According to one hypothesis, organisms that can regenerate e. Failures of host defense occur and fall into three broad categories: immunodeficiencies, [] autoimmunity, [] and hypersensitivities.

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.

AIDS and some types of cancer cause acquired immunodeficiency. Overactive immune responses form the other end of immune dysfunction, particularly the autoimmune diseases. 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. It is 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.

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.

These reactions are mediated by T cells , monocytes , and macrophages. Inflammation is one of the first responses of the immune system to infection, [44] but it can appear without known cause. The immune response can be manipulated to suppress unwanted responses resulting from autoimmunity, allergy, and transplant rejection , and to stimulate protective responses against pathogens that largely elude the immune system see immunization or cancer.

Immunosuppressive drugs are used to control autoimmune disorders or inflammation when excessive tissue damage occurs, and to prevent rejection after an organ transplant. Anti-inflammatory drugs are often used to control the effects of inflammation. Glucocorticoids are the most powerful of these drugs and can have many undesirable side effects , such as central obesity , hyperglycemia , and osteoporosis.

Lower doses of anti-inflammatory drugs are often used in conjunction with cytotoxic or immunosuppressive drugs such as methotrexate or azathioprine. Cytotoxic drugs inhibit the immune response by killing dividing cells such as activated T cells.

This killing is indiscriminate and other constantly dividing cells and their organs are affected, which causes toxic side effects.

Claims made by marketers of various products and alternative health providers , such as chiropractors , homeopaths , and acupuncturists to be able to stimulate or "boost" the immune system generally lack meaningful explanation and evidence of effectiveness.

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 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. Many vaccines are based on acellular components of micro-organisms, including harmless toxin components.

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; [] some are derived from oncogenic viruses like human papillomavirus , which causes cancer of the cervix , [] vulva , vagina , penis , anus , mouth, and throat , [] 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 for example, melanocytes into tumors called melanomas.

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.

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 hypoxia reduces the cytokine production for the anti-tumor response and progressively macrophages acquire pro-tumor M2 functions driven by the tumor microenvironment, including IL-4 and IL Some drugs can cause a neutralizing immune response, meaning that the immune system produces neutralizing antibodies that counteract the action of the drugs, particularly if the drugs are administered repeatedly, or in larger doses.

This limits the effectiveness of drugs based on larger peptides and proteins which are typically larger than Da. Computational methods have been developed to predict the immunogenicity of peptides and proteins, which are particularly useful in designing therapeutic antibodies, assessing likely virulence of mutations in viral coat particles, and validation of proposed peptide-based drug treatments.

Early techniques relied mainly on the observation that hydrophilic amino acids are overrepresented in epitope regions than hydrophobic amino acids; [] however, more recent developments rely on machine learning techniques using databases of existing known epitopes, usually on well-studied virus proteins, as a training set.

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.

Echinoderms , hemichordates , cephalochordates , urochordates. Many species, however, use mechanisms that appear to be precursors of these aspects of vertebrate immunity.

Immune systems appear even in the structurally simplest 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 is a type of defensive response used by plants that renders the entire plant resistant to a particular infectious agent.

Evolution of the adaptive immune system occurred in an ancestor of the jawed vertebrates. Many of the classical molecules of the adaptive immune system for example, immunoglobulins 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. 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.

These proteins are often used to shut down host defenses. An evasion strategy used by several pathogens to avoid the innate immune system is to hide within the cells of their host also called intracellular pathogenesis.

Here, a pathogen spends most of its life-cycle inside host cells, where it is shielded from direct contact with immune cells, antibodies and complement. Some examples of intracellular pathogens include viruses, the food poisoning bacterium Salmonella and the eukaryotic parasites that cause malaria Plasmodium spp.

and leishmaniasis Leishmania spp. Other bacteria, such as Mycobacterium tuberculosis , live inside a protective capsule that prevents lysis by complement. Such biofilms are present in many successful infections, such as the chronic Pseudomonas aeruginosa and Burkholderia cenocepacia infections characteristic of cystic fibrosis.

The mechanisms used to evade the adaptive immune system are more complicated. This is called antigenic variation. An example is HIV, which mutates rapidly, so the proteins on its viral envelope that are essential for entry into its host target cell are constantly changing.

