CHAPTER 18: Leukocyte Function and Non-malignant Leukocyte Disorders

Leukocytes constitute the cellular components of the innate and adaptive immune system and are critical for host defense. These cells mediate acute and chronic inflammation, modulate immune responses, and protect the host against numerous pathogens. It is no surprise that defects in leukocyte function predispose to various kinds of infection in proportion to the nature and severity of the defect.

Disorders affecting leukocytes can be divided broadly into malignant disorders (tumors of leukocytes or their progenitors) and non-malignant disorders. The malignant disorders are uncommon but clinically important entities that are covered in subsequent chapters. Here we consider the much more common non-malignant abnormalities that affect leukocytes. These can involve any of the leukocytes (neutrophils, eosinophils, basophils, monocytes, B cells, T cells, and natural killer cells), but the disorders of greatest clinical relevance affect neutrophils; these will be our major focus.

Like other formed elements of the peripheral blood, neutrophils are the progeny of earlier progenitors that acquire the capacity to function as they mature in the bone marrow (Fig. 18-1). Neutrophils are distributed within the body in several discrete and highly dynamic compartments or pools (Fig. 18-2). In the marrow, mitotically active myeloid progenitors mature over a period of 7 to 10 days to give rise to large numbers of neutrophils. Under steady-state conditions, most of these newly formed neutrophils die without ever being released into the blood, constituting a storage pool that can be called upon during times of increased need. Upon release from the marrow, neutrophils enter the circulating pool, the only pool that is measured clinically. On average, neutrophils circulate for only 3 to 6 hours before migrating into tissues, where they may survive for up to 3 days. At any given time, roughly half of the neutrophils in the blood are adherent to vessel walls; these cells are referred to as the marginal pool. Finally, some circulating neutrophils are sequestered in the spleen. Normally, <5% of the neutrophils in the body are in the circulating pool, which turns over quickly. As a result, stimuli that alter the release of neutrophils from the marrow, neutrophil margination, or the migration of neutrophils into tissues can have rapid and sometimes profound effects on the peripheral blood neutrophil count.

In most clinical laboratories, the normal white blood cell count ranges from 4500 to 11,000/μL, with approximately 60% of the cells being neutrophils. Leukocytosis, an elevation in the white blood cell count, usually stems from a pathologic insult that stimulates a systemic inflammatory reaction. By far the most common form of leukocytosis is granulocytosis, an increase in neutrophils also known as neutrophilia.

Acute inflammation of any cause can produce the rapid appearance of granulocytosis by mobilizing neutrophils from the marrow storage pool. In particularly severe bacterial infections, immature granulocytic forms, such as band cells or even earlier progenitors, may be released into the blood (a finding also called a left-shift), and in extreme cases the white blood cell count may rise to >50,000/mm³. In such instances the marked leukocytosis and the presence of immature cells may mimic the appearance of a myeloid leukemia (leukemoid reaction). Occasionally, the inflammatory process may involve the marrow itself and produce marrow fibrosis and distortion, leading not only to the release of immature myeloid elements but also nucleated erythroid progenitors and misshapen tear-drop red cells. This combination of features in a peripheral smear is referred to as leukoerythroblastosis. If the marrow involvement is extensive, myelophthisic anemia may result (Chapter 4). It should be emphasized, however, that in the vast majority of cases granulocytosis is a reaction to disorders involving tissues other than the marrow. In contrast, neutropenia, an abnormally low neutrophil count, is often caused by defects in neutrophil production that may require a bone marrow examination for evaluation.

Primary disorders associated with neutrophil dysfunction usually have a genetic basis. Although rare, they are of interest because unraveling of their pathogenesis has led to the identification of genes that regulate normal neutrophil development and function.

Granulocytosis can be primary or secondary (Table 18-1). Primary disorders associated with granulocytosis include myeloid neoplasms (discussed in subsequent chapters) and a variety of genetic disorders that merit brief mention.

• Hereditary neutrophilia is a rare form of chronic granulocytosis that usually exhibits an autosomal dominant pattern of inheritance. In some families it is caused by gain-of-function mutations in the granulocyte colonystimulating factor (G-CSF) receptor that lead to marrow hyperstimulation and increased neutrophil production.

