Immune system disorder, any of various failures in the body’s defense mechanisms against infectious organisms. Disorders of immunity include immune deficiency diseases, such as AIDS, that arise because of a diminution of some aspect of the immune response. Other types of immune disorders, such as allergies and autoimmune disorders, are caused when the body develops an inappropriate response to a substance—either to a normally harmless foreign substance found in the environment, in the case of allergies, or to a component of the body, in the case of autoimmune diseases. Lymphocytes (white blood cells of the immune system) can become cancerous and give rise to tumours called leukemias, lymphomas, and myelomas.

This article discusses various immune deficiencies, allergies, autoimmune disorders, and lymphocyte cancers. For additional information on leukemias, lymphomas, and myelomas, see the article cancer. A discussion of how the immune system works to prevent disease is found in the article immune system.

Immune deficiencies

Immune deficiency disorders result from defects that occur in immune mechanisms. The defects arise in the components of the immune system, such as the white blood cells involved in immune responses (T and B lymphocytes and scavenger cells) and the complement proteins, for a number of reasons. Some deficiencies are hereditary and result from genetic mutations that are passed from parent to child. Others are caused by developmental defects that occur in the womb. In some cases immune deficiencies result from damage inflicted by infectious agents. In others drugs used to treat certain conditions, or even the diseases themselves, can depress the immune system. Poor nutrition also can undermine the immune system. Limited contact with natural environmental factors, particularly with microorganisms found in biodiverse settings, also has been associated with increased risk of allergies, autoimmune disorders, and chronic inflammatory diseases.

Hereditary and congenital deficiencies

Immune deficiencies resulting from hereditary and congenital defects are rare, but they can affect all major aspects of the immune system. Luckily many of those conditions can be treated. In the rare hereditary disorder called X-linked infantile agammaglobulinemia, which affects only males, B lymphocytes are unable to secrete all classes of immunoglobulins. (An immunoglobulin is a type of protein, also called an antibody, that is produced by B cells in response to the presence of a foreign substance called an antigen.) The disease can be treated by periodic injections of large amounts of immunoglobulin G (IgG). The congenital, but not hereditary, T-cell deficiency disease called DiGeorge syndrome arises from a developmental defect occurring in the fetus that results in the defective development of the thymus. Consequently the infant has either no mature T cells or very few. In the most severe cases—i.e., when no thymus has developed—treatment of DiGeorge syndrome consists of transplantation of a fetal thymus into the infant. The group of disorders called severe combined immunodeficiency diseases result from a failure of precursor cells to differentiate into T or B cells. Bone marrow transplantation can successfully treat some of those diseases. The immune disorder called chronic granulomatous disease results from an inherited defect that prevents phagocytic cells from producing enzymes needed to break down ingested pathogens. Treatments include administration of a wide spectrum of antibiotics.

Deficiencies caused by infection

T cell infected with HIV [Credit: © NIBSC, Science Photo Library/Photo Researchers, Inc.]T cell infected with HIV© NIBSC, Science Photo Library/Photo Researchers, Inc.Damage to lymphocytes that is inflicted by viruses is common but usually transient. During infectious mononucleosis, for example, the Epstein-Barr virus infects B cells, causing them to express viral antigens. T cells that react against these antigens then attack the B cells, and a temporary deficiency in the production of new antibodies lasts until the overt viral infection has been overcome. Because antibodies already present in the blood are slowly broken down, failure to make new ones is important only if the infection persists for a long time, as it does occasionally. A much more serious viral infection is that caused by the human immunodeficiency virus (HIV), which is responsible for the fatal immune deficiency disease AIDS. HIV selectively infects helper T cells and prevents them from producing cytokines and from functioning in cell-mediated immunity. Persons with AIDS may be unable to overcome infections by a variety of microbes that are easily disposed of by persons uninfected with HIV. Severe infections by certain parasites, such as trypanosomes, also cause immune deficiency, as do some forms of cancer, but it is uncertain how that comes about. For example, Hodgkin disease, which attacks the lymphatic system, makes the patient more susceptible to infection.

Deficiencies caused by drug therapy

In countries with advanced medical services, immune deficiency often results from the use of powerful drugs to treat cancers. The drugs work by inhibiting the multiplication of rapidly dividing cells. Although the drugs act selectively on cancer cells, they also can interfere with the generation and multiplication of cells involved in immune responses. Prolonged or intensive treatment with such drugs impairs immune responses to some degree. Although the immune impairment is reversible, the physician must seek a balance between intentional damage to the cancer cells and unintentional damage to the immune system.

