The Distribution of Serum Immunoglobulins, Anti-Y-G Globulins (“Rheumatoid Factors”) and

Antinuclear Antibodies in White and Negro Subjects in Evans County, Georgia

By MAFSHALL A. LICHTMAN, JOHN H. VAUGHAN AND CURTIS G. HAMES

PIDEMIOLOGICAL STUDIES have confirmed the observation that serum gamma

globulin levels are significantly higher in Negro than in white subjects. The earliest observations describing this difference were made in African populations, where the Negro comparison group may have been selectively affected by important environmental factors. It was suggested that differential rates of infectious disease, particularly malaria, accounted for the racial difference in gamma globulin levels. Studies in the United States in the last decade have found little influence of social class on gamma globulin levels, and these observations have suggested that a major portion of the racial difference may be due to genic factors1-6 (Table 1).

In recent years, immunochemical tech- niques have allowed the separation of the electrophoretically slow-moving globulins into 3 major fractions (yG, yA and yM), and 2 minor fractions (yD and YE). Despite physicochemical and immunol- ogic differences, these plasma proteins have notable similarities in structure and in their function as antibody. The current state of knowledge regarding the structure, function and significance of these plasma globulins has been reviewed recently.’

The present study was undertaken to

examine the distribution of the major im- munoglobulin fractions in healthy individ- uals, randomly selected from a Southern community, and to investigate where among the major immunoglobulin classes the Negro-white difference in total gamma globulin level was located. Studies of the distribution of antinuclear antibodies and anti-yG globulins (“rheumatoid factors”) were also undertaken to determine what relationships these antibodies had to the levels of the various immunoglobulins in the population studied.

METHODS

Study Population. In 1960, in preparation for a study of cardiovascular morbidity in Evans County, Georgia, a census was performed to de- fine the population. The demographic informa- tion obtained was in close agreement with the Federal decennial census held a few months later. A description of the study population and the distribution of serum proteins in this rural population have been published previously.4*8 A random sample of 221 healthy subjects was chosen for the present study. Ten ml. of whole blood was withdrawn by venepuncture from each subject. After blood clotting the serum was sepa- rated and stored at - 20 C. until analyzed.

An additional sample of 100 healthy white men and women were studied. These subjects were skilled and semi-skilled workers from whom sera was obtained during pre-employment physi- cal examinations in Rochester, New York.

From the Department of Medicine, University of Rochester Medical Center, Rochester; New York, and the Evans County Health Department, Clax- ton, Georgia.

This study was supported in part by grants H E 3341, AM 02443, and 5T1 A1 28 from the United States Public Health Service.

MARSHALL A. LICHTMAN, M.D.: Senior In- structor in Medicine and Special Research Fellow

in Hematology, University of Rochester Medical Center, Rochester, New York. JOHN H. VAUGHAN, M.D.: Professor of Medicine and Head, Clinical Immunology Unit, University of Rochester Medi- cal Center, Rochester, New York. Recipient, Re- search Career Award from the National Institutes of Health. CURTIS G. HAMES, M.D.: Medical Practitioner and Co-chairman, Evans County, Georgia, Health Department, Claxton, Georgia.

204

ARTHRITIS AND RHEUMATISM, VOL. 10, No. 3 (June 1967)

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Immunological Assays. The three major irn- munoglobulins (yG, yA and y M ) were measured by single diffusion in agar.9 Antinuclear antibodies ( ANA) were assayed by immunofluorescence with leukocyte nuclei.10 Anti-yG globulins were de- tected by a modified latex fixation test (LFT), previously described.11

Statistical Methods. Reproducibility of the im- munoglobulin assay: Aliquots of pooled normal sera were measured throughout the assay period for use in assessment of the within-day and be- tween-day variability of the single diffusion method. The standard deviation (s.d.) obtained for the within-day sample was calculated from

Y i z j the formula s.d. I - .12 The s.d. for the n

hetween-day sample was-determined using the standard formula for this calculation.13 The co- efficient of variation (c.v.) was calculated from the formula, C.O. = (s.d./mean) X 100. This repre- sents a range of & 67 per cent about the sample mean and is not used here to represent the “per- cent error” of the method. Instead the more mean- ingful 95 per cent confidence limits or t X C.U. are used, where t is obtained from a table of t values for the appropriate degrees of freedom. In the case of the within-day sample the degrees of freedom are N/2, whereas in the between-day sample the degrees of freedom are N-1.12