• Down syndrome (trisomy 21) is the most common constitutional chromosomal disorder. About 10% of infants with Down syndrome develop a transient myeloproliferative disorder characterized by granulocytosis, circulating immature myeloid precursors, and hepatosplenomegaly. This disorder appears to be caused by acquired mutations in the transcription factor GATA-1 that result in deregulated growth of myeloid progenitors. In approximately 75% of patients, the disorder resolves spontaneously, but the remaining patients develop full-blown acute myeloid leukemia over a period of months to several years.

• Leukocyte adhesion deficiency (LAD) is a rare autosomal recessive disorder that is described later under functional disorders of neutrophils.

Secondary causes of granulocytosis are much more common than primary causes and can be sorted into the following pathogenic categories.

• Stress. Acute stress is associated with elevated levels of catecholamines and glucocorticoids, both of which cause rapid demargination of neutrophils. Psychic and physical stress, exercise, seizures, and pain can all cause neutrophilia through this mechanism.

• Infection. Early in infection, factors derived from inflammatory cells such as interleukin-1 and tumor necrosis factor increase the release of neutrophils from the marrow storage pool. When an inflammatory stimulus is sustained, a variety of cells including T cells and macrophages release growth factors such as granulocyte-macrophage colony-stimulating factor (GM-CSF) and G-CSF. These factors enhance the growth and survival of granulocyte progenitors in the marrow and, over a period of 5 to 7 days, increase the rate of neutrophil production. In some instances (eg, disseminated tuberculosis) chronic infections involve the marrow directly and produce a leukoerythroblastic picture in the peripheral blood, owing to marrow inflammation and fibrosis.

• Inflammation of noninfectious etiology can produce neutrophilia through the same mechanisms as those cited under infection.

• Drug reactions can produce neutrophilia through various mechanisms. β-Adrenergic drugs and glucocorticoids cause neutrophils to demarginate, and glucocorticoids also cause a modest increase in the release of neutrophils from the marrow storage pool. Anabolic steroids such as testosterone stimulate hematopoiesis and may cause mild granulocytosis. Lithium used in the treatment of bipolar disorder can cause granulocytosis by stimulating the release of colony-stimulating factors from marrow stromal cells.

• Marrow hyperstimulation. Granulocytosis stemming from elevated levels of hematopoietic growth factors and increased marrow production of neutrophils may be seen in hemolytic anemias and immune thrombocytopenic purpura or following treatment of megaloblastic anemias with folate or vitamin B₁₂. On occasion, cancers produce hematopoietic growth factors such as G-CSF that lead to paraneoplastic neutrophilia.

• Splenectomy. The asplenic state may be associated with mild granulocytosis due to decreased sequestration of neutrophils.

Neutrophilia is often detected during the evaluation of acutely ill patients suspected of having infections. In patients with acute infections, neutrophils often contain toxic granulation (persistence of primary granules), cytoplasmic vacuoles, and Döhle bodies, pale blue cytoplasmic aggregates of endoplasmic reticulum (Fig. 18-3). In severe sepsis, phagocytosed organisms occasionally may be seen within neutrophils. In acutely ill patients, the primary clinical focus is on the identification of infectious agents and the institution of appropriate antibiotic therapy.

The differential diagnosis of neutrophilia in sub-acutely or chronically ill patients is quite broad and includes numerous infectious, inflammatory, and neoplastic disorders. Mild neutrophilia in asymptomatic patients can be related to smoking or subtle inflammatory disease but requires evaluation to rule out important acquired diseases such as chronic myelogenous leukemia.

Neutrophilia is usually reactive and indicative of a normal-functioning bone marrow; consequently, bone marrow evaluation is usually not necessary. The exceptions are neutrophilias associated with either leukoerythroblastosis or the presence of immature myeloid precursors in the peripheral blood. Leukoerythroblastosis is indicative of an infiltrative disorder in the marrow, such as metastatic cancer, primary bone marrow disorders associated with fibrosis, or a disseminated granulomatous disease such as tuberculosis. The presence of immature myeloid precursors (often in concert with eosinophilia and basophilia) is characteristic of myeloid neoplasms such as chronic myelogenous leukemia and polycythemia vera, both of which can be distinguished from reactive processes by molecular tests described in Chapter 20. In unexplained persistent neutrophilia, a bone marrow examination may be informative diagnostically and should always include cultures for tuberculosis and fungal pathogens.