Medically induced suppression of the immune system also occurs when powerful drugs, which are designed to interfere with the development of T and B cells, are used to prevent the rejection of organ or bone marrow transplants or to damp down serious autoimmune responses. Although use of such drugs has greatly improved the success of transplants, it also leaves patients highly susceptible to microbial infections. Fortunately, most of those infections can be treated with antibiotics, but the immunosuppressive drugs have to be used with great care and for as short a period as possible.

Deficiencies caused by malnutrition

In countries where the diet, especially that of growing children, is grossly deficient in protein, severe malnutrition ranks as an important cause of immune deficiency. Antibody responses and cell-mediated immunity are seriously impaired, probably because of atrophy of the thymus and the consequent deficiency of helper T cells. The impairment renders children particularly susceptible to measles and diarrheal diseases. Fortunately, the thymus and the rest of the immune system can recover completely if adequate nutrition is restored.

Deficiencies associated with limited environmental exposure

hygiene hypothesis [Credit: Science in Seconds ( (A Britannica Publishing Partner)]hygiene hypothesisScience in Seconds ( (A Britannica Publishing Partner)The failure of regulatory T cells as a result of reduced exposure to microorganisms in the natural environment during early childhood has been associated with the development of certain allergic conditions, autoimmune disorders (e.g., type I diabetes and multiple sclerosis), and inflammatory bowel diseases. While the mechanism underlying T cell failure in this context remains unclear, it is known that normally harmless microorganisms that coevolved with humans can help prevent the body from generating inappropriate immune responses. This idea was first proposed in the late 1980s by American immunologist David P. Strachan in his hygiene hypothesis. The hypothesis suggested that small family size and increased personal hygiene reduced childhood exposure to infections and thereby resulted in the development of allergic disorders. Building on the hygiene hypothesis, scientists later proposed that the continued increase in the prevalence of allergic disorders and chronic inflammatory diseases in urban populations in the 21st century was linked to diminished contact with biodiverse environments and the microorganisms they contain.


The immune system recognizes and responds to almost any foreign molecule; it cannot discern between molecules that are characteristic of potentially infective agents and those that are not. In other words, an immune response can be induced by materials that have nothing to do with infection. The mechanisms brought into play, though beneficial for eliminating microbes, are not necessarily beneficial when otherwise innocuous substances are targeted. Furthermore, even initially protective mechanisms can cause secondary disorders when they operate on too great a scale or for a longer period than necessary, thereby damaging tissues remote from the infection. The terms allergy and hypersensitivity are commonly used to describe inappropriate immune responses that occur when an individual becomes sensitized to harmless substances. Allergic reactions do not as a rule cause symptoms to arise on the first exposure to an antigen. At the initial exposure reactive lymphocytes are generated that go into action only when the individual is reexposed to the antigen.

The manifestations of a particular allergic reaction depend on which of the immune mechanisms predominates in the response. Based on this criterion, immunologists use the Gell-Coombs classification system to recognize four types of hypersensitivity reactions. Types I, II, and III involve antibody-mediated mechanisms and are of rapid onset. The type IV reaction stems from cell-mediated mechanisms and has a delayed onset. It should be noted that the categorization, though useful, is an oversimplification and that many diseases involve a combination of hypersensitivity reactions.

Type I hypersensitivity

Type I, also known as atopic or anaphylactic hypersensitivity, involves IgE antibody, mast cells, and basophils.

Sensitization, activation, and effector phases

Type I hypersensitivity can be divided into three phases. The first is called the sensitization phase and occurs when the individual is first exposed to antigen. Exposure stimulates the production of IgE antibodies, which bind to mast cells and circulating basophils. The mast cells are found in tissues, often near blood vessels. The second phase is the activation phase, and it occurs when the individual is reexposed to the antigen. Reintroduction of the antigen causes IgE molecules to become cross-linked, which triggers the mast cells and basophils to release the contents of their granules into the surrounding fluids, initiating the third phase, called the effector phase, of the type I reaction. The effector phase includes all the body’s complex reactions to the potent chemicals from the granules. The chemicals include histamine, which causes small blood vessels to dilate and smooth muscle in the bronchial tubes of the lungs to constrict; heparin, which prevents blood coagulation; enzymes that break down proteins; signaling agents that attract eosinophils and neutrophils; and a chemical that stimulates platelets to adhere to blood vessel walls and to release serotonin, which constricts arteries. In addition the stimulated mast cells make chemicals (prostaglandins and leukotrienes) that have potent local effects; they cause capillary blood vessels to leak, smooth muscles to contract, granulocytes to move more actively, and platelets to become sticky.

