How the Immune System Works

The immune system is an organization of specialized cells, tissues, proteins, and organs that interact in a dynamic way to protect the human body against foreign invaders. Microbes, such as bacteria, parasites, and viruses are the principal targets of the immune system. Through the production of antigens (immune response) the body attacks organisms which may threaten the human system. The following is a brief explanation at how the immune system works.

The immune system has an efficient method to recognize its own (self) and foreign (nonself) cells. The body normally does not attack its own cells; however, when the system encounters organisms that carry nonself markers, it launches an attack to destroy them. Foreign organisms are called antigens, such as bacteria, fungi, parasites, and in some cases, tissues from another person (transplanted organs). An autoimmune disease may occur when the immune system is unable to recognize its own cells, attacking them.


The organs of the immune system are known as the lymphoid organs. The bone marrow, which is the soft tissue in the hollow center of bones, is the main manufacturer of blood cells, including the immune cells. The thymus, where the T lymphocytes mature, is located behind the breastbone. The spleen is a flattened organ located in the upper left of the stomach. The lymph nodes are distributed in various parts of the body with clusters in the neck, armpits, abdomen, and groin. Other lymphoid tissues include the tonsils, adenoids, and the appendix.

The principal cells in the immune system are lymphocytes (B and T cells), and phagocytes (neutrophils, macrophages, and dendritic cells). T cells mature in the thymus where they acquire their receptors for specific antigens. B cells mature into plasma cells in the bone marrow. B cells secrete substances known as antibodies. When an antigen specific antibody matches an antigen receptor, the B cell turns into a large plasma cell factory, producing identical copies of antibody molecules. Lymphocytes can move in and out of the bloodstream and lymphatic vessels, enabling them to monitor the body for invading antigens.

Neutrophils are produced in the bone marrow and travel through the blood stream to sites of infection where they kill microbes by phagocytosis after which they die.  Macrophages destroy antigens by phagocytosing them. Dendritic cells are antigen presenting cells. They trap antigens circulating in the lymph and blood and present it to the resident lymphocyte which then turns into a plasma cell, releasing millions of antibodies. They can also present antigens from non lymphoid tissue and present it to lymphocytes.

Layer defense:

The skin is the first natural protection the human body possesses against infection. The mucus membranes trap and expel the germs that might have entered the respiratory cavities. The chemical secretions of enzymes contained in the saliva, tears, and breast milk are antibacterial. Vaginal secretions and semen contain defenses against pathogens. Gastric acid and proteases contain chemicals that kill germs that might have been ingested along with food. The lungs and the respiratory tract get rid of pathogens by coughing or sneezing. When pathogens get through the first line of defense, they encounter another line of defense which is known as innate response.

Innate response:

The innate immune system responds to infection by secreting a series of chemical signals that cause inflammation, fever, and many other responses to injury or infection. Toll-like receptors (TLRs) are proteins that the body uses to defend itself against microbes. Innate immune response is a quick and general attack on pathogens and antigens. This type of response is encoded in the genes people inherit from their parents.

Acquired (adaptive) response:

The immune system has developed a method of immunological memory that recognizes a specific pathogen more effectively. Provided that the body has had a primary encounter with certain pathogen, the body acquires immunity for that same pathogen if it encounters it for a second time. Long-term immunity is acquired following infection by the activation of B and T cells. Immunity can also be induced by vaccination; the method of this process stimulates the immune system to produce antibodies which may prevent the development of disease by the same pathogen (immunity) in the future.

The immune system is an extraordinary defense against external invaders and it protects the human body so efficiently that not a single germ can pass through undetected; however, sometimes a germ can cause harm and make you feel sick. Therefore, it is important to maintain a well functioning immune system by improving your diet, taking the necessary vitamins and minerals, as well as engaging in proper physical activities.