The second most common form of leukocytosis is eosinophilia. Like neutrophilia, eosinophilia is much more often secondary than primary (Table 18-2). The most common causes of eosinophilia in the United States are disorders associated with Th2 immune responses, particularly asthma and reactions to allergens such as pollen and drugs. Worldwide, the most common cause of eosinophilia is helminthic worm infections, which are endemic in many parts of the developing world. Other causes of reactive eosinophilia include immune disorders such as systemic lupus erythematosus or vasculitis. Eosinophilia can be seen in some myeloid neoplasms, particularly certain acute myeloid leukemias and myeloproliferative disorders; in such cases, the eosinophils are usually a component of the malignant clone. Marked eosinophilia (absolute eosinophil counts >10,000/mm³) is particularly likely to be caused by certain types of myeloproliferative disorders, described further in Chapter 20. In other instances, eosinophilia is a reaction to some other kind of tumor. Hodgkin lymphoma (Chapter 23) is one notable example of a tumor that often produces reactive eosinophilia.

Basophilia is uncommon and usually is associated with eosinophilia. Mild basophilia is common in asthma and allergic conditions. More striking basophilia is sometimes seen in association with myeloid neoplasms, particularly chronic myelogenous leukemia and polycythemia vera, in which the basophils are part of the neoplastic clone.

Monocytosis is occasionally seen in association with tuberculosis or myeloid neoplasms, particularly those myeloproliferative disorders in which the monocytes and their precursors are a component of the malignancy.

Lymphocytosis most commonly stems from viral infections, particularly those caused by members of the gamma herpesvirus family, such as Epstein-Barr virus, the causative agent in infectious mononucleosis, and cytomegalovirus. The circulating lymphocytes are predominantly cytotoxic T cells, which are required for an effective immune response against Epstein-Barr virus and other herpesviruses. These T cells characteristically have abundant cytoplasm containing a few azurophilic granules and are sticky, often adhering to surrounding red cells (Fig. 18-4). Among bacterial infections, Bordetella pertussis (whooping cough) is unusual in causing lymphocytosis by releasing a toxin that inhibits the migration of lymphocytes out of the blood into lymph nodes. Much less commonly, lymphocytosis is a manifestation of a lymphocytic leukemia.

Leukopenia is defined as a decrease in the white blood cell count below the normal range. The most common and clinically significant form of leukopenia is neutropenia, defined as an absolute neutrophil count of <1500 cells/mm³. Patients with marked neutropenia (a state referred to as agranulocytosis) are highly susceptible to severe, sometimes fatal infections caused by bacteria and fungi. When neutropenia develops acutely, the risk of infection rises as neutrophil counts fall below 500/mm³; most patients who develop infections in this setting have neutrophil counts of <200/mm³. Chronic neutropenia appears to be compensated through uncertain mechanisms, so that even very low neutrophil counts (50-100/mm³) do not always result in infectious complications.

Neutropenia can be divided into congenital forms, which present in infancy or early childhood, and acquired forms, which present at any age (Table 18-3). Neutropenia in infants and very young children often, but not always, stems from genetic defects that cause an insufficiency of neutrophil production, whereas neutropenia in older children and adults may be caused by acquired defects affecting marrow production, neutrophil survival, or both.

The most common forms of congenital neutropenia are the following:

• Constitutional neutropenia. This lifelong neutropenia is usually discovered as an incidental laboratory abnormality and is of no clinical significance. By definition, patients have a modestly decreased neutrophil count (usually 500-1500/mm³) and no evidence of susceptibility to infection. Constitutional neutropenia is a reflection of genetic variation in the normal absolute neutrophil count, which, in certain ethnic groups, is shifted to a lower mean level. It is most common in people of African descent and in certain Arab populations. These individuals are asymptomatic, usually have other family members with low neutrophil counts, and respond normally to infectious challenges.

