Familial occurrence of complement dysfunction in Crohn's disease: correlation with intestinal symptoms and hypercatabolism of complement

Genetic analysis of four consanguineous multiplex families with inflammatory bowel disease

Background

Family studies support a genetic predisposition to inflammatory bowel diseases (IBD), but known genetic variants only partially explain the disease heritability. Families with multiple affected individuals potentially harbour rare and high-impact causal variants. Long regions of homozygosity due to recent inbreeding may increase the risk of individuals bearing homozygous loss-of-function variants. This study aimed to identify rare and homozygous genetic variants contributing to IBD.

Methods

Four families with known consanguinity and multiple cases of IBD were recruited. In a family-specific analysis, we utilised homozygosity mapping complemented by whole-exome sequencing.

Results

We detected a single region of homozygosity shared by Crohn's disease cases from a family of Druze ancestry, spanning 2.6 Mb containing the NOD2 gene. Whole-exome sequencing did not identify any potentially damaging variants within the region, suggesting that non-coding variation may be involved. In addition, affected individuals in the families harboured several rare and potentially damaging homozygous variants in genes with a role in autophagy and innate immunity including LRRK1, WHAMM, DENND3, and C5.

Conclusion

This study examined the potential contribution of rare, high-impact homozygous variants in consanguineous families with IBD. While the analysis was not designed to achieve statistical significance, our findings highlight genes or loci that warrant further research. Non-coding variants affecting NOD2 may be of importance in Druze patients with Crohn's disease.

Capturing the Biologic Onset of Inflammatory Bowel Diseases: Impact on Translational and Clinical Science

While much progress has been made in the last two decades in the treatment and the management of inflammatory bowel diseases (IBD)-both ulcerative colitis (UC) and Crohn's Disease (CD)-as of today these conditions are still diagnosed only after they have become symptomatic. This is a major drawback since by then the inflammatory process has often already caused considerable damage and the disease might have become partially or totally unresponsive to medical therapy. Late diagnosis in IBD is due to the lack of accurate, non-invasive indicators that would allow disease identification during the pre-clinical stage-as it is often done in many other medical conditions. Here, we will discuss what is known about the biologic onset and pre-clinical CD with an emphasis on studies conducted in patients' first degree relatives. We will then review the possible strategies to diagnose IBD very early in time including screening, available disease markers and imaging, and the possible clinical implications of treating these conditions at or close to their biologic onset. Later, we will review the potential impact of conducting translational research in IBD during the pre-clinical stage, especially focusing on the role of the microbiome in disease etiology and pathogenesis. Finally, we will highlight possible future developments in the field and how they can impact IBD management and our scientific knowledge of these conditions.

Antioxidant Supplements and Gastrointestinal Diseases: A Critical Appraisal

The gastrointestinal tract digests and absorbs dietary nutrients, protects the body against physical and chemical damage from contents in its lumen, provides immunity against external antigens, and keeps an optimum environment for the gut microbiota. These functions cannot be performed normally in several diseases of which the following are discussed here: irritable bowel syndrome and inflammatory bowel disease, which includes Crohn's disease and ulcerative colitis. Because these diseases are associated with oxidative stress, a host of antioxidant supplements are used for maintenance and recovery of the gut functions. However, the benefits of these supplements have not been established. The available 80 human trials were rated for levels of confidence and for benefits of the antioxidant supplements. For Crohn's disease, the supplements for which clear benefits occurred in at least 2 studies were allopurinol, Boswellia serrata (frankincense or shallaki), Artemesia species (wormwood), Tripterygium wilfordii (léi gōng téng), and omega-3 fatty acids. Similar beneficial supplements for ulcerative colitis were allopurinol, Matricaria chamomilla (chamomile), Curcuma longa (curcumin in turmeric), and omega-3 fatty acids. There was also a clear benefit for ulcerative colitis in 2 studies where a multiherbal Chinese medicine preparation and an Ayurvedic medicine preparation were used. For irritable bowel syndrome, there was only a marginal benefit of some of the antioxidant supplements. Thus, some antioxidant supplements may be beneficial at certain stages of specific diseases. This is consistent with the current concept that antioxidants act by inhibiting oxidative stress pathways in a tissue- and environment-specific manner and not by simply acting as scavengers.

