Every moment we are under attack by various disease-causing agents that could make us sick. Hence to stay alive and to thrive, the body is equipped with an immune system. The immune system consists of many parts including white blood cells (leucocytes), antibodies, bone marrow, lymph nodes, spleen, thymus and the tonsil. Here is a description of what they are and their function: 

  1. White Blood Cells: Cells that move through the blood looking for foreign bodies called antigens to destroy e.g. bacteria and viruses
  2. Antibodies (immunoglobulins): Specialized proteins produced by white blood cells that circulate in the blood looking for antigens to trap    
  3. Bone marrow: Spongy tissue inside the bone that produces red and white blood cells 
  4. Lymph nodes: “Look out points” for the immune system. Here, lymph fluid is screened by white blood cells before it is returned to the blood 
  5. Spleen:  Organ sitting on top of the stomach that helps to filter the blood. It has a large concentration of white blood cells and acts like a giant lymph node. As blood passes through the spleen, they are exposed to white blood cells
  6. Thymus: Organ in the upper part of the body just below where a necklace might sit. This is where certain white blood cells called T-cells develop
  7. Tonsil: These are organs located at the back of the mouth that contains white blood cells. They serve to destroy antigens that may be in our food or in the air we breath. Hence, they prevent lung and throat infections

Other components of the immune system are fever and inflammation. The heat associated with fever creates an unfavorable condition for the growth of certain infections such as bacteria. Inflammation is a response to infection involving certain typical symptoms such as pain, swelling, soreness, fever, rash and fatigue. These uncomfortable symptoms send signals to the body that it is under attack. The body responds by activating the immune system and sending help to the area that is affected.  

There are two major parts of the immune system, the innate or random immunity and the specific or adaptive immunity. The innate immune system randomly attacks and kills antigens without specificity. In other words, they are not like snipers picking out specific target, they kill just about any antigen in its path. The specific immune system on the other hand is very selective. It will kill only specific antigens. Let’s take a look at both of these and see how they work. 

The Innate (Random Immune System)

The innate system consists of the skin, mucous membranes, complementary proteins, natural killer cells, interferon proteins and white blood cells called phagocytes. The skin and mucous provides a natural barrier, preventing antigens from getting into the body. Phagocytes are white blood cells that engulf and destroy antigens. They may come in different forms such as neutrophils, monocytes, macrophages, mast cells, and dendritic cells. Dendritic cells are called “antigen-presenting cells” since they wear the proteins of the antigens they destroy on their surfaces and present them to T-cells (more about that in the next section). Complementary proteins provide backup to the phagocytes by lysing (bursting) pathogens and making them inactive. They also signal more phagocytes to come to the area to provide help. Natural Killer Cells command infected and cancer cells in the body to commit suicide so that the disease does not spread. Interferon proteins provides added protection to cells that are not yet infected, and enhance the activity of natural killer cells and phagocytes.

The Adaptive Immune System

The adaptive immune cells consists of B and T white blood cells called lymphocytes since they concentrate in the lymph node. B cells destroy antigens that are external to the cells and are trying to get into the cells while T-cells destroy antigens already in the cell such as viruses, tumors or body transplants that the body rejects. When B cells comes into contact with an antigen they are converted to “plasma cells”. These are cells that produce specific antibodies (also known as immunoglobulins) to fight against that specific antigen. Antibodies bind with the antigens preventing them from attacking body cells and making it easier for phagocytes to destroy them. Plasma cells can live only a few days (4-5) and so B cells also produce memory cells which has the ability to last your entire life. That means if you are infected by that antigen again, you will quickly be able to destroy it. Hence we say that you are “immune” to the infection. 

When a certain type of T cells called Helper T or CD4 cells comes into contact with dendritic cells, they ID the antigen on the surface of the cell and produce clones of themselves along with memory cells. The clones stimulate production of antibodies by plasma cells and also activate phagocyte and natural killer cell activity. Like the memory B cells, the memory T cells survive for many years providing life-time immunity.  Exposure to dendritic cells also stimulate another type of T-cells called CD8 cells to clone themselves and also produce memory T-cells. CD8 cells are called cytotoxic cells and are primarily responsible for destroying cancer and infected cells. 

Fig. Summary of how the adaptive immune system works

As we approach old age, our immune system does not work as well as it did in our youth. The thymus atrophies reducing the number of T Cells. This cause a reduction in antibodies and lower activity of phagocytes that as we learned earlier are stimulated when T cells multiply. This result in greater risk for disease including cancer cell development.

Courtney Simons
Courtney Simons is a food science professor. He holds a BS degree in food science and a Ph.D. in cereal science from North Dakota State University.
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