The percent variability between duplicate samples measured on the same day was & 11.5, 2 13.6 and 215.7 per cent for yA, yM and yG, respectively (Table 2) . These percent values represent the range above and below the sample mean within which duplicate aliquots measured on the same day will fall 95 per cent of the time, and represent the result5 of 20 paired samples. The percent variability of replicate samples measured on different days was k 20, & 27 and 2 27 per cent for yA, yM and yG, respectively (Table 2). The latter figures represent the range above and below the sample mean within which replicate aliquots measured on different days will fall 95 per cent of the time, and are based on the results of 47 aliquots of the same sera measured over a 6-month period. No systematic error in immunoglobulin results was noted over the 6- month period.

Population studies: Comparisons between groups were made using the t-test.13 Correlation coefficients, the statistical significance of correla- tion coefficients and linear regression equations were calculated using standard formulae.13

RESULTS Immzcnoglobulins

Negro subjects had higher levels of gamma globulins than did white subjects

in each immunoglobulin class. This differ- ence was significant at the 5 per cent level in the yG fraction only (Table 3) . The higher yG levels in Negroes were found in both sex groups and at all ages (Table 4) . Gamma-G levels were compared among white subjects dichotomized into high and low social class groupsf. The levels were comparable among high social class white (1073239.1 mg. per 100 ml.)* and low social class white (1172k45.2 mg. per 100 m1.)* subjects. Negro subjects had significantly higher levels of yG (1408 233.3 mg. per 100 ml.)* than either high or low social class whites. The cumulative frequency distribution of yG indicated that 85 per cent of Negro subjects were above the median yG value for whites (Fig. 1 ) . No significant difference in yG was present by age in any race-sex group. Gamma-G levels were similar when white men and women were compared. How- ever, yG was signifkantly higher among Negro women as compared to Negro men.

No significant difference in yA level was present when men and women were compared. An increase in yA concentra- tion was present in older aged subjects in all four race-sex groups. The latter obser- vation was confirmed by means of linear regression analyses (Fig. 2). The slope of the regression was significantly different from zero among Negro subjects ( F = 5.74, P< .05) and white subjects (Fz5.42 P<.O5), and the influence of age on yA level was very similar in each race group.

Gamma-M levels were not significantly influenced by age. Women in each race had higher yM values than did men (Table 5 ) .

The frequency distribution of the im- munoglobulins was unimodal with the ex- ception of yM among Negroes (Fig. 3 ) . A statistically significant, although low-

* The parentheses contain the mean and stand- ard error for the respective groups.

t Details of the methodology for social class scores and the rationale for these comparisons can be found in previous publications.4~~

WHITE AND NEGRO SUBJECTS IN EVANS COUNTY, GEORGIA 207

Table 2.-Reproducibility of Serum Immunoglobulin Assay

Y A YM Reproducibility of 20 Duplicate Samples

Assayed On The Same Day Over A 6-Month Period

YG

Coefficient

95% Confidence limits (c.v. X t) f 15.7% f 11.5% t 13.6%

of variation (c.v.) t 7.5% t 5.5% t 6.5%

Reproducibility of 47 Replicate Samples Measured On Different Days Over A 6-Month Period

Coefficient of variation (c.v.) f 13.5% t 10.0% 2 13.5% 95% Confidence limits (c.v. X t ) t 27.0% k 20.0% t 27.0%

Table 3.-Serum Zmmunoglobulin Concentrations* Among Men and Women b y Race (Mean and Standard Error)

Whites : Negroes

Whites (No.=112)

Negroes (No.= 109)

Significance of the Difference

(P)

YG 11 lZ(34.8 ) 1408(33.3, <.OOl YA 157 (8.9) 1730.7) <.10>.05

YM 120(6.6) 133 (6.5) <.lo> .05

*mg per 100 ml. The Negro subjects with yA measurements totaled 108.

I

\ NEGROES ,' (No409

20-

, 1 8 1

700 900 1100 1300 1500 1700 1900 2100 2300 mgm per loo ml

bG Fig. 1.-The cumulative percent frequency dis- tribution of yG by race group. The vertical lines indicate the median yG value for each group.

order correlation was present between y G and yA among both white and Negro subjects (Table 6 ) .

