How the immune system works

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Understanding Bacteria

Immune system

It originates from medicine and early studies on the causes of immunity to disease. The earliest known reference to immunity was during the plague of Athens in BC.

Thucydides noted that people who had recovered from a previous bout of the disease could nurse the sick without contracting the illness a second time. It was not until Robert Koch 's proofs , for which he was awarded a Nobel Prize in , that microorganisms were confirmed as the cause of infectious disease.

Immunology made a great advance towards the end of the 19th century, through rapid developments, in the study of humoral immunity and cellular immunity. The immune system protects organisms from infection with layered defenses of increasing specificity. In simple terms, physical barriers prevent pathogens such as bacteria and viruses from entering the organism. If a pathogen breaches these barriers, the innate immune system provides an immediate, but non-specific response. Innate immune systems are found in all plants and animals.

Here, the immune system adapts its response during an infection to improve its recognition of the pathogen. This improved response is then retained after the pathogen has been eliminated, in the form of an immunological memory , and allows the adaptive immune system to mount faster and stronger attacks each time this pathogen is encountered.

Both innate and adaptive immunity depend on the ability of the immune system to distinguish between self and non-self molecules. In immunology, self molecules are those components of an organism's body that can be distinguished from foreign substances by the immune system. One class of non-self molecules are called antigens short for anti body gen erators and are defined as substances that bind to specific immune receptors and elicit an immune response.

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 pattern recognition receptors , which recognize components that are conserved among broad groups of microorganisms, [15] or when damaged, injured or stressed cells send out alarm signals, many of which but not all are recognized by the same receptors as those that recognize pathogens.

The innate immune system is the dominant system of host defense in most organisms. Several barriers protect organisms from infection, including mechanical, chemical, and biological barriers. The waxy cuticle of most leaves , the exoskeleton of insects , the shells and membranes of externally deposited eggs , and skin are examples of mechanical barriers that are the first line of defense against infection.

In the lungs, coughing and sneezing mechanically eject pathogens and other irritants from the respiratory tract. The flushing action of tears and urine also mechanically expels pathogens, while mucus secreted by the respiratory and gastrointestinal tract serves to trap and entangle microorganisms.

Chemical barriers also protect against infection. Within the genitourinary and gastrointestinal tracts, commensal flora serve as biological barriers by competing with pathogenic bacteria for food and space and, in some cases, by changing the conditions in their environment, such as pH or available iron. However, since most antibiotics non-specifically target bacteria and do not affect fungi, oral antibiotics can lead to an "overgrowth" of fungi and cause conditions such as a vaginal candidiasis a yeast infection.

Inflammation is one of the first responses of the immune system to infection. Inflammation is produced by eicosanoids and cytokines , which are released by injured or infected cells. Eicosanoids include prostaglandins that produce fever and the dilation of blood vessels associated with inflammation, and leukotrienes that attract certain white blood cells leukocytes.

These cytokines and other chemicals recruit immune cells to the site of infection and promote healing of any damaged tissue following the removal of pathogens. The complement system is a biochemical cascade that attacks the surfaces of foreign cells. It contains over 20 different proteins and is named for its ability to "complement" the killing of pathogens by antibodies. Complement is the major humoral component of the innate immune response. In humans, this response is activated by complement binding to antibodies that have attached to these microbes or the binding of complement proteins to carbohydrates on the surfaces of microbes.

This recognition signal triggers a rapid killing response. After complement proteins initially bind to the microbe, they activate their protease activity, which in turn activates other complement proteases, and so on. This produces a catalytic cascade that amplifies the initial signal by controlled positive feedback.

This deposition of complement can also kill cells directly by disrupting their plasma membrane. Leukocytes white blood cells act like independent, single-celled organisms and are the second arm of the innate immune system. These cells identify and eliminate pathogens, either by attacking larger pathogens through contact or by engulfing and then killing microorganisms.

Phagocytosis is an important feature of cellular innate immunity performed by cells called phagocytes that engulf, or eat, pathogens or particles. Phagocytes generally patrol the body searching for pathogens, but can be called to specific locations by cytokines.

The pathogen is killed by the activity of digestive enzymes or following a respiratory burst that releases free radicals into the phagolysosome. Neutrophils and macrophages are phagocytes that travel throughout the body in pursuit of invading pathogens. During the acute phase of inflammation, particularly as a result of bacterial infection, neutrophils migrate toward the site of inflammation in a process called chemotaxis, and are usually the first cells to arrive at the scene of infection.

Macrophages are versatile cells that reside within tissues and produce a wide array of chemicals including enzymes, complement proteins , and cytokines, while they can also act as scavengers that rid the body of worn-out cells and other debris, and as antigen-presenting cells that activate the adaptive immune system. Dendritic cells DC are phagocytes in tissues that are in contact with the external environment; therefore, they are located mainly in the skin , nose , lungs, stomach, and intestines.

Dendritic cells serve as a link between the bodily tissues and the innate and adaptive immune systems, as they present antigens to T cells , one of the key cell types of the adaptive immune system.

