Every moment, our immune system is under attack by various foreign agents we call antigens. These include bacteria, viruses, fungi, and toxins. How does the immune system stand up to attack? In this article, I will explain how the immune system works. First, let’s look at the anatomy of the immune system and then talk about the two major components of the system. That is, the innate and the adaptive immune system.

Anatomy of the Immune System

You may not recognize this, but your first line of defense in protecting your body from outside invaders is your skin. Your skin is like a package. Just like how packaging materials protect food, your skin protects you. Inside of your body, the immune system starts with hematopoietic (from Greek, “blood-making”) stem cells in the bone marrow. These cells differentiate into red blood cells and the many types of white blood cells (leukocytes) that helps to destroy antigens. Once made in the bone marrow, leukocytes move into the blood and lymphatic system where they circulate looking for invaders. The lymphatic system is a network of tubes throughout the body similar to the circulatory system. It acts as a drainage system to capture and return fluid from the tissues back to the blood. It is also responsible for transporting absorbed fats to the blood after we eat. The lymphatic system consists of:

  1. A network of nodes throughout the body where you will find white blood cells known as lymphocytes. This includes the tonsils at the top and back of the throat. The tonsil prevents germs from the air, food, and water from getting in the body. A type of lymphocyte called B-cells is responsible for making antibodies (also called immunoglobulins) that are used to trap antigens.
  2. The thymus, a gland where T-cells (a type of lymphocytes) complete their maturation.
  3. The spleen, the largest organ in the lymphatic system, is a screen that removes cellular waste and old or damaged red blood cells. It is also a major site for eliminating germs using lymphocytes.

The Lymphatic System. Source. WikiCommons

Examples of White Blood Cells

The image below shows examples of white blood cells. They are not actually white but clear and transparent. They are stained purple for viewing under a microscope.

The Innate Immune System

The innate immune system consists of the skin, mucous membranes, interferon proteins, complementary proteins, and leukocytes which includes natural killer cells, phagocytes, and granulocytes. The role of each is listed below.

  • The skin and mucous: Provides a natural barrier, preventing antigens from getting into the body.
  • Interferon proteins: Sends signals to cells that are not yet infected, triggering them to prepare defenses against an infection. They also activate white blood cells to do their job more efficiently.
  • Natural killer cells: Command infected and cancer cells in the body to commit suicide so that they do not spread.
  • Phagocytes: Engulf and digest germs e.g. dendritic cells, monocytes, neutrophils, basophils, and mast cells
  • Granulocytes: Produce chemicals that destroy germs e.g., eosinophils, neutrophils, basophils, and mast cells. The best known chemical released by granulocytes is histamine. This chemical sets off an alarm that causes allergic and inflammatory responses such as itching, hives, red eyes, swelling, runny nose, and congestion. Antihistamine medicine helps to present these symptoms
  • Complementary proteins help to lyse (cut up) pathogens and making them inactive. They also signal more phagocytes to come to the area to provide help. 

Other components of the immune system are fever and inflammation. The heat associated with fever creates an unfavorable condition for the growth of certain infection agents such as bacteria. Inflammatory responses include pain, swelling, soreness, 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 immune cells to the area that is affected.  

The Adaptive Immune System

The adaptive immune cells consists of the two major lymphocytes, i.e. B and T-cells. There are two types of T-Cells. There is CD4 (helper) and CD8 (cytotoxic) cells. CD8 cells are able to poke holes by secreting perforins (proteins that create pores) into infected and cancer cells causing lysis. CD4 cells sense the presence of antigens in the body by binding to antigen-presenting cells such as dendritic cells and monocytes.

Previous to presenting themselves to CD4 cells, dendritic cells and monocytes destroy bacteria and then wear their protein on their surface. CD4 cells can “see” these proteins and immediately recognize what type of antigens are attacking. CD4 cells in turn activate phagocytes and B-cell activity. Some of the CD4 cells differentiate into memory cells so that they can mount a response quicker the next time they come in contact with the antigen.

How the Adaptive Immune System Works

Antigen-Antibody Complex

How CD8 Cells Kill Cancer Cells

List of Definitions to Remember

  1. Antigen: A foreign substance that produces an immune response
  2. Antibody: A protein produced in response to an antigen that is capable of counteracting that specific antigen
  3. Stem cell: An undifferentiated cell that is capable of giving rise to mature cells of different kinds
  4. Leukocyte: A white blood cell
  5. Lymphatic system: A network of vessels through which lymph drains from tissues to the blood
  6. Lymphocyte: A white blood cell in the lymphatic system (namely B and T cells)
  7. Thymus: A lymphoid organ in the neck where T cells mature
  8. Spleen: A lymphoid organ that is used to filter the blood of waste and dead blood cells, and is part of the immune system
  9. Phagocyte: A type of white blood cell that is capable of engulfing and digesting antigens
  10. Granulocyte: A type of white blood cell that has granules which can secrete chemicals and destroy antigens
  11. Inflammation: A process by which your immune system protects you from invaders by sending out “first responders” i.e., white blood cells that begins to attack the pathogens. The process usually cause redness, swelling, fever, and pain
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. He also holds Masters degrees in both Environmental Science and Instructional Design from Wright State University.
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