The mean and variance of serum yM and YG levels among the sample of white- and blue-colIar workers in Ro- chester, New York, were very similar to the results obtained in white subjects in Evans County, Georgia. The mean yA concentration was significantly lower ( P < . O l ) in the New Yorker (129k5.1 mg. per 100 ml. ) * as compared to the Georgian subjects, (157k8.9 mg per 100 ml)' a dif- ference accountable to age (see below).

Anti-yG Globulins and Antinuclear Anti- bodies

The overall prevalence of a positive LFT in this population was 6.8 per cent (Table 7) . The frequency of a positive

ard error for the respective groups. * The parenthesis contain the mean and stand-

208 LICHTMAN, VAUGHAN AND HAMES

Table 4.-Eerum yG Concentration* by Age, Race and Sex

Whites : Negroes Whites Negroes Significance of

the Difference Mean? S.E.

Sex Aae (Yr.) No. Mean+-S.E. No. hfeankS.E.

Men

Women

15-34 35-54 55-74 Total

15-34 35-54 55-74 Total

17 1121+73.2 17 1081259.0 20 1094.t74.0 54 1098239.5

19 1060k62.5 19 1228%33.6 20 1086260.7 58 1124233.0

21 19 20 60

1304kM.6 1347k89.5 1332k58.2 1328k41 .O

=.05 <.05 < .02 <.001

18 15 16 49

1559279.8 1537e63.6 1417k96.3 1506246.9

<.001 < .05 <.01 <.00l

*mg per 100 ml.

2501

200-

. F 2 150- . L

501 0 I I I I I

0 10 20 30 40 50 60 70 80

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Fig. 2.-The linear regression of yA on age. The slope of the regression lines is significantly different from zero among Negro and white subjects (P<.05). The higher yA levels among Negroes of all ages is also demonstrated. The shaded areas encompass 1 s.e. of the re- spective regression lines.

was noted with increasing age (Table 7 ) . In the majority of cases (53 per cent) with a positive LFT a titer equal to or less than 1:80 was found (Table 7 ) . The im- munoglobulin levels among white subjects with and without a positive LFT did not differ significantly. Negro subjects with a positive LFT had somewhat higher yA and yM levels than those with a negative LFT (Table 8).

None of the 221 subjects examined had identifiable serum ANA by the methods employed.

DISCUSSION

Immunoglobulins The major difference in total serum

gamma globulin concentration when

Table 5.-Serum yM Concentration* b y Race and Sex

Males :Females : MALES FEMALES Significance

of the Difference

No. Mean+S.E. No. Mean3zS.E. (P) Whites 54 10327.1 58 135-1- 7.6 <.01 Negroes 60 11127.5 49 160+10.3 <.01

*mg per 100 ml.

LFT was similar among Negro men, Negro and white subjects are compared Negro women, and white women. White rests in the YG fraction. This difference men had one-third to one-fifth the preval- was present at all ages studied and in ence of the other three race-sex groups. both sexes (Table 4). The difference was An increasing frequency of a positive LFT due to an elevation among Negro subjects

WHITE AND NEGRO SUBJECTS IN EVANS COUNTY, GEORGIA 209

Table 6.-Association of Immunoglobulin Fractions (Correlation Coeficients)

y A vs yG y A vs y M y M ZIS yG

Whites .27 * .03 .05 (No.=112)

Negroes .18 ** .02 .14 (No.=lO8)

* P<.O1 ** P<.O5

All other correlations are not statistically sig- nificant a t the five percent level.

One Negro subject did not have a yA level de- termined and therefore was omitted from this analysis.

Table 7.-Frequency of a Positive Latex Fixation Test b y Age, Race and Sex

White White Negro Negro Men Women Men Women Total

Ase (Yr) 15-34 35-54 55-74 Total

NO. % NO. % NO. 70 NO. % NO. % 0/17 0 1/19 5.3 1/21 4.8 1/18 5.6 3/75 4.0 0/17 0 3/19 15.7 1/19 5.3 1/15 6.7 5/70 7.1 1/20 5.0 1/20 5.0 2/20 10.0 3/16 18.7 7/76 9.2 1/54 1.9 5/58 8.6 4/60 6.7 5/49 10.2 15/221 6.8

Strength of Reaction : Titer No. NO. No. No. No.

