Immune System

The immune system is a system of biological structures and tissue.

Pathogens can rapidly evolve and adapt to avoid detection and neutralization by the immune system. As a result, multiple defense mechanisms have also evolved to recognize and neutralize pathogens. Even simple unicellular organisms such as bacteria possess a rudimentary immune system, in the form of enzymes that protect against bacteriophage infections. Other basic immune mechanisms evolved in ancient eukaryotes and remain in their modern descendants, such as plants and insects. These mechanisms include phagocytosis, antimicrobial peptides called defensins, and the complement system. Jawed vertebrates, including humans, have even more sophisticated defense mechanisms,^[1] including the ability to adapt over time to recognize specific pathogens more efficiently. Adaptive (or acquired) immunity creates immunological memory after an initial response to a specific pathogen, leading to an enhanced response to subsequent encounters with that same pathogen. This process of acquired immunity is the basis of vaccination.

Disorders of the immune system can result in Immunology covers the study of all aspects of the immune system.

Contents 1 History of immunology 2 Layered defense 3 Surface barriers 4 Innate immune system 4.1 Humoral and chemical barriers 4.1.1 Inflammation 4.1.2 Complement system 4.2 Cellular barriers 5 Adaptive immune system 5.1 Lymphocytes 5.1.1 Killer T cells 5.1.2 Helper T cells 5.1.3 γδ T cells 5.1.4 B lymphocytes and antibodies 5.1.5 Alternative adaptive immune system 5.2 Immunological memory 5.2.1 Passive memory 5.2.2 Active memory and immunization 6 Disorders of human immunity 6.1 Immunodeficiencies 6.2 Autoimmunity 6.3 Hypersensitivity 7 Other mechanisms 8 Tumor immunology 9 Physiological regulation 9.1 Nutrition and diet 10 Manipulation in medicine 11 Manipulation by pathogens 12 See also 13 References 14 External links

[edit] History of immunology

[8]

Immunology made a great advance towards the end of the 19th century, through rapid developments, in the study of humoral immunity and cellular immunity.^[9] Particularly important was the work of Paul Ehrlich, who proposed the side-chain theory to explain the specificity of the antigen-antibody reaction; his contributions to the understanding of humoral immunity were recognized by the award of a Nobel Prize in 1908, which was jointly awarded to the founder of cellular immunology, Elie Metchnikoff.^[10]

[edit] Layered defense

The immune system protects organisms from [12]

Both innate and adaptive immunity depend on the ability of the immune system to distinguish between self and non-self [14]

[edit] Surface barriers

Several barriers protect organisms from infection, including mechanical, chemical, and biological barriers. The waxy [15]

Chemical barriers also protect against infection. The skin and respiratory tract secrete proteases serve as powerful chemical defenses against ingested pathogens.

Within the genitourinary and gastrointestinal tracts, [25]

[edit] Innate immune system

Microorganisms or toxins that successfully enter an organism encounter the cells and mechanisms of the innate immune system. The innate response is usually triggered when microbes are identified by [11]

[edit] Humoral and chemical barriers

[edit] Inflammation

Inflammation is one of the first responses of the immune system to infection.^[32]

[edit] Complement system

The complement system is a [35]

In humans, this response is activated by complement binding to antibodies that have attached to these microbes or the binding of complement proteins to [33]

[edit] Cellular barriers

Leukocytes ([12]

[41]

Neutrophils and macrophages are phagocytes that travel throughout the body in pursuit of invading pathogens.^[12]

Dendritic cells (DC) are phagocytes in tissues that are in contact with the external environment; therefore, they are located mainly in the [45]

Mast cells reside in [48]

[edit] Adaptive immune system

The adaptive immune system evolved in early vertebrates and allows for a stronger immune response as well as immunological memory, where each pathogen is “remembered” by a signature antigen.^[49] The adaptive immune response is antigen-specific and requires the recognition of specific “non-self” antigens during a process called antigen presentation. Antigen specificity allows for the generation of responses that are tailored to specific pathogens or pathogen-infected cells. The ability to mount these tailored responses is maintained in the body by “memory cells”. Should a pathogen infect the body more than once, these specific memory cells are used to quickly eliminate it.