• Benign neutropenia of childhood describes a disorder in which neutrophil counts may be quite low (<100 cells/mm³), but infections nevertheless tend not to be life threatening. Infants typically present at 6 to 12 months of age with an increased number or abnormal persistence of typical childhood infections. The neutrophil count is usually lower than 500 cells/mm³ and may be near zero, yet patients usually have a surprisingly benign course. They are treated with prophylactic antibiotics and are supported acutely with G-CSF in the setting of severe infection. The disorder likely has an autoimmune basis, as antibodies against neutrophil-specific antigens are often present. This form of neutropenia resolves completely within 2 years in over 95% of these patients.

• Severe congenital neutropenia encompasses a rare group of genetic disorders associated with life-threatening infections. Inheritance may be autosomal dominant, autosomal recessive, or X-linked; a significant fraction of cases are sporadic. Over 60% of the autosomal dominant cases are caused by mutations in neutrophil elastase (also known as ELA2), a serine protease present in azurophilic primary granules that cleaves elastin and many other protein substrates. The pathogenesis of the neutropenia is unknown; it is currently postulated that the mutant elastase folds abnormally and accumulates in the endoplasmic reticulum. This in turn is believed to induce the unfolded protein response, which activates certain intracellular pathways that lead to apoptotic cell death. A bone marrow examination typically reveals a hypercellular marrow with an apparent maturation arrest at the promyelocyte stage of differentiation. A significant percentage of patients with severe congenital neutropenia (20% at 10 years), especially those due to elastase mutations, develop acute myeloid leukemia. These leukemias are usually associated with acquired gain-of-function mutations in the G-CSF receptor, which presumably contribute to the proliferative drive of the neoplasm.

• Cyclic neutropenia is a fascinating disorder in which the neutrophil count in the peripheral blood oscillates from near normal to close to zero over a cycle that averages 21 days. The bone marrow typically shows a decrease in mature granulocytes and a maturation arrest at the myelocyte stage of development during periods of severe neutropenia. Patients are prone to infections, particularly when their neutrophil counts are low, but these tend to be superficial (eg, periodontitis) and less serious than those in severe congenital neutropenia. Cyclic neutropenia is usually inherited in an autosomal dominant fashion and is nearly always caused by mutations in ELA2. It is unknown why elastase mutations produce cyclic neutropenia in some patients and severe congenital neutropenia in others. Patients with cyclic neutropenia do not appear to have any increased susceptibility to myeloid leukemia.

• Other rare congenital forms of neutropenia are recognized. One that merits brief mention is Chédiak-Higashi syndrome, an autosomal recessive disorder caused by loss-of-function mutations in LYST, a gene that is essential for the proper formation of lysosomal granules in cells such as granulocytes, cytotoxic lymphocytes, and melanocytes. A clue to the diagnosis is the presence of abnormal granules in neutrophils and lymphocytes (Fig. 18-5). Patients are prone to bacterial infections due to neutropenia and neutrophil dysfunction. Death often follows an accelerated phase associated with hepatosplenomegaly, pancytopenia, and hemophagocytosis that may be triggered by Epstein-Barr virus infection, which persists because of the dysfunction of cytotoxic T cells.

Acquired neutropenias are encountered much more frequently than the congenital forms and have diverse causes, including the following:

• Prescription drugs are probably the most common cause of acquired isolated neutropenia. Drugs can produce neutropenia through several mechanisms. Certain chemotherapeutic agents are cytotoxic and cause a predictable transient decrease in neutrophil production that is associated with marrow hypoplasia. In contrast, other drugs suppress the production of neutrophils in an unpredictable, idiosyncratic fashion. In many instances, this second class of drugs appears to induce antibodies that destroy granulocytes and suppress the growth of marrow progenitors. Bone marrow examination often shows a maturation arrest at the promyelocyte or myelocyte stage of granulocyte differentiation.