Type I allergic reactions

anaphylaxis: response to bee venom [Credit: Encyclopædia Britannica, Inc.]anaphylaxis: response to bee venomEncyclopædia Britannica, Inc.The overall result of the type I reaction is an acute inflammation marked by local seepage of fluid from and dilation of the blood vessels, followed by ingress of granulocytes into the tissues. This inflammatory reaction can be a useful local protective mechanism. If, however, it is triggered by an otherwise innocuous antigen entering the eyes and nose, it results in swelling and redness of the linings of the eyelids and nasal passages, secretion of tears and mucus, and sneezing—the typical symptoms of hay fever. If the antigen penetrates the lungs, not only do the linings of the bronchial tubes become swollen and secrete mucus, but the muscle in their walls contracts and the tubes are narrowed, making breathing particularly difficult. These are the symptoms of acute asthma. If the antigen is injected beneath the skin—for example, by the sting of an insect or in the course of some medical procedure—the local reaction may be extensive. Called a wheal-and-flare reaction, it includes swelling, produced by the release of serum into the tissues (wheal), and redness of the skin, resulting from the dilation of blood vessels (flare). If the injected antigen enters the bloodstream and interacts with basophils in the blood as well as with mast cells deep within the tissues, the release of active agents can cause hives, characterized by severe itching. If the antigen enters through the gut, the consequences can include painful intestinal spasms and vomiting. Local reaction with mast cells increases the permeability of the mucosa of the gut, and in many cases the antigen enters the bloodstream and also produces hives. Regardless of whether the allergen is injected or ingested, if it ends up in the bloodstream, it can induce anaphylaxis, a syndrome that in its most severe form is characterized by a profound and prolonged drop in blood pressure accompanied by difficulty in breathing. Death can occur within minutes unless an injection of epinephrine is administered immediately. This type of severe allergic reaction can occur in response to foods, drugs such as penicillin, and insect venom.

Another feature of type I hypersensitivity reactions is that, once the immediate local reaction to the allergen has taken its course, there may occur an influx of more granulocytes, lymphocytes, and macrophages at the site. If the allergen is still present, a more prolonged form of the same reaction—the so-called late-phase reaction, which lasts a day or two rather than minutes—may supervene. This is a feature of asthmatic attacks in some subjects, in whom repeated episodes also lead to increased sensitivity of the air passages to the constrictive action of histamine. If such persons can escape exposure to the allergen for several weeks, subsequent exposure causes much less severe attacks. A prolonged IgE-induced reaction also causes atopic dermatitis, a skin condition characterized by persistent itching and scaly red patches. These often develop at sites where the skin is bent, such as the elbows and knees. The persistence is due to the influx of mast cells stimulated by the continued presence of the allergen, which is often a harmless substance such as animal hair or dander.

Typical type I allergens

Most people are not unduly susceptible to hay fever or asthma. Those who are—about 10 percent of the population—are sometimes described as atopic (from the term atopy, meaning “uncommon”). Atopic individuals have an increased tendency to make IgE antibodies. This tendency runs in families, though there is no single gene responsible as there is in some hereditary diseases such as hemophilia. Although many innocuous antigens can stimulate a small amount of IgE antibody in the atopic individual, some antigens are much more likely to do so than others, especially if they are repeatedly absorbed in very small amounts through mucosal surfaces. Such antigens are often termed allergens. These substances are usually polypeptides that have carbohydrate groups attached to them. They are resistant to drying, but no special characteristic is known that clearly distinguishes allergens from other antigens. Allergens are present in many types of pollen (which accounts for the seasonal incidence of hay fever), in fungal spores, in animal dander and feathers, in plant seeds (especially when finely ground) and berries, and in what is called house dust. The main allergen in house dust has been identified as the excreta of mites that live on skin scales; other mites (those that live in flour, for example) also excrete potent allergens. This list is far from exhaustive. Sensitivities to chocolate, egg whites, oranges, or cow’s milk are not uncommon.

The amount of allergen needed to trigger an acute type I hypersensitivity reaction in a sensitive person is very small: less than one milligram can produce fatal anaphylaxis if it enters the bloodstream. Medical personnel should inquire about any history of hypersensitivity before administering drugs by injection, and if necessary they should inject a test dose into (rather than through) the skin to ensure that hypersensitivity is absent. In any case, a suitable remedy should be at hand.