Preclinical and Undiagnosed Crohn's Disease: The Submerged Iceberg

Little is known on the natural history of Crohn's disease (CD) before diagnosis. By the time the patient is diagnosed, the disease has often produced considerable damage to the intestinal mucosa and sometimes other organs. Such period before diagnosis might involve both a silent and a symptomatic phase. The silent phase, or preclinical CD, might last several years after the biological disease onset. Evidence is accumulating that the symptomatic phase might also go undiagnosed for months or years. In fact, for each established case of CD, there are probably several undiagnosed cases, a classic iceberg phenomenon of disease. Such status quo--lagging behind diagnostic standards for many other diseases--effectively hampers efforts to block disease evolution and the development of complications. This is no longer tenable because CD is a debilitating, severe, and costly affection, whose incidence is rapidly rising worldwide. Here, we will review what is currently known on preclinical and undiagnosed CD and what could be done to improve accuracy and timeliness of diagnosis.

The complement system in inflammatory bowel disease

Complement is well appreciated to be a potent innate immune defense against microbes and is important in the housekeeping act of removal of apoptotic and effete cells. It is also understood that hyperactivation of complement, or the lack of regulators, may underlie chronic inflammatory diseases. A pipeline of products to intervene in complement activation, some already in clinical use, is being studied in various chronic inflammatory diseases. To date, the role of complement in inflammatory bowel disease has not received a lot of research interest. Novel genetically modified laboratory animals and experiments using antagonists to complement effector molecules have kindled important research observations implicating the complement system in inflammatory bowel disease pathogenesis. We review the evidence base for the role and potential therapeutic manipulation of the complement cascade in inflammatory bowel disease.

Tissue studies in screened first-degree relatives reveal a distinct Crohn's disease phenotype

BACKGROUND

First-degree relatives of patients with Crohn's disease (CD) are at risk of developing the disease with 5% to 15% reported to be affected over time. Yet, a much greater proportion of them (>40%) shows features of "subclinical inflammation" with elevated intestinal inflammatory markers such as fecal calprotectin. The meaning of these findings is unclear in the absence of tissue data.

METHODS

Thirty-eight asymptomatic first-degree relatives of patients with CD underwent ileocolonoscopy and other tests including fecal calprotectin. All known causes of intestinal inflammation were carefully excluded. Age and gender-matched controls consisted of 10 individuals who underwent colonoscopy for other reasons. Histology was scored based on known methods.

RESULTS

Compared with controls, the relatives had significantly greater median values for fecal calprotectin and histological scores. In relatives, endoscopy identified 3 different phenotypes: (1) normal, (2) with minor lesions (aphthae or small superficial erosions), and (3) with typical CD inflammation. Based on the histological scores, the clustering analysis produced 3 corresponding highly separated clusters (61%, 26%, and 13% of the total, respectively) with divisive coefficient D = 0.94. When followed up (on the average for 53 mo), individuals in the second cluster had histological scores similar to baseline values (P = 0.12).

CONCLUSIONS

Tissue studies in first-degree relatives of patients with CD reveal 3 distinct groups: normal, with minimal inflammation, and with frank disease. The second cluster represents a novel phenotype, which does not seem to develop the disease over time. These findings explain previous observations of "subclinical inflammation" in such population.

Inflammatory bowel diseases in patients with adaptive and complement immunodeficiency disorders

Crohn's disease and ulcerative colitis are idiopathic chronic inflammatory diseases that primarily affect the gastrointestinal tract. The underlying causes remain poorly understood, but there is a growing body of evidence advocating a likely primary pathogenic role for immunodeficiency in the development of Crohn's lesions. Concordantly, a number of congenital immunodeficiencies disrupting the cellular innate immune system strongly predispose to noninfectious, Crohn's-like inflammatory bowel disease. There are case reports and series suggesting that the same may be true for some of the congenital adaptive and complement immunodeficiencies. This review considers and critiques these potential associations.

High normal serum levels of C3 and C1 inhibitor, two acute-phase proteins belonging to the complement system, occur more frequently in patients with Crohn's disease than ulcerative colitis

Few data are available on measurements of serum concentrations of complement proteins in inflammatory bowel disease (IBD). Therefore we measured serum levels of C3, C4, and C1-esterase inhibitor (C1-INH) as well as C-reactive protein (CRP) in 167 patients with Crohn's disease (CD) and 111 patients with ulcerative colitis (UC). Median serum concentrations of C3 and C1-INH were significantly higher in CD than in UC. According to multiple logistic regression analysis adjusted to age, sex, activity of disease, and presence of extraintestinal manifestations, IBD patients with high-normal (> or = 128%, > or = 75th percentile ) C1-INH concentrations had significantly (0.0275) higher odds ratio to have a diagnosis of CD than UC. Patients with high-normal C3 (> or = 1.40 g/liter) and high (> or =20 mg/liter) CRP concentrations had an even higher odds ratio of a CD diagnosis (P = 0.0132). Our findings indicate that measurement of C3, C1-INH, and CRP can be used as an additional marker to pANCA/ASCA for distinguishing patients with CD and UC.