• Viral and bacterial infections caused by a variety of agents can result in neutropenia. Mild neutropenia may accompany typical childhood viral infections as well as infectious mononucleosis (acute Epstein-Barr virus infection), viral hepatitis, parvovirus B19 infection, and human immunodeficiency virus (HIV) infection. In viral infections, several mechanisms contribute to the neutropenia, including increased margination, increased migration, and decreased production, the latter due to the suppressive effects of cytokines such as interferon-gamma on the marrow. On occasion, Epstein-Barr virus, hepatitis, or HIV infections cause prolonged severe neutropenia, sometimes by inducing autoantibodies. Bacterial infections generally cause neutrophilia, but exceptions to this rule include typhoid fever (Salmonella typhi), brucellosis, rickettsial infections, and disseminated tuberculosis. These infections are commonly associated with neutropenia through a variety of mechanisms. Sepsis may be associated with neutropenia, particularly in infants, the elderly, and patients with severe alcoholism. Neutropenia in the setting of sepsis may stem from increased peripheral destruction or decreased marrow production (marrow exhaustion) and is understandably associated with a poor prognosis.

• Autoimmune neutropenia may be isolated or occur as part of a generalized autoimmune disorder, particularly systemic lupus erythematosus. The basis of the neutropenia is not clear, because many patients with anti-neutrophil antibodies have normal neutrophil counts, suggesting that cell-mediated neutrophil destruction also plays a role. The neutropenia is often mild and benign and serves mainly as a marker of the activity of the underlying disease.

• Hematologic malignancies can produce neutropenia in several ways. Acute leukemias, myelodysplastic syndromes, and other marrow-based neoplasms often displace normal marrow progenitors, leading to insufficient marrow production. Certain lymphoid neoplasms (eg, chronic lymphocytic leukemia) may be associated with the production of autoantibodies directed against neutrophils and marrow progenitors. One uncommon neoplasm associated with isolated neutropenia is large granular lymphocyte leukemia, a neoplasm of cytotoxic T cells that suppresses granulocytic progenitors in the bone marrow through uncertain mechanisms.

• Nutritional deficiencies of folate and vitamin B₁₂ are often associated with neutropenia that is generally not as severe as the megaloblastic anemia seen with these deficiencies.

• Hypersplenism may cause mild neutropenia due to sequestration of neutrophils.

• Adult patients sometimes present with neutropenia of uncertain etiology (idiopathic neutropenia). Some with very mild neutropenia (1000-1500 neutrophils/mm³) may have an underlying inflammatory disorder associated with cytokine production that suppresses neutrophil production. In others, the neutropenia is severe (50-100 neutrophils/mm³). These patients should have a bone marrow examination and cytogenetic studies to rule out a myelodysplastic syndrome (Chapter 20). If the marrow is normal, the disorder tends to follow a benign course.

It is often helpful to obtain serial neutrophil counts, both to confirm the presence of neutropenia and to follow trends in the neutrophil count over time. A careful history can determine whether the neutropenia is likely to be acute or chronic. In the pediatric age group, it is important to ask parents about a family history of neutropenia or increased susceptibility to infection. The patient should be examined for evidence of infection, with particular attention paid to the oropharynx, lymph nodes, spleen, and perianal region.

In those with severe bacterial infections and sepsis, the clinical focus is on identifying the organism and its likely source and treatment with broad-spectrum antibiotics. In other settings, certain laboratory tests may be helpful. The peripheral smear sometimes contains clues; for example, the presence of increased numbers of large lymphocytes with cytoplasmic granules should raise suspicion of acute viral infection or large granular lymphocytic (LGL) leukemia, a neoplasm usually of cytotoxic T cell origin, both of which can be evaluated with additional tests. Bone marrow examination often reveals findings that point to particular etiologies. Drug-related and autoimmune neutropenias are frequently associated with a maturation arrest in the marrow. Dysplasia (abnormal and disordered maturation of bone marrow progenitors) might be evidence of a myelodysplastic syndrome (see Chapter 20), which can be evaluated further by cytogenetic analysis of marrow cells. In newborns, infants, and young children, special genetic tests may reveal elastase mutations or other inherited defects associated with congenital neutropenias.

Initial treatment is directed toward mitigating the complications of neutropenia. In patients with severe neutropenia, fever is an indication for prompt treatment with broad-spectrum antibiotics that cover bacterial and fungal infections, following appropriate diagnostic cultures. It is important to remember that neutrophils are responsible for many of the clinical manifestations of infection and that infected neutropenic patients cannot produce pus or characteristic localizing signs. Therefore, fever may be the only manifestation of infection. Even if cultures are negative, patients should continue receiving antibiotics until the neutrophil count recovers (in drug-induced or other acute neutropenias) or for a full 7 to 10 day course (in the setting of refractory neutropenia).