These frequent changes in antigens may explain the failures of vaccines directed at this virus. In HIV, the envelope that covers the virion is formed from the outermost membrane of the host cell; such "self-cloaked" viruses make it difficult for the immune system to identify them as "non-self" structures.

Immunology is a science that examines the structure and function of the 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. Although he explained the immunity in terms of "excess moisture" being expelled from the blood—therefore preventing a second occurrence of the disease—this theory explained many observations about smallpox known during this time.

These and other observations of acquired immunity were later exploited by Louis Pasteur in his development of vaccination and his proposed germ theory of disease. 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. Köhler and César Milstein for theories related to the immune system.

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Further information: History of immunology. Nature Reviews. doi : PMC PMID Current Opinion in Immunology. S2CID British Medical Bulletin. Current Topics in Microbiology and Immunology.

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Retrieved 8 January Revue d'histoire des sciences et de leurs applications. Trends in Immunology. Nature Medicine. org Retrieved on 8 January Additionally, commensal bacteria can help create conditions in the local environment that keep infectious agents from causing problems.

For example, commensal bacteria may release chemicals that are toxic to other types of bacteria. Evidence for the importance of these bacteria can be seen after taking oral antibiotics.

You may have loose stools or intestinal cramping for a few days. This is because antibiotics, such as penicillin, can kill many different types of bacteria — good and bad. A final way that the innate immune system works is through immune system cells. These cells are not specific in their search for invaders.

The most important cells associated with innate immune responses are:. Watch this short video showing how the innate immune system works. When pathogens get past the non-specific mechanisms of protection afforded by the innate immune system, the adaptive immune system takes over.

Memory cells monitor the body to stop or lessen the impact of future infections by the same pathogen. If a second infection occurs at all, it is typically shorter in duration and less severe than a first encounter. Vaccines allow us to leverage the advantages of immunologic memory without the risks involved with a first encounter.

Sticking to our police force example, vaccines are like the practice drills that officers complete in an effort to be ready for an actual event. The adaptive immune response is driven by the activities of cells called antigen-presenting cells APCs.

Three cell types can serve as APCs — dendritic cells, macrophages and B cells. Of these, dendritic cells are the most common and powerful APC type.

They are considered to be the bridge between the innate and adaptive immune responses. Dendritic cells are produced in bone marrow and migrate through the blood to tissues where they monitor for pathogens. As this happens, the dendritic cell migrates from the tissue to the nearest lymph node where these surface signals, called antigens, help to activate T cells.

Dendritic cells can process and present most types of pathogens, such as viruses, bacteria, fungi and parasites. Whereas antigen presentation is the primary function of dendritic cells, macrophages and B cells are capable APCs, but this is not their primary function.

Macrophages, as described in the innate immune system section, primarily destroy pathogens, signal the innate immune response, and cause inflammation. When they function as APCs, it is typically to present antigens from pathogens they have ingested that have evolved so that they are not killed by typical innate immune responses.

Similar to dendritic cells, macrophages and B cells, acting as APCs, must travel to the draining lymph node to activate the adaptive immune response. When antigen is presented in draining lymph nodes, the adaptive immune response starts in earnest. The actions are wide-reaching, but can include growing, changing, reproducing, or interacting with other cells.

More than 50 kinds of cytokines have been identified. Different types of cells have different receptors, and, therefore, can be more or less affected by particular cytokines.

Additionally, some cytokines cause more than one action, and multiple cytokines can cause similar actions. It also allows for people born with immune deficiencies to survive. In addition to the cytokines and APCs, two primary cell types are central to the efforts of the adaptive immune response — T cells and B cells.

These cells are important in moderating the adaptive immune response. You can think of them like the police chiefs and sergeants making sure the appropriate numbers of staff are responding to a situation. Three types of T cells each have distinct roles:. Once activated, B cells start to reproduce, quickly increasing in number.

In our example, B cells are the troops of officers that descend on the crime scene. And, like the weapons troopers carry, B cells are also armed. The sole purpose of most B cells is to secrete large quantities of antibodies. B cells that secrete antibodies are also known as plasma cells.

Antibodies secreted by B cells are a crucial weapon of the adaptive immune response. They are specific for the pathogen that is attacking, so they can bind to and neutralize it. Five different classes of antibodies, also known as immunoglobulins Ig , exist in people: IgG, IgM, IgA, IgE, and IgD.

Each has unique characteristics and roles. Watch this short video about how antibodies work. Most of the cells that are activated during an infection die during or shortly afterward.