Recent advances in inflammatory bowel disease

The last decade has seen tremendous advances in our knowledge, which has led to genuine improvements in our understanding of the pathogenesis and management of inflammatory bowel disease (IBD). The combined power of cellular and molecular biology has begun to unveil the enigmas of IBD, and, consequently, substantial gains have been made in the treatment of IBD. Refinements in drug formulation have provided the ability to target distinct sites of delivery, while enhancing the safety and efficacy of older agents. Simultaneous progress in biotechnology has fostered the development of new agents that strategically target pivotal processes in disease pathogenesis. This article addresses our current understanding of the pathogenesis of IBD, including the latest developments in animal models and covers agents currently used in the treatment of IBD as well as emerging therapies.

Elevated basal intestinal mucosal cytokine levels in asymptomatic first-degree relatives of patients with Crohn’s disease

AIM: To determine levels of cytokines in colonic mucosa of asymptomatic first degree relatives of Crohn’s disease patients.

METHODS: Cytokines (Interleukin (IL) 1-Beta, IL-2, IL-6 and IL-8) were measured using ELISA in biopsy samples of normal looking colonic mucosa of first degree relatives of Crohn’s disease patients (n = 9) and fro m normal controls (n = 10) with no family history of Crohn’s disease.

RESULTS: Asymptomatic first degree relatives of patients with Crohn’s disease had significantly higher levels of basal intestinal mucosal cytokines (IL-2, IL-6 and IL-8) than normal controls. Whether these increase d cytokine levels serve as phenotypic markers for a genetic predisposition to de veloping Crohn’s disease later on, or whether they indicate early (pre-cli nical) damage has yet to be further defined.

CONCLUSION: Asymptomatic first degree relatives of Crohn’s disease patients have higher levels of cytokines in their normal-looking intestinal mucosa compared to normal controls. This supports the hypothesis that increased cytokines may be a cause or an early event in the inflammatory cascade of Crohn’s disease and are not merely a result of the inflammatory process.

Anti-Saccharomyces cerevisiae mannan antibodies in familial Crohn's disease

OBJECTIVE

Anti-Saccharomyces cerevisiae mannan antibodies (ASCA) are associated with Crohn's disease. The aim of this study was to determine the prevalence of ASCA in families in which at least two members were affected with Crohn's disease.

METHODS

A total of 20 families including two (n=15) or more (n=5) patients with Crohn's disease were tested for ASCA with use of an ELISA method. Overall, 51 affected members, 66 healthy first degree relatives, and 163 healthy control subjects were studied.

RESULTS

ASCA were detected in 35 of 51 (69%) patients with Crohn's disease and in 13 of 66 (20%) healthy relatives versus one of 163 healthy control subjects (p < 0.0001 and p < 0.001). ASCA-positive relatives were distributed in 12 of 20 families. ASCA were present in eight healthy parents and four healthy siblings. The prevalence of ASCA in relatives did not depend on the ASCA status of affected members.

CONCLUSION

ASCA in 20% of healthy first degree relatives of patients with Crohn's disease suggest that these antibodies might be a subclinical marker for Crohn's disease in families. Whether ASCA reflect environmental or genetic factors or a combination of both is unknown.

Ulcerative colitis: a genetic disease?

A number of lines of evidence support the hypothesis that ulcerative colitis is an inherited disorder in a proportion of cases. First, there is a pattern of familial aggregation. Second, there are differences in the prevalence of the disease in different ethnic groups. Finally, the concordance rate in monozygotic twin pairs is higher than that of dizygotic twin pairs, although not as high as the concordance rates observed in Crohn's disease. Genetic models of the inheritance patterns suggest that ulcerative colitis is probably caused by one major gene, although that gene (or genes) remains to be identified. While at least one localization for susceptibility to Crohn's disease now seems certain, efforts to localize and characterize the susceptibility genes involved in the inheritance of ulcerative colitis are still underway. While the genes of the major histocompatibility complex have been imputed as causal in susceptibility to ulcerative colitis, a consensus of proof continues to elude us.