[edit] Lymphocytes

The cells of the adaptive immune system are special types of leukocytes, called lymphocytes. B cells and T cells are the major types of lymphocytes and are derived from hematopoietic stem cells in the bone marrow.^[35] B cells are involved in the humoral immune response, whereas T cells are involved in cell-mediated immune response.

Both B cells and T cells carry receptor molecules that recognize specific targets. T cells recognize a “non-self” target, such as a pathogen, only after antigens (small fragments of the pathogen) have been processed and presented in combination with a “self” receptor called a [50]

In contrast, the B cell antigen-specific receptor is an antibody molecule on the B cell surface, and recognizes whole pathogens without any need for antigen processing. Each lineage of B cell expresses a different antibody, so the complete set of B cell antigen receptors represent all the antibodies that the body can manufacture.^[35]

[edit] Killer T cells

[52]

[edit] Helper T cells

[54] These cells have no cytotoxic activity and do not kill infected cells or clear pathogens directly. They instead control the immune response by directing other cells to perform these tasks.

Helper T cells express T cell receptors (TCR) that recognize antigen bound to Class II MHC molecules. The MHC:antigen complex is also recognized by the helper cell’s [56]

[edit] γδ T cells

[50]

[edit] B lymphocytes and antibodies

A [61]

[edit] Alternative adaptive immune system

Although the classical molecules of the adaptive immune system (e.g., antibodies and T cell receptors) exist only in jawed vertebrates, a distinct lymphocyte-derived molecule has been discovered in primitive jawless vertebrates, such as the lamprey and hagfish. These animals possess a large array of molecules called variable lymphocyte receptors (VLRs) that, like the antigen receptors of jawed vertebrates, are produced from only a small number (one or two) of genes. These molecules are believed to bind pathogenic antigens in a similar way to antibodies, and with the same degree of specificity.^[62]

[edit] Immunological memory

When B cells and T cells are activated and begin to replicate, some of their offspring become long-lived memory cells. Throughout the lifetime of an animal, these memory cells remember each specific pathogen encountered and can mount a strong response if the pathogen is detected again. This is “adaptive” because it occurs during the lifetime of an individual as an adaptation to infection with that pathogen and prepares the immune system for future challenges. Immunological memory can be in the form of either passive short-term memory or active long-term memory.

[edit] Passive memory

Newborn [65]

[edit] Active memory and immunization

Long-term active memory is acquired following infection by activation of B and T cells. Active immunity can also be generated artificially, through vaccination. The principle behind vaccination (also called immunization) is to introduce an antigen from a pathogen in order to stimulate the immune system and develop specific immunity against that particular pathogen without causing disease associated with that organism.^[14] This deliberate induction of an immune response is successful because it exploits the natural specificity of the immune system, as well as its inducibility. With infectious disease remaining one of the leading causes of death in the human population, vaccination represents the most effective manipulation of the immune system mankind has developed.^[35][66]

Most viral [67]

[edit] Disorders of human immunity

The immune system is a remarkably effective structure that incorporates specificity, inducibility and adaptation. Failures of host defense do occur, however, and fall into three broad categories: immunodeficiencies, autoimmunity, and hypersensitivities.