However, a small subset of both B and T cells remain indefinitely. They are called memory cells. These memory cells recognize specific antigens.

For example, most of us have memory B and T cells that monitor our body for influenza. Whether our first encounter with influenza was an infection or the result of vaccination, our immune system went through the process of becoming activated and responding to the assault.

This first response is called the primary immune response. The memory cells that remain after a primary infection serve as guards watching for influenza to appear again. If it does, these cells will quickly activate allowing the immune system to produce a faster and more efficient immune response to this second or third or fourth, etc.

Immunologic responses driven by memory cells are called secondary responses. In our police example, think of memory responses as experienced officers. Those officers with more experience are likely to anticipate what is happening allowing them to respond more quickly, confidently and efficiently.

In the same way, memory cells allow the adaptive immune system to ramp up its attack more quickly. This preparedness decreases the response time by several days.

The results can be realized in a few ways. Some people may not have any symptoms and not even realize they were exposed the second time. Some people will have symptoms, but they will not have as severe of symptoms. They are likely to be sick for fewer days as well.

Watch this short video about how the adaptive immune system works. Materials in this section are updated as new information and vaccines become available.

The Vaccine Education Center staff regularly reviews materials for accuracy. You should not consider the information in this site to be specific, professional medical advice for your personal health or for your family's personal health.

You should not use it to replace any relationship with a physician or other qualified healthcare professional. For medical concerns, including decisions about vaccinations, medications and other treatments, you should always consult your physician or, in serious cases, seek immediate assistance from emergency personnel.

Parts of the Immune System. Contact Us Online. Organs and tissues Organs and tissues important to the proper functioning of the immune system include the thymus and bone marrow, lymph nodes and vessels, spleen, and skin.

Bone marrow and thymus If the immune system is a police force, the bone marrow is the police academy because this is where the different types of immune system cells are created. Lymph nodes and vessels Lymph nodes are tissues full of immune cells.

Two vessel systems are critical to the immune function of lymph nodes: Blood vessels — Lymph, a fluid rich in immune system cells and signaling chemicals, travels from the blood into body tissues via capillaries.

Lymphatic fluid collects pathogens and debris in the tissues. Then the lymphatic fluid containing immune cells enters draining lymph nodes where it is filtered. Lymphatic vessels — Once filtration is complete, lymph vessels carry this fluid toward the heart.

Depending on where the filtered lymph arrives from, it enters either the thoracic duct on the left side of the heart, or a similar, but smaller duct on the right side of the heart. The thoracic duct collects lymph from the whole body except the right side of the chest and head.

The lymph from these areas drains to the smaller duct. From here, the lymph and its immune cells are returned to the bloodstream for another trip through the body. Spleen The spleen is the largest internal organ of the immune system, and as such, it contains a large number of immune system cells.

Skin Sometimes the skin is described as the largest organ of the immune system because it covers the entire body. Physical barriers Our bodies physically ward off many potential pathogens. Chemical barriers Mucus not only provides a physical barrier, it also contains chemicals that help protect us from pathogens.

Partnerships Bacteria live in and on us. Non-specific cellular responses A final way that the innate immune system works is through immune system cells. The most important cells associated with innate immune responses are: Neutrophils — These are the most numerous type of innate immune responder cells.

Their primary job is to destroy pathogens. Neutrophils circulate in the blood, but enter different parts of the body where an invader has been identified. When a neutrophil finds a pathogen, it surrounds and ingests it — a process called phagocytosis. Neutrophils only survive a few days.

Macrophages — These long-lived cells are present in virtually all tissues of the body where they use phagocytosis to trap invaders found in the tissue. While the phagocytic activity of macrophages is an important part of innate immunity, these cells are even more important for their role in activating other parts of the immune system.

Macrophages that have ingested a pathogen secrete chemical signals, called cytokines, which help recruit other immune cells to the area — this leads to inflammation. Inflammation is important for a few reasons. First, it establishes an environment in which cells traveling in the blood can move into the affected tissue.

Second, it allows for clotting factors to become activated in an effort to contain the infection, and third, it promotes tissue repair.

Many cells and organs work together to protect the body. White blood cells, also called leukocytes LOO-kuh-sytes , play an important role in the immune system.