Familial aggregation of inflammatory bowel disease in a Mediterranean area

To evaluate familial aggregation of inflammatory bowel disease (IBD) in the Mediterranean area and to estimate the disease risk in first degree relatives. 427 patients with IBD were consecutively interviewed in order to obtain a complete pedigree of first degree relatives. Sufficient information was obtained in 98% of 2,685 family members. The prevalence ratio of IBD in family members was estimated and compared to the prevalence ratio of IBD in general population; the ratio was then standardized by age since the prevalence of the disease is age-dependent. The lifetime risk was assessed by the Kaplan Meier method. Thirty index cases (7%) had at least one affected first degree relative. As compared with the general population, first degree relatives of the 427 patients with IBD had a 4.38-fold increase in the age corrected risk of having the same disease. The kaplan-Meier curve showed a higher risk at 25 years of age for offsprings (3%) than for parents (1%) and siblings (1%) whereas the crude ratio showed a higher risk for siblings (1.9%) compared to parents (0.8%) and offsprings (1%). In the Mediterranean area, the familial prevalence of IBD is higher than in the general population and comparable to North European rates.

Aetiology and pathogenesis of chronic inflammatory bowel disease

While Crohn's disease and ulcerative colitis are both conditions characterized by intestinal inflammation, with some overlap in their clinical and histological features, they are essentially different in pathogenesis. Crohn's disease appears to be primarily a condition of chronic T-lymphocyte activation, with tissue damage induced by secondary macrophage activation. What activates the T-cells is unknown. In this chapter we look at the evidence for and against cell-wall deficient mycobacteria species, viral infection of vascular endothelium and luminal contents as potential mechanisms of chronic activation. In ulcerative colitis, by contrast, there is no strong evidence for T-cell activation, and humoral mechanisms predominate. While the finding of atypical anti-neutrophil cytoplasmic antibodies (P-ANCAs) may be useful in screening, the only novel pathogenetic discovery is the co-localization of a 40 kD colonic autoantibody with immunoglobulins and complement on the apical enterocyte surface. Despite the fundamental differences in initiating mechanisms, the two conditions have many 'downstream' inflammatory processes in common. We discuss the evidence for local production of cytokines, arachidonic acid metabolites and reactive oxygen and nitrogen radicals, highlighting the potential adverse consequences for intestinal vascular integrity.

Intestinal permeability in patients with Crohn's disease and ulcerative colitis and their first degree relatives

Increased intestinal permeability in patients with Crohn's disease and their first degree relatives has been proposed as an aetiological factor. The nine hour overnight urinary excretion of polyethyleneglycol-400 (PEG-400) and three inert sugars (lactulose, l-rhamnose, and mannitol) was used to test the permeation in 47 patients with Crohn's disease of whom 18 had at least one first degree relative with inflammatory bowel disease (2BD) and 52 patients with ulcerative colitis of whom 16 had at least one first degree relative with IBD. A total of 17 first degree relatives with IBD and 56 healthy first degree relatives were included. Thirty one healthy subjects not related to patients with IBD served as controls. No significant differences in PEG-400 permeation were found between the groups of patients, relatives, and controls, or between diseased and healthy relatives. The permeability to lactulose, rhamnose, and mannitol similarly did not differ between the three groups. This study challenges the previously reported findings of increased PEG-400 permeation in patients with Crohn's disease and in their healthy and diseased first degree relatives. There was no increase in permeability in a similar group of ulcerative colitis patients and their families.

Familial aggregation of inflammatory bowel disease in northern Italy: a multicenter study. The Gruppo di Studio per le Malattie Infiammatorie Intestinali (IBD Study Group)

To assess the familial aggregation of inflammatory bowel disease (IBD) in Italy, the family pedigrees of 411 patients with ulcerative colitis (UC) and 241 patients with Crohn's disease (CD) seen at 14 participating hospitals were studied. Sufficient information was obtained on 97% of 3752 first-degree relatives, 80% of 8869 second-degree relatives, and 74% of 5791 cousins. Thirty-six propositi (5.52%) had a total of 44 affected relatives (16 CD, 28 UC). The prevalence of IBD was higher in first- than in second-degree relatives and cousins (791, 112, and 163 in 100,000, respectively). A strong intrafamilial disease concordance was observed, with 26 cases of UC and 6 of CD among relatives of UC patients and 10 cases of CD and 2 of UC among relatives of CD patients. The prevalence of UC among first-degree relatives of UC patients and that of CD among first-degree relatives of CD patients was 680 and 531 in 100,000, respectively. In conclusion, there is a high degree of familial aggregation for IBD in Italy, with a strong intrafamilial disease concordance.

