24.1:
What is the Immune System?
The immune system is the body's natural system of the innate and adaptive defenses, in charge of providing resistance to diseases.
In the innate system, the first barriers, the skin and associated mucus membranes keep invading microorganisms out of the body via a number of protective mechanisms.
For example, skin is both acidic and contains various bactericidal chemicals to inhibit bacterial growth and elements like mucus-coated hairs in the nose trap inhaled particles to prevent passage through the nasal passages.
Although these surface barriers are rather effective, when they are compromised, the second line of internal defense is activated.
Now, phagocytic cells such as macrophages and mast cells, along with other non-specific cells and chemicals are in charge. They mount an inflammatory response to prevent the spread of infection.
The third line of defense, the adaptive system, responds by employing specific cells called lymphocytes to precisely destroy the infectious agent themselves or through circulating antibodies.
However, before such protection can be provided, the system must be primed, that is it must have previously encountered the foreign substance, forming a memory to produce a stronger attack.
24.1:
What is the Immune System?
Overview
The immune system comprises diverse biological structures and processes that protect the body from disease. These processes can be classified into innate and adaptive immunity. To work effectively, the immune system needs to detect pathogens by distinguishing the body’s own structures from foreign elements. If this determination fails, autoimmune diseases occur in which the immune system reacts against the body’s own tissue.
The Innate Immune System Acts Fast and Non-specifically
The innate immune system is the first line of defense against infection. It comprises physical barriers and a variety of cells that act quickly and non-specifically against elements that are foreign to the host (i.e., non-self). Examples of physical barriers in mammals are skin, the lining of the gastrointestinal tract, and secretions, such as mucus or saliva. Once an invader overcomes physical barriers, cells of the inflammatory response are recruited to the entry site: mast cells release a plethora of chemicals that attract other cells of the innate immune system and activates the adaptive immune system. Phagocytic cells, such as neutrophils and macrophages, ingest and destroy pathogens. Natural killer cells, a special type of white blood cell, destroy virus-infected cells. Together, cells of the innate immune system eradicate the invader or hinder its spread, and activate the adaptive immune system.
Pattern Recognition Receptors Enable Organisms to Distinguish Self from Non-self
How can an organism distinguish its own tissue (self) from a foreign element or invader (non-self)? This ability is conferred by pattern recognition receptors (PRRs). These receptors recognize microbe-associated molecular patterns (MAMPs) that are unique to bacteria, viruses, parasites, or fungi. Examples are parts of the bacterial outer membrane or double-stranded RNA of viruses. MAMPs are not specific to a distinct species or pathogen variant but represent a hallmark of a broad class of pathogens (i.e., gram-negative bacteria or fungi). The innate immune system, therefore, acts non-specifically against pathogens.
The Adaptive Immune System Is Highly Specific
Vertebrates evolved the adaptive immune system, which stores a “memory” of a previous attack and can subsequently mount a stronger response against specific pathogens. While the innate immune system employs a broad range of cell types, the adaptive immune system relies on two kinds of white blood cells to target pathogens: B cells and T cells. While T cells are part of the cell-mediated immunity, B cells constitute the humoral branch of adaptive immunity.
B cells can directly destroy a foreign particle or differentiate into plasma cells that release antibodies. Antibodies then target the invader for destruction by other cells. T cells perform several functions, depending on their surface receptor composition and chemical arsenal. All T cells carry surface receptors that are each specific to a single antigen. After encountering the antigen, T cells can stimulate other parts of the immune system or actively destroy infected or cancerous cells. Some B and T cells remain available long after the infection has been cleared and, upon repeated exposure to the same foreign element, mount a stronger and faster immune response.
While the innate immune system acts within minutes to hours of a threat or infection, the adaptive immune system responds over days. Only the adaptive immune system “learns” (i.e., adapts) within the lifetime of an organism.
Autoimmune Diseases
Autoimmune diseases occur when the immune system does not function properly. Autoimmune diseases, generally, develop when the immune system is unable to differentiate the body’s healthy tissue from a foreign element. Typically, one of three pathological effects arises from an autoimmune disease: damage or destruction of tissues, altered organ growth, or altered organ function.
One example of such an autoimmune disease is Type 1 diabetes mellitus (T1DM). During the onset of T1DM, usually in children through early adulthood, immune cells produce antibodies that attack the insulin-producing cells of the pancreas, so-called β cells. The result is the destruction of β cells and subsequently the inability to regulate the blood sugar level. T1DM currently cannot be cured, but insulin treatment, a special diet and exercise help patients manage the disease. It is essential to diagnose the disease early and start management before all insulin-producing cells are destroyed.
Suggested Reading
Riera Romo, Mario, Dayana Pérez‐Martínez, and Camila Castillo Ferrer. “Innate Immunity in Vertebrates: An Overview.” Immunology 148, no. 2 (June 2016): 125–39. [Source]
Yatim, Karim M., and Fadi G. Lakkis. “A Brief Journey through the Immune System.” Clinical Journal of the American Society of Nephrology 10, no. 7 (July 7, 2015): 1274–81. [Source]