Mast cells reside in connective tissues and mucous membranes , and regulate the inflammatory response. They secrete chemical mediators that are involved in defending against parasites and play a role in allergic reactions, such as asthma. Natural killer cells , or NK cells, are lymphocytes and a component of the innate immune system which does not directly attack invading microbes.

It is now known that the MHC makeup on the surface of those cells is altered and the NK cells become activated through recognition of "missing self". Normal body cells are not recognized and attacked by NK cells because they express intact self MHC antigens. 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. 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. 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.

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 major histocompatibility complex MHC molecule.

There are two major subtypes of T cells: In addition there are regulatory T cells which have a role in modulating immune response. These two mechanisms of antigen presentation reflect the different roles of the two types of T cell. 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.

Killer T cells are a sub-group of T cells that kill cells that are infected with viruses and other pathogens , or are otherwise damaged or dysfunctional. Recognition of this MHC: The T cell then travels throughout the body in search of cells where the MHC I receptors bear this antigen. When an activated T cell contacts such cells, it releases cytotoxins , such as perforin , which form pores in the target cell's plasma membrane , allowing ions , water and toxins to enter.

The entry of another toxin called granulysin a protease induces the target cell to undergo apoptosis. Helper T cells regulate both the innate and adaptive immune responses and help determine which immune responses the body makes to a particular pathogen. They instead control the immune response by directing other cells to perform these tasks. Helper T cells have a weaker association with the MHC: Helper T cell activation also requires longer duration of engagement with an antigen-presenting cell.

Cytokine signals produced by helper T cells enhance the microbicidal function of macrophages and the activity of killer T cells. On the other hand, the various subsets are also part of the innate immune system, as restricted TCR or NK receptors may be used as pattern recognition receptors. A B cell identifies pathogens when antibodies on its surface bind to a specific foreign antigen. This combination of MHC and antigen attracts a matching helper T cell, which releases lymphokines and activates the B cell.

These antibodies circulate in blood plasma and lymph , bind to pathogens expressing the antigen and mark them for destruction by complement activation or for uptake and destruction by phagocytes. Antibodies can also neutralize challenges directly, by binding to bacterial toxins or by interfering with the receptors that viruses and bacteria use to infect cells. Evolution of the adaptive immune system occurred in an ancestor of the jawed vertebrates. Many of the classical molecules of the adaptive immune system e.

However, 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. 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. Newborn infants have no prior exposure to microbes and are particularly vulnerable to infection. Several layers of passive protection are provided by the mother.

During pregnancy , a particular type of antibody, called IgG , is transported from mother to baby directly through the placenta , so human babies have high levels of antibodies even at birth, with the same range of antigen specificities as their mother.

This passive immunity is usually short-term, lasting from a few days up to several months. In medicine, protective passive immunity can also be transferred artificially from one individual to another via antibody-rich serum.

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.

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. Most viral vaccines are based on live attenuated viruses, while many bacterial vaccines are based on acellular components of micro-organisms, including harmless toxin components. 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 occur when one or more of the components of the immune system are inactive. The ability of the immune system to respond to pathogens is diminished in both the young and the elderly , with immune responses beginning to decline at around 50 years of age due to immunosenescence. Additionally, the loss of the thymus at an early age through genetic mutation or surgical removal results in severe immunodeficiency and a high susceptibility to infection.

Immunodeficiencies can also be inherited or ' acquired'. AIDS and some types of cancer cause acquired immunodeficiency. Overactive immune responses comprise the other end of immune dysfunction, particularly the autoimmune disorders. Here, the immune system fails to properly distinguish between self and non-self, and attacks part of the body.

Under normal circumstances, many T cells and antibodies react with "self" peptides. Hypersensitivity is an immune response that damages the body's own tissues. 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.

This is also called antibody-dependent or cytotoxic hypersensitivity, and is mediated by IgG and IgM antibodies. 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. Inflammation is one of the first responses of the immune system to infection, [28] but it can appear without known cause. It is likely that a multicomponent, adaptive immune system arose with the first vertebrates , as invertebrates do not generate lymphocytes or an antibody-based humoral response. Immune systems appear even in the structurally most simple forms of life, with bacteria using a unique defense mechanism, called the restriction modification system to protect themselves from viral pathogens, called bacteriophages.

Pattern recognition receptors are proteins used by nearly all organisms to identify molecules associated with pathogens. Antimicrobial peptides called defensins are an evolutionarily conserved component of the innate immune response found in all animals and plants, and represent the main form of invertebrate systemic immunity. Ribonucleases and the RNA interference pathway are conserved across all eukaryotes , and are thought to play a role in the immune response to viruses.

Unlike animals, plants lack phagocytic cells, but many plant immune responses involve systemic chemical signals that are sent through a plant. Systemic acquired resistance SAR is a type of defensive response used by plants that renders the entire plant resistant to a particular infectious agent.