Some types of white blood cells, called phagocytes FAH-guh-sytes , chew up invading organisms. Others, called lymphocytes LIM-fuh-sytes , help the body remember the invaders and destroy them.

One type of phagocyte is the neutrophil NOO-truh-fil , which fights bacteria. When someone might have bacterial infection, doctors can order a blood test to see if it caused the body to have lots of neutrophils. Other types of phagocytes do their own jobs to make sure that the body responds to invaders.

The two kinds of lymphocytes are B lymphocytes and T lymphocytes. Lymphocytes start out in the bone marrow and either stay there and mature into B cells, or go to the thymus gland to mature into T cells. B lymphocytes are like the body's military intelligence system — they find their targets and send defenses to lock onto them.

T cells are like the soldiers — they destroy the invaders that the intelligence system finds. When the body senses foreign substances called antigens , the immune system works to recognize the antigens and get rid of them.

B lymphocytes are triggered to make antibodies also called immunoglobulins. These proteins lock onto specific antigens. After they're made, antibodies usually stay in our bodies in case we have to fight the same germ again.

That's why someone who gets sick with a disease, like chickenpox, usually won't get sick from it again. The B cells are activated by the T helper cells: T helper cells contact B cells that match the same germs that they do. This activates the B cells to multiply and to transform themselves into plasma cells.

These plasma cells quickly produce very large amounts of antibodies and release them into the blood. Because only the B cells that match the attacking germs are activated, only the exact antibodies that are needed will be produced.

Some of the activated B cells transform into memory cells and become part of the "memory" of the adaptive immune system. The various cells of the adaptive immune system communicate either directly or via soluble chemical messengers such as cytokines small proteins.

These chemical messengers are mostly proteins and are produced by different cells in the body. Antibodies are compounds of protein and sugar that circulate in the bloodstream. They are created by the immune system to fight germs and foreign substances. Antibodies can quickly detect germs and other potentially harmful substances, and then attach to them.

This neutralizes the "intruders" and attracts other immune system cells to help. Antibodies are produced by the B lymphocytes. Germs and other substances that can provoke the creation of antibodies are also referred to as "antigens.

An antibody only attaches to an antigen if it matches exactly, like a key in the lock of the antibody. That is how antibodies detect the matching germs to initiate a fast response from the adaptive immune system.

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Show details Cologne, Germany: Institute for Quality and Efficiency in Health Care IQWiG ; Search term. The innate and adaptive immune systems Last Update: July 30, ; Next update: The innate immune system: Fast and general effectiveness The innate immune system is the body's first line of defense against germs entering the body.

The innate immune system consists of Protection offered by the skin and mucous membranes. Protection offered by the skin and mucous membranes All outer and inner surfaces of the human body a key part of the innate immune system.

Protection offered by the immune system cells defense cells and proteins The innate immune system activates special immune system cells and proteins if germs get past the skin and mucous membranes and enter the body.

What happens during an inflammation? Certain proteins enzymes are also activated to help in the immune response see below. Scavenger cells: Neutralizing germs Bacteria or viruses that enter the body can be stopped right away by scavenger cells phagocytes.

The role of proteins Several proteins enzymes help the cells of the innate immune system. The tasks of these enzymes include: marking germs as targets for scavenger cells,. destroying bacteria cell walls to kill them, and. fighting viruses by destroying the viral envelope the outermost layer of a virus or cells that have been infected with viruses.

Natural killer cells: Searching for changed body cells The natural killer cells are the third major part of the innate immune system. However, a small subset of both B and T cells remain indefinitely.

They are called memory cells. These memory cells recognize specific antigens. For example, most of us have memory B and T cells that monitor our body for influenza. Whether our first encounter with influenza was an infection or the result of vaccination, our immune system went through the process of becoming activated and responding to the assault.

This first response is called the primary immune response. The memory cells that remain after a primary infection serve as guards watching for influenza to appear again. If it does, these cells will quickly activate allowing the immune system to produce a faster and more efficient immune response to this second or third or fourth, etc.

Immunologic responses driven by memory cells are called secondary responses. In our police example, think of memory responses as experienced officers. Those officers with more experience are likely to anticipate what is happening allowing them to respond more quickly, confidently and efficiently.

In the same way, memory cells allow the adaptive immune system to ramp up its attack more quickly. This preparedness decreases the response time by several days.

The results can be realized in a few ways. Some people may not have any symptoms and not even realize they were exposed the second time. Some people will have symptoms, but they will not have as severe of symptoms.