Other treatment options aim to restore the neutrophil count and vary depending on the underlying cause. Drugs are always chief suspects and should be stopped unless essential. Drug-induced neutropenia usually resolves about 7 to 10 days after the drug is stopped, although some drugs can cause prolonged neutropenia that persists for several weeks. In those with severe drug-related neutropenia, recovery can be speeded by administration of G-CSF. With symptomatic congenital neutropenias, the two main therapeutic options are G-CSF and bone marrow transplantation. Before the advent of G-CSF, nearly all patients with severe congenital neutropenia died in infancy. High-dose therapy with G-CSF now allows these patients to survive to adulthood but has revealed a propensity for the development of myeloid leukemia. The majority of patients with chronic benign neutropenia are managed without G-CSF, except in the setting of severe infections. Autoimmune neutropenia can be treated with a variety of immunosuppressive agents such as glucocorticoids. Large granular lymphocyte leukemia often responds very well to treatment with glucocorticoids or low-dose methotrexate.

Lymphopenia is the second most common form of leukopenia. Most of the normal lymphocytes found in peripheral blood are T cells, mainly CD4-positive helper cells and smaller numbers of CD8-positive cytotoxic cells. Lymphopenia may be generalized (affecting B and T cells) or involve only T cells or T-cell subsets.

The most common forms of lymphopenia are acquired. One of the most important causes is infection with human immunodeficiency virus (HIV), which has a specific tropism for T-helper cells expressing CD4. In advanced HIV (acquired immunodeficiency syndrome, or AIDS), this leads to a profound deficiency of CD4-positive T-helper cells, lymphopenia, and frequent secondary infections by opportunistic pathogens. Another common acquired cause is treatment with glucocorticoid-type steroids, which are often used to treat autoimmune diseases, inflammatory conditions, and lymphoid neoplasms. At high doses, glucocorticoids induce the apoptosis of lymphocytes and their progenitors through mechanisms that remain poorly understood.

Primary causes of lymphopenia include rare congenital immunodeficiency states such as severe combined immunodeficiency, in which the production of both T and B cells is markedly compromised. Severe combined immunodeficiency may be inherited in an autosomal dominant or X-linked fashion. It is caused by loss-of-function mutations in several different genes, including those needed for antigen receptor gene rearrangement (such as the recombination activating genes RAG-1 and RAG-2). Primary causes of isolated T-cell lymphopenia include thymic aplasia (DiGeorge syndrome), which is usually associated with germline deletions involving chromosome 22q11.

The susceptibility of neutropenic patients to bacterial and fungal infections highlights the importance of neutrophils in killing these pathogens. To do their job properly, circulating neutrophils must be able to execute a sequential series of tightly regulated cellular functions.

• Adhesion to endothelium and migration into tissues. A variety of substances released from affected tissues, including histamine (from mast cells), components of microorganisms, and inflammatory mediators released from tissue macrophages initiate this process by activating the endothelial cells that line blood vessels. Inflamed endothelium expresses surface molecules that promote the adhesion of neutrophils, such as E-selectin and other ligands for β₂-integrins (Fig. 18-6). Once firmly adherent, neutrophils migrate into tissues through gaps that appear between endothelial cells.

• Chemotaxis. To find their prey, neutrophils actively move in response to chemoattractants such as bacterial products, proteolytic fragments of complement (C5a), and mediators such as the cytokine interleukin-8 that are released from inflammatory cells already on the scene, all of which bind to G-coupled receptors expressed on neutrophils. These receptors produce signals that orient the cytoskeletal machinery of the neutrophil, allowing the cell to move toward the site where the pathogens are located.

• Recognition and phagocytosis of pathogens. Neutrophils express receptors for bacterial products (eg, endotoxin) and opsonins, host proteins such as antibodies, and proteolytic fragments of complement (eg, C3b) that are deposited on bacteria. Of these, opsonins are particular effective at inducing phagocytosis, the process by which the pathogen is engulfed in a plasma membrane vesicle known as a phagosome and taken up into the neutrophil.