Treatment of type I allergic responses



Type II hypersensitivity





Type III hypersensitivity

Type III, or immune-complex, reactions are characterized by tissue damage caused by the activation of complement in response to antigen-antibody (immune) complexes that are deposited in tissues. The classes of antibody involved are the same ones that participate in type II reactions—IgG and IgM—but the mechanism by which tissue damage is brought about is different. The antigen to which the antibody binds is not attached to a cell. Once the antigen-antibody complexes form, they are deposited in various tissues of the body, especially the blood vessels, kidneys, lungs, skin, and joints. Deposition of the immune complexes causes an inflammatory response, which leads to the release of tissue-damaging substances, such as enzymes that destroy tissues locally, and interleukin-1, which, among its other effects, induces fever.

Immune complexes underlie many autoimmune diseases, such as systemic lupus erythematosus (an inflammatory disorder of connective tissue), most types of glomerulonephritis (inflammation of the capillaries of the kidney), and rheumatoid arthritis.

Type III hypersensitivity reactions can be provoked by inhalation of antigens into the lungs. A number of conditions are attributed to this type of antigen exposure, including farmer’s lung, caused by fungal spores from moldy hay; pigeon fancier’s lung, resulting from proteins from powdery pigeon dung; and humidifier fever, caused by normally harmless protozoans that can grow in air-conditioning units and become dispersed in fine droplets in climate-controlled offices. In each case, the person will be sensitized to the antigen—i.e., will have IgG antibodies to the agent circulating in the blood. Inhalation of the antigen will stimulate the reaction and cause chest tightness, fever, and malaise, symptoms that usually pass in a day or two but recur when the individual is reexposed to the antigen. Permanent damage is rare unless individuals are exposed repeatedly. Some occupational diseases of workers who handle cotton, sugarcane, or coffee waste in warm countries have a similar cause, with the sensitizing antigen usually coming from fungi that grow on the waste rather than the waste itself. The effective treatment is, of course, to prevent further exposure.

The type of allergy described in the preceding paragraph was first recognized as serum sickness, a condition that often occurred after animal antiserum had been injected into a patient to destroy diphtheria or tetanus toxins. While still circulating in the blood, the foreign proteins in the antiserum induced antibodies, and some or all of the symptoms described above developed in many subjects. Serum sickness is now rare, but similar symptoms can develop in people sensitive to penicillin or certain other drugs, such as sulfonamides. In such cases the drug combines with the subject’s blood proteins, forming a new antigenic determinant to which antibodies react.

The consequences of antigen-and-antibody interaction within the bloodstream vary according to whether the complexes formed are large, in which case they are usually trapped and removed by macrophages in the liver, spleen, and bone marrow, or small, in which case they remain in the circulation. Large complexes occur when more than enough antibody is present to bind to all the antigen molecules, so that these form aggregates of many antigen molecules cross-linked together by the multiple binding sites of IgG and IgM antibodies. When the ratio of antibody to antigen is enough to form only small complexes, which can nevertheless activate complement, the complexes tend to settle in the narrow capillary vessels of the synovial tissue (the lining of joint cavities), the kidney, the skin, or, less commonly, the brain or the mesentery of the gut. The activation of complement—which leads to increased permeability of the blood vessels, release of histamine, stickiness of platelets, and attraction of granulocytes and macrophages—becomes more important when the antigen-antibody complexes are deposited in blood vessels than when they are deposited in the tissues outside the capillaries. The symptoms, depending on where the damage occurs, are swollen, painful joints, a raised skin rash, nephritis (kidney damage, causing blood proteins and even red blood cells to leak into the urine), diminished blood flow to the brain, or gut spasms.

The formation of troublesome antigen-antibody complexes in the blood can also result from subacute bacterial endocarditis, a chronic infection of damaged heart valves. The infectious agent is often Streptococcus viridans, normally a harmless inhabitant of the mouth. The bacteria in the heart become covered with a layer of fibrin, which protects them from destruction by granulocytes, while they continue to release antigens into the circulation. These can combine with preformed antibodies to form immune complexes that can cause symptoms resembling those of serum sickness. Treatment involves eradication of the heart infection by a prolonged course of antibiotics.

Type IV hypersensitivity



Tuberculin-type hypersensitivity



Contact hypersensitivity and dermatitis




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