Neutrophil autoantibodies in ulcerative colitis: familial aggregation and genetic heterogeneity

The possibility that the neutrophil autoantibodies associated with ulcerative colitis represent a genetic marker of susceptibility was investigated by determining their prevalence in unaffected relatives of patients. Neutrophil autoantibodies were detected using an enzyme-linked immunosorbent assay, and positive values were confirmed by indirect immunofluorescence. An increased prevalence of neutrophil antibodies was found not only in the probands (68%, 26/38) but also in their clinically unaffected family members (15.7%, 17/108) compared with controls (2.9%, 1/35) (P less than 0.0001 and P less than 0.05, respectively). These results were confirmed with sera from a second center, where 86.4% (19/22) of probands were positive and 20.9% (9/43) of their relatives were positive. The prevalence of neutrophil autoantibodies in the relatives of probands who were antibody positive (21.4%) was significantly greater than the prevalence in relatives of probands who were antibody negative (7%; P less than 0.05). The findings are consistent with these antibodies being a potential marker of genetic susceptibility to ulcerative colitis and suggest the possibility of genetic heterogeneity within this disease.

New insights into the pathogenesis of appendicitis based on immunocytochemical analysis of early immune response

Although appendicitis is the most common condition requiring emergency surgery, the pathogenesis of this disease is poorly understood. In this study, local immune responses in acute appendicitis were investigated, using an experimental rabbit model, as well as in different grades of appendicitis in humans. Polymorphonuclear leukocyte (PMN) and plasma cell isotypes infiltration was measured in 24 New Zealand-bred white rabbits following experimental obstruction of the appendiceal lumen for periods ranging from 30 minutes to 24 hours. Significant infiltration of PMNs, IgM, and IgG plasma cells was noted at 30 minutes following obstruction, and IgA plasma cells were noted at 2 hours after obstruction. The immunopathological features observed in the study of human appendicitis clearly demonstrated the role of plasma cell infiltration in the immunopathogenesis of appendicitis. IgA and IgG plasma cells were significantly increased in the lamina propria of both focal and acute suppurative appendicitis, compared with controls (P less than .001). In focal appendicitis, in which the focus of inflammation is confined to only a few serial sections, the increased IgA and IgG plasma cell infiltration was present throughout the entire length of the resected organ. The possibility, as seen in this study, that antigen stimulation of the effector immune system could give rise to appendicitis, provides new insights into the diversity of inflammatory responses contributing to diseases of the intestinal tract. This previously unrecognised concept in the pathogenesis of appendicitis requires further elucidation of the stimulating agent, and in particular the T-cell recognition process.

Familial empiric risk estimates of inflammatory bowel disease in Ashkenazi Jews

Genetic factors have been implicated in the etiology of inflammatory bowel disease (IBD) because of the increased occurrence of IBD in relatives. To further characterize the familial aggregation of IBD, we obtained family histories by interview on 188 IBD patients, including 154 Ashkenazi Jews (82%), ascertained through a Los Angeles gastroenterology practice. Thirty-three index cases (17.6%) had at least one affected first-degree relative; an additional 11 had more distant affected relatives. Thus, 23.4% of our sample had a positive family history. The quantification of empiric risk estimates for various classes of relatives has been quite limited and has been reported in only a few series. An important goal of our study was the determination of the specific empiric risk figures for relatives. We obtained uncorrected risk estimates of 2.5% to off-spring, 5.2% to siblings, and 2.9% to parents. Although the highest risk we observed is to siblings, IBD has a variable and often late age of onset, and it is likely that many relatives, particularly offspring, of patients in this sample have not reached the age at which they will manifest clinical disease. Thus, these uncorrected risks as well as those reported in the literature are an underestimate of the true empiric risks. To provide an estimate of the true lifetime risks, we utilized age-specific incidence data to calculate the following age-corrected empiric risk estimates for IBD: 8.9% to offspring, 8.8% to siblings, and 3.5% to parents. It is these latter age-corrected estimates that are most appropriate for both genetic counseling and genetic modeling.

Risk factors in inflammatory bowel disease

A patient with inflammatory bowel disease may have several risk factors, including host defence and familial and environmental factors. Host defence factors include neutrophil and complement abnormalities and increased intestinal permeability. This may explain why elemental diets are effective treatment in Crohn's disease. Food is a major factor affecting the intestinal tract, and the considerable change in dietary habits during this century may explain why Crohn's disease has become more common. There is a geographic difference in the distribution of the disease. This could be due to difference in fish consumption. Fish oil has been shown to be effective in an uncontrolled study in the treatment of ulcerative colitis. This needs to be confirmed by ongoing controlled studies. Very few data have been published on the dietary habits of different countries. There is a need for a uniform dietary assessment that can be applied in a multicentre study. These studies would involve much co-operation but would yield important clues in the aetiology of inflammatory bowel disease.