• Degranulation. Newly formed phagosomes rapidly fuse with the neutrophil's cytoplasmic granules, a type of specialized lysosome, to create phagolysosomes.

• Pathogen killing. Pathogens consumed and transferred into phagolysosomes are killed through several mechanisms. One involves the direct action of preformed factors found in granules, such as proteases (eg, elastase) and lysozyme, an enzyme that digests bacterial cell walls. However, the most important mechanism of pathogen killing is the generation of reactive oxygen and nitrogen species within the phagolysosome. This so-called respiratory burst depends on the assembly of a multiprotein enzyme, NADPH oxidase, which reduces O₂ to superoxide (O₂.). Additional reactions involving O₂. in turn produce hypochlorite (OCl., the active ingredient in bleach) and other highly reactive free radicals that damage the membranes and cell walls of phagocytosed pathogens, inducing sufficient injury to kill them.

The importance (and many of the molecular details) of these processes were made clear by rare patients who suffer recurrent bacterial and fungal infections despite apparently normal or even increased numbers of circulating neutrophils. Studies of such patients led to the identification of specific defects in neutrophils stemming from loss-of-function mutations in genes required for one or more of the steps outlined earlier. Several of these disorders are of pathophysiologic interest.

• Disorders of leukocyte adhesion. Leukocyte adhesion deficiency (LAD) (Fig. 18-6) is an autosomal recessive disorder caused by mutations that have deleterious effects on the function of β₂ integrin (LAD I) or the selectin ligand PSGL-1 (LAD II). Patients often have elevated neutrophil counts, particularly in the face of infection, due to an inability of neutrophils to adhere to endothelium and migrate into infected tissues.

• Disorders of phagolysosome function. One of the best known of these disorders is chronic granulomatous disease, which is caused by mutations that impair the activity of NADPH oxidase, the enzyme required for the respiratory burst. The failure to kill phagocytosed bacteria or fungi effectively allows infections to persist. As infections become chronic, activated macrophages accumulate and cluster into poorly formed granulomas, a feature that gives this disorder its name.

• Disorders affecting neutrophil granules. The prototype of these rare diseases is Chédiak-Higashi syndrome, which has already been discussed under neutropenia. The abnormal granules in this disorder result in impaired bacterial killing.

Histiocyte (literally tissue cell) is an archaic term still applied to tissue macrophages. Disorders associated with macrophage dysfunction are uncommon. One that deserves mention due to its severity bears the unfortunate name hemophagocytic lymphohistiocytosis (HLH). HLH occurs in a variety of settings and has a number of triggers. The common feature in all is the activation of macrophages throughout the body due to overstimulation by factors released by T effector cells. The activated macrophages phagocytose hematopoietic cells and release additional cytokines and chemokines, producing a systemic inflammatory state associated with cytopenias.

HLH occurs in both familial and acquired forms. The familial forms usually present in infants and are caused by autosomal recessive mutations in at least four different genes. One of the most commonly mutated genes encodes perforin, a component of the granules of cytotoxic T cells. The acquired forms can occur in children or adults and may be a complication of autoimmune disorders (where it has been termed macrophage activation syndrome), T-cell lymphoma, or infection, especially with Epstein-Barr virus. Regardless of the cause, patients with HLH typically present with fever, splenomegaly, and pancytopenia. An examination of the bone marrow reveals macrophages phagocytosing both mature and immature marrow elements (Fig. 18-7). Laboratory abnormalities typically include a very high ferritin level (>10,000 μg/L), a finding that is quite sensitive and specific for HLH.

Primary forms of HLH often respond to chemotherapy and immunosuppressive agents but inevitably recur and are ultimately fatal unless bone marrow transplantation is performed. Secondary forms of HLH may be treated with therapy for the underlying illness (eg, chemotherapy for a patient with T-cell lymphoma) but often also require regimens directed at the HLH, which are similar to those used in primary disease. Recognition and treatment of cases of secondary HLH (eg, those associated with Epstein-Barr virus) with chemotherapy regimens that include the topoisomerase II inhibitor etoposide appear to increase survival, particularly in pediatric patients.