Another important role of the immune system is to identify and eliminate tumors. This is called immune surveillance. The transformed cells of tumors express antigens that are not found on normal cells. To the immune system, these antigens appear foreign, and their presence causes immune cells to attack the transformed tumor cells. The antigens expressed by tumors have several sources; [90] some are derived from oncogenic viruses like human papillomavirus , which causes cervical cancer , [91] while others are the organism's own proteins that occur at low levels in normal cells but reach high levels in tumor cells.

One example is an enzyme called tyrosinase that, when expressed at high levels, transforms certain skin cells e. 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. This allows killer T cells to recognize the tumor cell as abnormal. Clearly, some tumors evade the immune system and go on to become cancers.

Paradoxically, macrophages can promote tumor growth [] when tumor cells send out cytokines that attract macrophages, which then generate cytokines and growth factors such as tumor-necrosis factor alpha that nurture tumor development or promote stem-cell-like plasticity.

The immune system is involved in many aspects of physiological regulation in the body. The immune system interacts intimately with other systems, such as the endocrine [] [] and the nervous [] [] [] systems. The immune system also plays a crucial role in embryogenesis development of the embryo , as well as in tissue repair and regeneration. Hormones can act as immunomodulators , altering the sensitivity of the immune system.

For example, female sex hormones are known immunostimulators of both adaptive [] and innate immune responses. By contrast, male sex hormones such as testosterone seem to be immunosuppressive. When a T-cell encounters a foreign pathogen , it extends a vitamin D receptor. This is essentially a signaling device that allows the T-cell to bind to the active form of vitamin D , the steroid hormone calcitriol.

T-cells have a symbiotic relationship with vitamin D. Not only does the T-cell extend a vitamin D receptor, in essence asking to bind to the steroid hormone version of vitamin D, calcitriol, but the T-cell expresses the gene CYP27B1 , which is the gene responsible for converting the pre-hormone version of vitamin D, calcidiol into the steroid hormone version, calcitriol. Only after binding to calcitriol can T-cells perform their intended function.

Other immune system cells that are known to express CYP27B1 and thus activate vitamin D calcidiol, are dendritic cells , keratinocytes and macrophages. It is conjectured that a progressive decline in hormone levels with age is partially responsible for weakened immune responses in aging individuals. As people age, two things happen that negatively affect their vitamin D levels.

First, they stay indoors more due to decreased activity levels. This means that they get less sun and therefore produce less cholecalciferol via UVB radiation. Second, as a person ages the skin becomes less adept at producing vitamin D. The immune system is affected by sleep and rest, [] and sleep deprivation is detrimental to immune function. When suffering from sleep deprivation, active immunizations may have a diminished effect and may result in lower antibody production, and a lower immune response, than would be noted in a well-rested individual.

Additionally, proteins such as NFIL3 , which have been shown to be closely intertwined with both T-cell differentiation and our circadian rhythms, can be affected through the disturbance of natural light and dark cycles through instances of sleep deprivation, shift work, etc.

As a result, these disruptions can lead to an increase in chronic conditions such as heart disease, chronic pain, and asthma. In addition to the negative consequences of sleep deprivation, sleep and the intertwined circadian system have been shown to have strong regulatory effects on immunological functions affecting both the innate and the adaptive immunity. First, during the early slow-wave-sleep stage, a sudden drop in blood levels of cortisol , epinephrine , and norepinephrine induce increased blood levels of the hormones leptin, pituitary growth hormone, and prolactin.

These signals induce a pro-inflammatory state through the production of the pro-inflammatory cytokines interleukin-1, interleukin , TNF-alpha and IFN-gamma. These cytokines then stimulate immune functions such as immune cells activation, proliferation, and differentiation. It is during this time that undifferentiated, or less differentiated, like naïve and central memory T cells, peak i.

It usually infects the warm, moist areas of the body like the eyes, throat, vagina, anus, and urethra. Gonorrhea infection is passed through unprotected oral, anal, or vaginal sex. This STD requires an immediate and thorough treatment. Gonorrhea, if not treated in time, can have serious complications and can also be life-threatening. It also holds the risk of transmission through sexual partners, if and when not treated. In the above treatments for gonorrhea, you have to make sure that you follow the medication as prescribed and do not miss out on any doses.

It is also important that your sexual partner is also treated for the same, so as to prevent the risk of further transmission of the disease. In such cases, one needs to get retested for the STD and thereby should follow up with an alternate treatment of antibiotics. Some of these antibiotics that do not work in killing the gonorrhea virus include penicillin, tetracycline, quinolones, and sulfa drugs.

The above-mentioned treatments for gonorrhea can be availed from any sexual health care clinic near you. While medication can stop the infection from causing any further damages, it cannot reverse any lasting damage that has already been caused by the disease. One of the best ways to boost the immune system and keep your body running on a healthy course is zinc. This essential mineral can increase the number of antibodies and white blood cells, while also directly affecting the bacterial infection of gonorrhea.

Garlic and its important active ingredient, allicin, is another popular home remedy for gonorrhea. This is one of the most easily accessible and popular antibacterial foods that you can add to your diet, and it also happens to be versatile in culinary preparations.

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