1 :20-1 :80 1 3 2 2 8 1 :160-1:640 0 2 1 1 4 1:1280-1:2560 0 0 1 2 3 Total 1 5 4 5 15

Table 8.-Zmmunoglobulin Concentration* Among Subjects With Positive and Negative Latex Fixation Tests

YG 7-4 YM Mean Median Mean Median Mean Median

White Latex Positive 1297 1190 207 123 128 112 Subjects : ( N o 7 6 )

Latex Negative 1101 1040 154 138 120 103 (No.=106)

Negro Latex Positive 1358 1375 196 208 148 153 Subjects : (No.=9)

Latex Negative 1414 1372 170 165 131 120 (No.=100)

* mg per 100 ml.

generally and not to a small subgroup of individuals with markedly elevated values (Figure 1).

gamma globulin levels among apparently

healthy Negroes when this variable is used in clinical medicine as an index of specific disease processes (i.e. sarcoidosis, collagen

One should be aware of the higher disease and other chronic diseases). Two recent studies of sarcoidosis have observed

810 LICHTMAN, VAUGHAK AND HAMES

a significantly higher frequency of ele- vated gamma globulins among Negroes with sarcoidosis as compared to whites with ~arcoidosis .~~J~ This may reflect an elevated baseline value in Negro subjects and/or a hyperresponsiveness of im- munoglobulin synthesis in Negroes with this disease.

The reason for the difference in serum gamma globulins and more particularly YG levels when Negro and white subjects are compared has not been established. Certainly environmental factors ( e.g., antigen) play an important role in gamma globulin synthesis rates. It has been dem- onstrated, however, that in an equivalent environmental ( and presumably anti- genic) milieu gamma globulin levels in the Negro are significantly higher than in white s~ibjects.~-~ In Evans County, Geor- gia, a detailed assessment of the possible influence of way of life, infectious dis- ease, nutrition and other variables on the racial difference in serum gamma globulin level revealed significantly higher levels of gamma globulin in Negro than white subjects when matched for social class (Table 1). Furthermore, social class did not appear to influence gamma globulin levels in either white or Negro subjects? In addition, increased synthesis rates of gamma globulin have been observed in Negroes as compared to whites,ls although in that study the question of an environ- mental determinant for the finding was not entirely excluded.

The epidemiological observations here and in earlier studies have made it nec- essary to consider genic factors in the racial differences noted. Similar conclu- sions have been reached by African in- vestigator~.~' Such an hypothesis implies the presence of genic regulation of im- munoglobulin synthesis rates. Whether such mechanisms exist is currently un- known. If, as is theorized, the Jacob and Monod model l6 or another modulator system controlling protein synthesis lo can

mgm per IOOml

Fig. 3.-The percent frequency distribution of the immunoglobulin fractions by race group.

be applied to human immunoglobulin syn- thesis, normal and pathological intra- species quantitative deviations in gamma globulin levels may be related to controller genic mechanisms. Analogous reasoning has been used to explain im- munoglobulin deficiency states where ex- perimental evidence had indicated that structural genes may not be primarily involved.20J1

Sex differences in gamma globulin levels have not been prominent in the few reports referring to this subject. Higher values among women have been seen]." but this finding has been incon~istent.~ In the present study women had higher levels of yM than men (Table 5 ) and Negro women had higher levels of yG than Negro men (Table 4 ) . Claman and Merrill have also observed higher YM levels among women as compared to

WHITE AND NEGRO SUBJECTS IN EVANS COUNTY, GEORGIA 211

men.-02 The possibility of a sex-linked in- fluence on gamma globulin synthesis has been raised by family studies of congenital hypogamma g l ~ b u l i n e m i a . ~ ~ ~ ~ Regulation of gamma globulin synthesis by the X-chromosome, as is implied by the afore- mentioned observations, would not ac- count for elevated gamma globulin levels among women, however, if X-chromosome inactivation influenced an area of the X-chromosome governing gamma globulin synthesis. Indeed, Stiehm and Fudenberg have found no associated abnormalities in gamma globulin levels in patients with congenital monosomy or trisomy involving the X-chr0mosorne.2~ It is possible that the observed differences in immunoglobulin levels between normal men and women are related to a somatic effect of estrogens, like the enhancement of antibody produc- tion by estrogens that has been reported in animal s t ~ d i e s . ~ ~ , ~ ~

It has been suggested that gamma globulin concentrations increase with age in the adult. This however, has not been a uniform observation in studies of this question. It is of interest in the current study that yA levels increased with age in all 4 race-sex groups studied. Such an increase may not be noticeable when only total serum globulin levels are measured. No consistent changes by age were evi- dent in this study in yM or yG levels.

