Parts of the innate system include: leucocytes and phagocytes.

1.         Leukocytes – Leukocytes are more commonly referred to as white blood cells.  There are a number of different leukocytes, and their functions vary from one to another.

a.         Basophils – Basophils have large granules and a double or triple lobed nucleus.  They are approximately 12-15 µm in diameter.  They function in allergic reactions where they release histamine.

b.         Eosinophils – Eosinophils have a double lobed nucleus and are about 10-12 µm in diameter.  They function primarily with allergic reactions, hay fever and asthma.  In some instances, they also function in fighting off certain parasites.

c.         Natural Killer Cells – Natural Killer Cells (NK Cells) are cytotoxic leukocytes.  They contain small granules and contain a protein called perforin and granzymes.  They function to attack tumors and virally infected cells.  When they approach a tumor cell or a cell that is infected by a virus, the NK cells move close to the infected cell.  It releases the perforin that dissolves through the cell membrane creating larger pores.  The granzymes are then released into the cell where they cause cell apoptosis (cell death).  Apoptosis not only leads to cell death but also to the destruction of the virus or tumor components

2.         Phagocytes – Phagocytes are a group of leukocytes that function as the Pac-Men of the immune system by ingesting bacteria, virus, dead cells, dying cells and any other foreign particle that does not belong in the human body.

a.         Dendritic Cells – Dendritic cells are easily identified by the numerous outgrowths called dendrites branching out like fibrous root systems.  They use these dendrites to capture invading bacteria and other microbes.  They are generally found near the skin, nasal membranes, lungs, stomach and intestines, where they interact with T & B cells to initiate the adaptive immune system.

b.         Macrophages – Macrophages are large, 21 µm diameter, phagocytes that arise from other mature macrophages or from monocytes.  They form and remain near an external wound.  Based upon which signal the macrophage receives, it will function as a killer cell, producer of an antigen or just collect and ingest waste material that is detrimental to the body.

c.         Neutrophils – Neutrophils are also called polymorphonuclear leukocytes because they have a multilobed nucleus.  They are about 10-12 µm in diameter and contain granules in the cytoplasm.  They function by ingesting invading bacteria.  Neutrophils comprise over 50% of white blood cells and can be found in large quantities in the pus that develops on wounds and sores.

d.         Monocytes – Monocytes contain no granules, have a kidney shaped nucleus and range about 14-17 µm in diameter.  Monocytes not only function as a phagoctic cell, but they can actually leave the bloodstream and enter other body tissues.  They search out damaged, old and dead cells in the tissue and ingest them as they would a bacterium.

e.         Mast Cells – Mast cells work in conjunction with dendritic, T and B cells where it helps to regulate the adaptive immune system.  They generally seek out and ingest gram negative bacteria such as salmonella.  They also release cytokines that signal additional phagocytes to the location of the infection.

B.        The Adaptive Immune System – The adaptive immune system is a more specific system that composed of more specialized cells called lymphocytes that target specific pathogens.

1.         Lymphocytes – Lymphocytes are a type of small while blood cell that is found primarily in the lymphatic system.  There are two main types of lymphocytes: B cells and T cells.

a.         B Cells – B cells are manufactured in the bone marrow, hence the name B cell.  Upon maturing, they will produce antibodies that attack specific bacteria and toxins.

i.          Plasma Cells – Certain chemical signals will cause a B cell to mature into plasma cells.  Plasma cells produce large amounts of proteins called antibodies.  They have short lifespans of 6 – 24 hours.

ii.         Memory B Cells – Memory B cells are formed in response to a specific antigen attacking the body.  Unlike the short lived plasma cells, memory cells can live for years and some even last a life time.  It is these cells that give us the ability to ward off future infections of the same antigen.

b.         T Cells – T cells are called T cells because the immature cells migrate to the thymus gland where they await their signal to mature and go into action.  Once activated by the hormone thymosin, the T cells target specific pathogens to destroy them.  They also produce chemicals called cytokines that stimulates further immune system response to the targeted pathogen or infection.

II.         Organs and Tissues of the Immune System

A.        Bone Marrow – The bone marrow produces immature B cells, thymocytes and other lymphatic cells.  Some will mature within the bone marrow while most will circulate to different parts of the body to await maturation.

B.        Spleen – The spleen has two sections: a red pulp and white pulp section.

1.         Red Pulp – The primary function of the red pulp section of the spleen is to filter and store blood.

2.         White Pulp – The white pulp section of the spleen is composed of lymphoid tissues within the spleen.  As the spleen’s lymphoid tissue detects antigens in the blood, it launches an immunological attack against it by sending out the appropriate signals for the type of antigen detected.

C.        Lymph Nodes – Lymph nodes are small glands located from the neck to the groin area.  They are part of the extensive lymphatic system that circulates interstitial fluid and plays a primary role in the immune system.  They act as filtering agents for body fluids.  In the process of filtration they will launch the appropriate immune cells to any bacteria, virus or other pathogens that are detected.  The major lymph nodes involved are grouped by location:

1.         Cervical Lymph Nodes – Lymph nodes located in the head and neck.

2.         Axillary Lymph Nodes – Lymph nodes located in the underarm area.

3.         Supraclavicular Lymph Nodes – Lymph nodes located mainly in the hollow area just above the clavicle or collarbone.

4.         Mediastinal Lymph Nodes – Lymph nodes located between the air sacs of the lungs.

5.         Mesenteric Lymph Nodes – Lymph nodes located in the lower abdominal region.

6.         Inguinal Lymph Nodes – Lymph nodes located in the groin region.

7.         Femoral Lymph Nodes – Lymph nodes located along the femoral veins in the upper thigh.

D.        Thymus – The thymus is a butterfly shaped gland in the chest between the heart and the sternum.  Immature T cells collect here until they are triggered to maturity and action by the thymus gland.

E.        Tonsils – The tonsils are located on each side of the back of the mouth.  They are composed of lymphatic tissues and considered to be one of the first lines of defense against pathogens that enter the mouth.

F.         Appendix – Also known as the vermiform appendix is a short tube-like sac attached to the caecum.  Long thought to be a vestigial organ, more recent studies have revealed that the appendix contains lymphoid cells that function as part of the immune system, specifically in helping to target pathogens that enter the intestinal tract.

G.        Mucosal Associated Lymphoid Tissues (MALT) – The mucosal associated lymphoid tissue is found adjacent to the mucous linings of the digestive, respiratory and urogenital tracts.  MALT consists of plasma cells that will produce antibodies to ward off the pathogens that frequently invade these mucous linings.

H.        Cutaneous Associated Lymphoid Tissues (CALT) – The largest organ in the human body is our skin.  The skin acts as a barrier against many pathogens and toxins.  CALT can be found near the skin in various locations and assists in the defense of any pathogen that penetrates the skin barrier.

The immune system is a very complicated system that involves tissues, specialized cells and organs throughout the body.  There are other organs and tissues in the body that play a role in the immune system that are not mentioned above.

Have you ever wondered how or why such a complex defense system would evolve?  At what stage of evolution did an organism decide it needed to evolve an immune system?  There is no logical explanation for the evolution of an immune system, especially one so complex and involving so many different cells, tissues and organs.

The only logical explanation is that it was specifically designed by our Creator God to help protect us from so many of the physical effects and diseases that developed as a result of the Fall.

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