[edit] Immunodeficiencies

[70]

Immunodeficiencies can also be inherited or ‘[72]

[edit] Autoimmunity

Overactive immune responses comprise the other end of immune dysfunction, particularly the [59]

[edit] Hypersensitivity

Hypersensitivity is an immune response that damages the body’s own tissues. They are divided into four classes (Type I – IV) based on the mechanisms involved and the time course of the hypersensitive reaction. Type I hypersensitivity is an immediate or anaphylactic reaction, often associated with allergy. Symptoms can range from mild discomfort to death. Type I hypersensitivity is mediated by IgE, which triggers degranulation of mast cells and basophils when cross-linked by antigen.^[74] Type II hypersensitivity occurs when antibodies bind to antigens on the patient’s own cells, marking them for destruction. This is also called antibody-dependent (or cytotoxic) hypersensitivity, and is mediated by IgG and IgM antibodies.^[74] Immune complexes (aggregations of antigens, complement proteins, and IgG and IgM antibodies) deposited in various tissues trigger Type III hypersensitivity reactions.^[74] Type IV hypersensitivity (also known as cell-mediated or delayed type hypersensitivity) usually takes between two and three days to develop. Type IV reactions are involved in many autoimmune and infectious diseases, but may also involve contact dermatitis (poison ivy). These reactions are mediated by T cells, monocytes, and macrophages.^[74]

[edit] Other mechanisms

It is likely that a multicomponent, adaptive immune system arose with the first [77]

[78]

Unlike animals, plants lack phagocytic cells, but many plant immune responses involve systemic chemical signals that are sent through a plant.^[81]

[edit] Tumor immunology

Another important role of the immune system is to identify and eliminate [88]

The main response of the immune system to tumors is to destroy the abnormal cells using killer T cells, sometimes with the assistance of helper T cells.^[87]

Clearly, some tumors evade the immune system and go on to become cancers.^[92]

Paradoxically, macrophages can promote tumor growth ^[94] when tumor cells send out cytokines that attract macrophages, which then generate cytokines and growth factors that nurture tumor development. In addition, a combination of hypoxia in the tumor and a cytokine produced by macrophages induces tumor cells to decrease production of a protein that blocks metastasis and thereby assists spread of cancer cells.

[edit] Physiological regulation

[99]

When a T-cell encounters a foreign [101]

It is conjectured that a progressive decline in hormone levels with age is partially responsible for weakened immune responses in aging individuals.^[104]

The immune system is affected by sleep and rest,^[108]

[edit] Nutrition and diet

Overnutrition is associated with diseases such as diabetes and obesity, which are known to affect immune function. More moderate malnutrition, as well as certain specific trace mineral and nutrient deficiencies, can also compromise the immune response.^{[109][page needed]}

Foods rich in certain [111]

[edit] Manipulation in medicine

The immune response can be manipulated to suppress unwanted responses resulting from autoimmunity, allergy, and [112]

[114]

Larger drugs (>500 proteomics) involved in the immune response.

[edit] Manipulation by pathogens

The success of any pathogen depends on its ability to elude host immune responses. Therefore, pathogens evolved several methods that allow them to successfully infect a host, while evading detection or destruction by the immune system.^[121]

An evasion strategy used by several pathogens to avoid the innate immune system is to hide within the cells of their host (also called [124]

The mechanisms used to evade the adaptive immune system are more complicated. The simplest approach is to rapidly change non-essential [127]

[edit] See also

Wikimedia Commons has media related to: Immunology

• Clonal selection
• Hapten
• Human physiology
• Immunoproteomics
• Immunostimulator
• Original antigenic sin
• Tumor antigens
• Immune system receptors
• Polyclonal response
• Plant disease resistance
• Immune network theory

[edit] References

[edit] External links

• Immune System – from the University of Hartford (high school/undergraduate level)
• Microbiology and Immunology On-Line Textbook – from the University of South Carolina School of Medicine (undergraduate level)
• Immunobiology; Fifth Edition – Online version of the textbook by Charles Janeway (Advanced undergraduate/graduate level)

v t e adaptive immune system and complement Lymphoid Antigens Antigen Superantigen Allergen Hapten Epitope Linear Conformational Mimotope Dendritic cell Macrophage B cell Antibodies Antibody Monoclonal antibodies Polyclonal antibodies Autoantibody Microantibody Polyclonal B cell response Allotype Isotype Idiotype Immune complex Paratope Immunity vs. tolerance action: Immunity Autoimmunity Alloimmunity Allergy Hypersensitivity Inflammation Cross-reactivity inaction: Tolerance Central Peripheral Clonal anergy Clonal deletion Tolerance in pregnancy Immunodeficiency Immunogenetics Somatic hypermutation Clonal selection) V(D)J recombination Junctional diversity Immunoglobulin class switching HLA Lymphocytes T cell) Humoral (B cell) NK cell Substances Cytokines Opsonin Cytolysin Complement Anaphylatoxins LMC igrc tumr 4)

