What Is the Immune System?
The immune system is a complex network of cells, tissues, and organs that work harmoniously to defend the body against foreign invaders. Primarily, the foreign invaders are microbes that can cause infection (bacteria, parasites, or fungi). The immune system works to keep foreign invaders out of the body, or if they do enter the body, to find and destroy them.
How Does the Immune System Work?
The immune system operates like a sophisticated communications system. When a foreign invader enters the body, the immune system is alerted. At that point, the immune system cells are activated and start to produce powerful chemicals. Immune cells communicate by direct physical contact or they can communicate by releasing chemical messengers.
The skin serves as an initial barrier to invading microbes. Invaders can gain entry through cuts or cracks in the skin, though. The digestive and respiratory tracts also can be points of entry for foreign invaders but they, too, have their own means to protect against invaders (for example, mucus in the nose, cough or sneeze to keep invaders out of nose and lungs, stomach acid destroys invaders in the gut). If microbes do penetrate these initial barriers, they still must make it through the walls of the digestive, respiratory, or urogenital passages to reach underlying cells. The passages are lined with epithelial cells covered in a layer of mucus to help block the transport of invaders into deeper cell layers.
The mucosal surfaces secrete IgA, often the first type of antibody to encounter an invading microbe. Underneath the epithelial layer, various immune cells, including macrophages, B cells, and T cells, wait for invaders that might get beyond the barriers at the surface. Once past the surface, the invaders then must get beyond general defenses of the innate immune system (patrolling phagocytes, natural killer T cells, and complement). If the invaders make it past the general defenses, they meet up with specific weapons of the adaptive immune system, primarily antibodies and T cells which have receptors that direct them to their targets.
What Is the Role of Immune Cells?
The immune system has an army of cells at the ready (including lymphocytes and phagocytes). While certain immune cells attack all invaders, others are trained to respond to specific targets only. All immune cells are derived from immature stem cells in the bone marrow. The immature cells, in response to different cytokines and other chemical signals, develop into specific immune cell types (T cells, B cells, or phagocytes).
B cells and T cells are types of lymphocytes. B cells secrete antibodies into the body's fluids. Antibodies attack foreign invaders (acting as antigens) that are found circulating in body fluids but antibodies are not able to penetrate cells. T cells, on the other hand, have specialized antibody-like receptors on their surface that recognize fragments of antigens on infected cells. T cells can direct and regulate immune responses, or they can directly attack infected or cancerous cells.
Phagocytes are large white cells that consume foreign invaders or foreign particles. Monocytes are a type of phagocyte that circulates in the bloodstream. When monocytes migrate into tissues, they turn into into macrophages. As macrophages, they are able to rid the body of old cells and debris. Macrophages also can display bits of foreign antigen to attract matching lymphocytes. They also produce chemical signals which are essential for the immune response. Granulocytes, mast cells, platelets, and dendritic cells also have significant roles in the immune response.
Immune system cells communicate with each other by releasing and responding to chemical messengers, known as cytokines. The cytokines, which include interleukins, interferons, and growth factors, are proteins which are secreted by immune cells to act on other cells, producing the immune response to foreign invaders.
Immunity Should Safeguard Good Health
While we have described how the immune response protects us from foreign invaders and the consequence of illness, immune tolerance is essential to preventing illness as well. Immune tolerance describes how T or B lymphocytes ignore the body's own tissues while searching out foreign invaders. Immune tolerance is essential to prevent the immune system from attacking the body's own cells.
When the body's immune system fails to function properly, the body produces T cells and antibodies directed against antigens in its own cells and tissues, in other words, against itself. When this occurs, healthy cells and tissues are damaged, and autoimmune disease can develop. Rheumatoid arthritis and lupus are examples of autoimmune diseases. Aside from autoimmune reactions, the immune system can be responsible for allergic disease, immune complex disorders, and immune deficiency disorders.
Molecular and Cellular Basis of Immunity and Immunological Diseases. Primer on the Rheumatic Diseases. Klippel J. Pages 94-97. Published by the Arthritis Foundation. Thirteeth Edition.