They are likely to be sick for fewer days as well. Watch this short video about how the adaptive immune system works. Materials in this section are updated as new information and vaccines become available. The Vaccine Education Center staff regularly reviews materials for accuracy.

You should not consider the information in this site to be specific, professional medical advice for your personal health or for your family's personal health. You should not use it to replace any relationship with a physician or other qualified healthcare professional.

For medical concerns, including decisions about vaccinations, medications and other treatments, you should always consult your physician or, in serious cases, seek immediate assistance from emergency personnel. Parts of the Immune System.

Contact Us Online. Organs and tissues Organs and tissues important to the proper functioning of the immune system include the thymus and bone marrow, lymph nodes and vessels, spleen, and skin. Bone marrow and thymus If the immune system is a police force, the bone marrow is the police academy because this is where the different types of immune system cells are created.

Lymph nodes and vessels Lymph nodes are tissues full of immune cells. Two vessel systems are critical to the immune function of lymph nodes: Blood vessels — Lymph, a fluid rich in immune system cells and signaling chemicals, travels from the blood into body tissues via capillaries. Lymphatic fluid collects pathogens and debris in the tissues.

Then the lymphatic fluid containing immune cells enters draining lymph nodes where it is filtered. Lymphatic vessels — Once filtration is complete, lymph vessels carry this fluid toward the heart.

Depending on where the filtered lymph arrives from, it enters either the thoracic duct on the left side of the heart, or a similar, but smaller duct on the right side of the heart. The thoracic duct collects lymph from the whole body except the right side of the chest and head. The lymph from these areas drains to the smaller duct.

From here, the lymph and its immune cells are returned to the bloodstream for another trip through the body. Spleen The spleen is the largest internal organ of the immune system, and as such, it contains a large number of immune system cells.

Skin Sometimes the skin is described as the largest organ of the immune system because it covers the entire body. Physical barriers Our bodies physically ward off many potential pathogens.

Chemical barriers Mucus not only provides a physical barrier, it also contains chemicals that help protect us from pathogens. Partnerships Bacteria live in and on us. Non-specific cellular responses A final way that the innate immune system works is through immune system cells.

The most important cells associated with innate immune responses are: Neutrophils — These are the most numerous type of innate immune responder cells. Their primary job is to destroy pathogens. Neutrophils circulate in the blood, but enter different parts of the body where an invader has been identified.

When a neutrophil finds a pathogen, it surrounds and ingests it — a process called phagocytosis. Neutrophils only survive a few days. Macrophages — These long-lived cells are present in virtually all tissues of the body where they use phagocytosis to trap invaders found in the tissue.

While the phagocytic activity of macrophages is an important part of innate immunity, these cells are even more important for their role in activating other parts of the immune system.

Macrophages that have ingested a pathogen secrete chemical signals, called cytokines, which help recruit other immune cells to the area — this leads to inflammation. Inflammation is important for a few reasons.

First, it establishes an environment in which cells traveling in the blood can move into the affected tissue. Second, it allows for clotting factors to become activated in an effort to contain the infection, and third, it promotes tissue repair.

Pain, redness and swelling at the site of a wound are indicative of the inflammatory response induced by macrophages. Dendritic cells — These cells have long tentacles and also phagocytose pathogens in tissues. However, the main purpose of dendritic cells is not to destroy pathogens like neutrophils or to alert the immune system to cause inflammation like macrophages.

Instead, dendritic cells serve to bridge the innate and adaptive immune responses. Unlike neutrophils, macrophages, and dendritic cells — all of which employ phagocytosis — NK cells attach to an infected cell and release chemicals into it to kill it.

Natural killer cells are also known for their ability to fight tumor cells. Preparing for battle When antigen is presented in draining lymph nodes, the adaptive immune response starts in earnest.

T cells These cells are important in moderating the adaptive immune response. Three types of T cells each have distinct roles: Helper T cells oversee cytokine signaling to activate B cells and increase the efficiency of other immune cells, such as macrophages.

Cytotoxic T cells are important in viral infections in that they kill cells that have been infected by viruses. Regulatory T cells regulate the immune response. They signal for increased activity early in an infection, and conversely, signal for a decrease in the response as the infection is brought under control.

B cells Once activated, B cells start to reproduce, quickly increasing in number.