Evans County, Georgia is a rural com- munity and hence the comparison of urban skilled workers in Rochester, New York, with rural Georgians is of interest. No difference in yG or yM levels was pres- ent between the 2 groups, supporting the finding of two previous studies in the United States, that social class has a negligible influence on serum gamma globulin level^.^.^ The higher mean yA level found in Georgians as compared to New Yorkers was explicable by the dif- ference in age between these groups. The New York subjects had a mean age 20 years younger than the Georgians, and

hence, according to the regression equa- tion in Fig. 2, would be expected to have a mean yA level approximately 20 mg. per cent less than the Georgian subjects.

When the individual immunoglobulin levels were correlated with each other, a statistically significant association was ob- served between yA and yG among white and Negro subjects, It is therefore of cor- responding interest that the rather frequent occurrence of patients with hypoimmuno- globulin G and A in the presence of nor- mal levels of yM have suggested to Barth and associates a linked regulator gene def ect.20

In the physiological range in the present study a significant positive or negative cor- relation between yG and yM, or yA and yM globulin was not present. It is possible that: 1) there is no association between yG and yM, or yA and yM; 2) the association of these immunoglobulins is not best eval- uated by a correlation coefficient with its assumption of a linear relationship; and 3) in ranges of immunoglobulins that included pathological values significant correlations would nevertheless develop. The hetero- geneity of the immunoglobulins may pre- dude associations developing in the absence of pathological stimuli.

Anti-yG Globulins and Antinuclear Anti- bodies

It is now generally accepted that a few percent of apparently healthy individuals have plasma anti-yG globulins ( “rheuma- toid factors”) detected by their ability to react with latex particles coated with YG globulins ( LFT) . The frequency with which a positive LFT had been found in healthy subjects has been variable and in part has depended on methodology and the threshold titer that is considered positive. These considerations have been reviewed in detail by Singer.28

The increased frequency of positive LFT in older individuals noted in this study has also been observed by other investiga-

212 LICHTMAN, VAUGHAN AND HAMES

The precise role of aging on the heightening of LFT reactivity is uncertain. It may be pertinent to note that anti-yG globulin capable of giving LFT reactions can be provoked in experimental animals by the injection of autologous yG globu- l i n ~ : ~ autologously derived antigen-anti- body c0mplexes,3~ bacterial antigens 35 and by chronic i n f e ~ t i o n . ~ ~ The effect of aging may be, therefore, the means whereby in- dividuals are sufficiently exposed to such influences with resultant stimulation of anti-yG globulins. In the current study, the distribution of the LFT reactions in rela- tion to immunoglobulin levels provide no strong data either to support or to refute such an hypothesis.

Since this and the aforementioned stud- ies have been cross-sectional and not longitudinal (cohort analysis ), factors other than aging could be responsible for the increased prevalence of positive LFT in older individuals.

Whether LFT reactivity in normal per- sons marks those who are predisposed to diseases like rheumatoid arthritis is not yet known. Inferences might be drawn about the relationship of anti-yG globulin in the plasma of healthy subjects to the develop- ment of rheumatoid disease by comparing the frequency of these antibodies by age, race, sex and other demographic factors to the frequency of rheumatoid disease by these same factors. In the current study

the sex and age distribution of “rheuma- toid factors” is similar to the generally ac- cepted sex and age distribution of clinical rheumatoid arthritis. The greater fre- quency of positive LFT among Negroes may be matched by a greater frequency of clinical rheumatoid arthritis among Negroes?? Such inferences are placed at risk, however, by the difficulty in estab- lishing criteria for determining the real prevalence of rheumatoid disease and “rheumatoid factors”. For example, other investigators have not observed a sex dif- ference in positive LFT in normal sub- j e c t ~ . ~ ~ ~ ~ ~ ~ ~ ~ It should be emphasized that the “pattern fitting” discussed above can only suggest hypotheses to test, since only a prospective study of LFT-positive indi- viduals can determine the risk that such reactivity carries. Studies such as the Tecumseh, Michigan Community Health Study32 may be the beginning of the ac- quisition of such needed information in this country.

