The Immune System in Simple Language

I am so fascinated by everything about the immune system, how it works 24/7 and keeps us safe while we are surrounded by millions of microorganisms, like bacteria, viruses, and fungi. One way to understand the immune system is by viewing the human body as a castle protected by high walls and by soldiers. An antigen, an intruder (bacteria, viruses, and fungus) and an antibody is our defense, a protector.

Line of Defenses

1. Physical Barrier
Skin: The skin, mucous membrane, and secretions of the nose, eyes, and mouth.
Lungs: Thick, sticky mucus that is produced by cells in the respiratory tract, the brushes (cilia) trap dust and microorganisms to prevent them from entering our lungs.
Stomach: The acid in the stomach is mixed with a variety of digestive enzymes, all of which kill most of the microorganisms.

2. Chemical Barriers
When pathogens (intruders) breach the body’s first line of defense (i.e. the skin or mucous membrane), within minutes innate immune system white blood cells are activated as our second defense. They travel throughout the body in pursuit of invaders. Neutrophils found in the bloodstream are the most abundant type of WBC representing 50% to 60% of total circulating leukocytes. These cells are similar to specialized units in police departments that run to the scene of the crime. They travel through the bloodstream to increase blood flow to affected areas and draw a great number of fighters to fight the intruders. They also send special police units like the "professionals" phagocytes and dendritic cells to identify and eliminate pathogens, either by attacking pathogens through contact or by engulfing and then killing microorganisms. Phagocytosis is an important feature of patrolling the body in search of pathogens and engulfing these pathogens or particles. If pathogens penetrate the second line of defense, the third line of defense will act. The third line of defense is the adaptive immune response with lymphocytes like B cells and T cells being the major types of lymphocytes and being derived from the bone marrow. B cells are involved in the humoral immune response, whereas T cells are involved in the cell-mediated immune response. They normally are in a resting state and get activated. B cells are highly specific and will attack and destroy infected cells. Some of these B cells also become memory cells, which will help the body "remember" the intruder and prevent its reinfection. T cells,  known as Helper T cells, activate B cells to attract cytokines (a chemical weapon), so actively killing the cell response is then retained after the pathogen is eliminated, in the form of immunological memory. The T cell travels throughout the body in search of intruders. When an activated T cell contacts infected cells, it releases cytotoxins, which form pores in the target cell allowing water and toxins to enter the target cell to undergo apoptosis. T cell killing of host cells is particularly important in preventing the replication of viruses. The B cells have no cytotoxic activity and do not kill infected pathogens directly. B cells begin to divide their offspring (plasma cells) and secrete millions of copies of the antibody. These antibodies circulate in the blood and bind to pathogens expressing complement activation and destruction. Throughout the lifetime, these memory cells remember each specific pathogen encountered and can mount a strong response if the pathogen is detected again.