Immune status in healthy relatives of patients with familial Crohn's disease

Immune-mediated mechanisms and genetic factors are believed to be involved in the pathogenesis of Crohn's disease. We studied T- and B-cell subpopulation proportions and various functional assays, including proliferative responses to PHA and Con A, Con A-induced suppressive activity, and natural killer cell assay toward the K562 cell line, in the peripheral blood of 22 patients with inactive familial Crohn's disease and their 35 healthy relatives including nine families. HLA-A, -B, and -DR antigens were determined in all the subjects. With the exception of minor abnormalities of suppressor cell activity present in some relatives of two families, neither significant impairments of immunological parameters in patients or their relatives nor concordant segregation of HLA haplotypes and disease were observed. These data indicate that peripheral immune abnormalities previously described in patients with Crohn's disease do not constitute primary factors involved in the disease itself and that familial incidence in Crohn's disease cannot be linked to immunological markers presently studied.

Abnormal metabolism of arachidonic acid in chronic inflammatory bowel disease: enhanced release of leucotriene B4 from activated neutrophils

The metabolism of endogenous arachidonic acid P(AA) was investigated in activated neutrophils from 20 patients with Crohn's disease, 20 with ulcerative colitis, and 25 healthy volunteers. 1-14C-P(AA) was incorporated into intracellular pools of phospholipids prior to activation of the cells with ionophore A23187 and analyses of released arachidonic acid metabolites by thin layer chromatography. Total release of radioactivity expressing the release of arachidonic acid and its metabolites, was equal in the experimental and control groups, which suggests a normal substrate availability. In contrast, there was a marked increase in the relative release of leucotriene B4 (LTB4) and its omega-oxidation products, 20-hydroxy-LTB4 (20-OH-LTB4) and 20-carboxy-LTB4 (20-COOH-LTB4), with LTB4 values exceeding the reference interval in seven of 20 patients with Crohn's disease, median 8.7%, and in six of 20 patients with ulcerative colitis, median 7.7%, as compared with a median of 5.3% in healthy volunteers. Furthermore, a decreased release of unmetabolised arachidonic acid, correlating inversely with the release of LTB4 in all experimental and control groups, and normal values for the production of other metabolites of arachidonic acid--for example, 5-hydroxyeicosatetraenoic acid (5-HETE) and 12-hydroxyheptadecatrienoic acid (HHT), point to an enzymatic abnormality such as increased activity of leucotriene B synthetase. An increased capacity for release of LTB4, the major pro-inflammatory metabolite of arachidonic acid lipoxygenation by polymorphonuclear leucocytes, may contribute to perpetuation of the inflammation and to tissue destruction in chronic inflammatory bowel disease. Our findings agree with previous reports of an increased release of LTB4 by the colonic mucosa in this condition.

Complement in the pathophysiology and diagnosis of human diseases

Complement is a humoral effector system composed of 21 plasma proteins that was identified initially because of its cytolytic effects. In addition to cytolysis, complement has a number of different functions related to inflammatory and other host defense processes. The description of the reaction mechanism includes: (1) activation of the classical pathway through recognition of IgG and IgM antibodies by C1q, (2) activation of the alternative pathway which is usually achieved without participation of immunoglobulins, (3) generation of proteolytic enzymes composed of heteropolymers that cleave certain precursor proteins, (4) formation of the membrane attack complex (MAC), and (5) participation of control mechanisms. Methodologies for studying protein concentration and functional activities of complement components include not only the classical hemolytic techniques but also the extremely sensitive new radioimmunoassays and enzyme immunoassays for measuring the products of complement activation that are generated in vivo. Examples of genetically controlled complement deficiencies have been published for most complement components. The symptomatology of some of these patients serves to emphasize the protective role of complement. Acquired deficiencies are significant not only as laboratory aids in diagnosis and to evaluate the course of certain diseases, but also to indicate possible pathogenic disease mechanisms. Recently, it has been recognized that the complement proteins with genes located in the HLA region are polymorphic. Certain variants of proteins C2, C4, and factor B occur with higher frequencies in certain diseases than in the general population, which appears to be of great practical importance in laboratory medicine.