v t e 9) Primary lymphoid organs Bone marrow Hematopoietic stem cell Thymus Cortex Medulla Hassall’s corpuscles Secondary lymphoid organs Spleen (process blood) structural: Hilum Trabeculae Diaphragmatic surface of spleen Visceral surface of spleen Red pulp Cords of Billroth Marginal zone White pulp Periarteriolar lymphoid sheaths Germinal center blood flow: Trabecular arteries Trabecular veins Tonsils/Waldeyer’s tonsillar ring Palatine Lingual Pharyngeal Tubal Tonsillar crypts Lymph nodes (process extracellular fluid) lymph flow: Afferent lymph vessels Cortical sinuses Medullary sinuses Efferent lymph vessels High endothelial venules B cells: Primary follicle/Germinal center Mantle zone Marginal zone layers: Trabeculae Subcapsular sinus Cortex Paracortex Medullary cord) Hilum MALT (process mucosa) GALT Peyer’s patch Germinal center LMO devp tumr proc

v t e organs TA 2–4: MS Skeletal system Bone Carpus Collar bone (clavicle) Thigh bone (femur) Fibula Humerus Mandible Metacarpus Metatarsus Ossicles Patella Phalanges Radius Skull (cranium) Tarsus Tibia Ulna Rib Vertebra Pelvis Sternum Cartilage Joints Fibrous joint Cartilaginous joint Synovial joint Muscular system Muscle Tendon Diaphragm TA 5–11: splanchnic/ viscus mostly Thoracic Respiratory system URT Nose Nasopharynx Larynx LRT Trachea Bronchus Lung mostly Abdominopelvic Digestive system+ adnexa Mouth Salivary gland Tongue upper GI Oropharynx Laryngopharynx Esophagus Stomach lower GI Small intestine Appendix Colon Rectum Anus accessory Liver Biliary tract Pancreas Urinary system Kidney Ureter Bladder Urethra Reproductive system Female Uterus Vagina Vulva Ovary Placenta Male Scrotum Penis Prostate Testicle Seminal vesicle Endocrine system Pituitary Pineal Thyroid Parathyroid Adrenal Islets of Langerhans TA 12–16 Circulatory system Cardiovascular system peripheral Artery Vein Lymphatic vessel Heart Lymphatic system primary Bone marrow Thymus secondary Spleen Lymph node Nervous system Brain Spinal cord Nerve Sensory system Ear Eye Integumentary system Skin Subcutaneous tissue Breast Mammary gland Blood (Non-TA) Myeloid Myeloid immune system Lymphoid Lymphoid immune system General superficial anatomy of limbs

v t e immune receptors Antibody receptor: Fc receptor Epsilon (ε) FcεRI · (FcεRII is C-type lectin) Gamma (γ) Neonatal Alpha (α)/mu (μ) FcαRI · Fcα/μR Secretory Polymeric immunoglobulin receptor Antigen receptor B cells Antigen receptor BCR Co-receptor stimulate: CD81 inhibit: CD22 Accessory molecules CD79) T cells Ligands MHC class II) Antigen receptor TRG@ Co-receptors CD8 (with two glycoprotein chains CD8α and CD8β) · CD4 Accessory molecules TCRζ) Cytokine receptor see cytokine receptors Killer-cell IG-like receptors KIR3DS1 Leukocyte IG-like receptors LILRA5 tlrp) LMC igrc tumr 4)

 

This article uses material from the Wikipedia article Immune System , which is released under the Creative Commons Attribution-Share-Alike License 3.0.