The total absence of detectable ANA in this healthy population group was un- expected, since this laboratory has previ- ously reported a 4 per cent prevalence of positive reactors in presumably normal sera.lo In this respect it is of note that other investigators have reported 0 to 1.5 per cent ANA positive reactions among normal subjects .3R-40

SUMMARY AND CONCLUSIONS The distribution of immunoglobulin lev-

els in a random sample of healthy subjects aged 15 to 74 years was studied. A higher concentration of gamma globulin was pres- ent in Negro as compared to white subjects in all three of the major immunoglobulin classes but was of statistical significance in the yG fraction only. An increase in yA in older subjects o€ both races was noted. An increase in yM among women was ob- served. A low-order, but significant, posi- tive correlation between yG and yA was

present among white and Negro subjects. Gamma-G and yM levels were virtually identical when a sample of white urban New Yorkers and white rural Georgians were compared. Gamma-A concentrations were higher in the rural subjects and this difference was related to the older mean age of the Georgians.

Anti-yG globulins ( “‘rheumatoid fac- tors”) were detected in increasing fre- quency with age and with greater frequency among Negroes and white women than

WHITE AND NEGRO SUBJECTS IN EVANS COUNTY, GEORGIA 213

among white men. Possible correlations in healthy Negroes. Until the reason for the between anti-y G globulins and immuno- racial difference in immunoglobulin levels globulin levels were found only among is more certain, in vivo and in vitro studies Negro subjects. of immune systems should control for the

Antinuclear antibodies were not detecta- racial constitution of experimental groups. ble in any of the sera.

In the use of electrophoretically or im- munologically determined serum gamma ACKNOWLEDGMENTS

globulin concentrations as diagnostic vari- The authors acknowledge fie able technical ables in clinical medicine, cognizance assistance of Mrs. Kay Carlisi, Mrs. Doris Kreckman should be taken of the elevated levels found and Miss Diane McCauley.

SUMMARY

Immunoglobulin levels in a healthy biracial population sample indicated that Negroes had higher levels of the three major immunoglobulins than did whites, although the most prominent difference was in the yG fraction. No urban-rural difference in im- munoglobulin levels was noted. Women had higher yM levels than did men. Gamma-A concentration increased with age. Anti-yG globulins were detected with increased fre- quency in older subjects. Negroes, and white women, had a higher prevalence of anti- yG globulins than did white men. Antinuclear antibodies were not detected.

SUMMARIO IN INTERLINGUA

Le nivellos de immunoglobulina in un specimen de un normal population biracial indicava que le negros habeva plus alte nivellos del tres major immunoglobulinas que le blancos, ben que le plus prominente differentia esseva in le fraction yG. Nulle dif- ferentia urban-rural in le nivellos de immunoglobulinas esseva notate. Feminas habeva plus alte nivellos de yM que masculos. Le concentration de yA accresceva con le etate del subjectos. Globulinas anti yG esseva detegite con augmentate frequentia in subjectos de etate plus avantiate. Negros e femininas blanc habeva un plus alte prevalentia de globulinas anti yG que masculos blanc. Anticorpore anti nucleo non esseva detegite.

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J. G.: Serum protein concentration in the North American Negroid. Science 123:991, 1956.

2. Pollack, V. E., Mandema, E., Doig, A. B., Moore, M., and Kark, R. B.: Observations on electrophoresis of serum proteins from healthy North American Caucasians and Negro subjects and from patients with sys- temic lupus erythematosus. J. Lab. Clin. Med. 58:353, 1961.

3. Klein, G. C., Cumming, M. M., and Ham- merstein, J. R.: Differences between the normal serum protein patterns of Amer- ican Indian, Negro and Caucasian sub- jects. Proc. SOC. Exp. Biol. Med. 111:298, 1962.

4. Lichtman, M. A., Hames, C. G., and Mc- Donough, J. R.: Serum protein electropho- retic fractions among Negro and white sub- jects in Evans County Georgia. Amer. J.

Clin. Nutr. 16:492, 1965. 5. Siegel, M., Lee, S. L., Ginsberg, V., Schultz,

F., and Wong, W.: Racial differences in serum gamma globulin levels: Compara- tive data for Negroes and Puerto Ricans and other Caucasians. J. Lab. Clin. Med. 66:715, 1965.

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