The Immune System

Immunology, the study of the immune system, is an extremely important field and is key to future advancements in cancer, autoimmune diseases, and combating diseases like coronavirus. So what exactly is the immune system?

The immune system is composed of the innate immune system and the adaptive immune system.  The innate immune response consists of the physical, chemical, and cellular defenses against pathogens. The main purpose is to immediately prevent the spread and movement of foreign pathogens throughout the body.  This type of response is nonspecific, which means that it does not target a specific pathogen. The purpose of a nonspecific response is that it protects the body from all infections.  An example of an innate immune response is seen when a foreign substance, such as a thorn, pierces your first line of defense.  This line of defense is your skin, mucous, hair, and chemicals in your blood. If that barrier is breached, then there is a second line of defense, which elicits an inflammatory response.  An inflammatory response begins when a pathogen causes an increase in blood flow to the infected area. Blood vessels expand, and white blood cells leak from the vessels to invade the infected tissue. Phagocytes, which are WBCs, engulf and destroy bacteria. This causes the infected area to redden and swell, and your body temperature may also rise. 

If the pathogen bypasses the innate immune system, the adaptive immune system comes into play.  This system relies on B and T cells, and uses specific antigens to strategically mount an immune response.  B cells are formed and mature in the bone marrow, and at this stage it is considered a naive B cell.  They express antibodies on the cell surface, which can also be called membrane-bound antibodies.  Once naive B cells encounter an antigen, maturation begins. It then quickly divides and becomes a memory B cell or an effector B cell.  Memory B cells express the same membrane-bound antibody as the original naive B cell.  Effector B cells secrete antibodies, which are not membrane bound.  These cells work to identify free-floating pathogens.  T cells are formed in the bone marrow, and then migrate to the thymus to mature.  There, they start to express T cell receptors (TCRs) and other receptors called CD4 and CD8 receptors.  T cell receptors can only recognize antigens that are bound to certain receptor molecules, called Major Histocompatibility Complex class 1 (MHC I) and class 2 (MHC II). These MHC molecules are membrane-bound surface receptors on antigen-presenting cells, like dendritic cells and macrophages. CD4 and CD8 play a role in T cell recognition and activation by binding to either MHC I or MHC II.

The immune system has many parts. First off, we have the bone marrow, which is a sponge-like tissue inside the bone. .  This tissue is responsible for the production of defense cells, and these cells multiply in number until they migrate from the bone marrow into the bloodstream.  At birth, your bones contain red bone marrow, which build your immune system’s defense cells.  However, as you age, the red bone marrow turns into fat tissue.  The next organ is the thymus, which is located behind the breast bone and above the heart.  Like red bone marrow, it is fully developed in children, but turns into fat tissue as you grow older.  This is where T lymphocytes, or T cells, mature and learn which structures on cell surfaces are self or non-self.  The thymus is also responsible for the production of the hormones thymosin and thymopoietin during childhood, which regulates the maturation of your defense cells. Our next organ are the lymph nodes, and they exchange substances between the blood and tissue.  They filter and clean the lymph, which will eventually enter the bloodstream.  These nodes also contain defense cells, which trap pathogens and activate specific antibodies in the blood.  If the lymph nodes become swollen or hard, then that is a sign that you are experiencing an active defense reaction.  The spleen is located beneath your diaphragm, and it stores macrophages, scavenger cells, and other defense cells.  It also removes erythrocytes, or RBCs.  Next we have your tonsils, which are located at your throat and palate, and because of its location, it comes into contact with pathogens quickly, and it is the one that activates the immune system immediately.  Tonsils contain mainly lymphocytes. Then we have the bowel, where over half of antibody producing cells are found.  These cells recognize pathogens and mark and destroy them.  And last but not least, we have the mucosa associated lymphoid tissue, which prevents bacteria and viruses from attaching.   It can be found where pathogens may enter easily, such as your airways. 

Citations:

https://www.youtube.com/watch?v=2DFN4IBZ3rI&t=513s

https://www.youtube.com/watch?v=rd2cf5hValM&t=507s

https://www.hopkinsmedicine.org/health/conditions-and-diseases/the-immune-system

https://www.medicalnewstoday.com/articles/320101

https://www.niaid.nih.gov/research/immune-system-overview

Image Credit:

https://my.clevelandclinic.org/health/articles/21196-immune-system