Depression of thymus-dependent immunity in wasting protein-energy malnutrition does not depend on an altered ratio of helper (CD4+) to suppressor (CD8+) T cells or on a disproportionately large atrophy of the T-cell relative to the B-cell pool

The Tolerance Model of Non-Inflammatory Immune Competence in Acute Pediatric Malnutrition: Origins, Evidence, Test of Fitness and Growth Potential

The tolerance model rests on the thesis of a physiologically regulated, albeit unsustainable, systemic attempt to adapt to the catabolic challenge posed by acute prepubescent malnutrition even in its severe forms. The model centers on the immunological component of the attempt, positing reorientation toward a non-inflammatory form of competence in place of the classic paradigm of immunological attrition and exhaustion. The foundation of the model was laid in 1990, and sixteen years later it was articulated formally on the basis of a body of evidence centered on T cell cytokines and interventions with cytokine and hormonal mediators. The benefit originally suggested was a reduced risk of autoimmune pathologies consequent to the catabolic release of self-antigens, hence the designation highlighting immune tolerance. Herein, the emergence of the tolerance model is traced from its roots in the recognition that acute malnutrition elicits an endocrine-based systemic adaptive attempt. Thereafter, the growth of the evidence base supporting the model is outlined, and its potential to shed new light on existing information is tested by application to the findings of a published clinical study of acutely malnourished children. Finally, some knowledge gaps pertinent to the model are identified and its potential for growth consonant with evolving perceptions of immunobiology is illustrated.

Fidelity in Animal Modeling: Prerequisite for a Mechanistic Research Front Relevant to the Inflammatory Incompetence of Acute Pediatric Malnutrition

Inflammatory incompetence is characteristic of acute pediatric protein-energy malnutrition, but its underlying mechanisms remain obscure. Perhaps substantially because the research front lacks the driving force of a scholarly unifying hypothesis, it is adrift and research activity is declining. A body of animal-based research points to a unifying paradigm, the Tolerance Model, with some potential to offer coherence and a mechanistic impetus to the field. However, reasonable skepticism prevails regarding the relevance of animal models of acute pediatric malnutrition; consequently, the fundamental contributions of the animal-based component of this research front are largely overlooked. Design-related modifications to improve the relevance of animal modeling in this research front include, most notably, prioritizing essential features of pediatric malnutrition pathology rather than dietary minutiae specific to infants and children, selecting windows of experimental animal development that correspond to targeted stages of pediatric immunological ontogeny, and controlling for ontogeny-related confounders. In addition, important opportunities are presented by newer tools including the immunologically humanized mouse and outbred stocks exhibiting a magnitude of genetic heterogeneity comparable to that of human populations. Sound animal modeling is within our grasp to stimulate and support a mechanistic research front relevant to the immunological problems that accompany acute pediatric malnutrition.

Feeding the immune system

A well-functioning immune system is key to providing good defence against pathogenic organisms and to providing tolerance to non-threatening organisms, to food components and to self. The immune system works by providing an exclusion barrier, by identifying and eliminating pathogens and by identifying and tolerating non-threatening sources of antigens, and by maintaining a memory of immunological encounters. The immune system is complex involving many different cell types distributed throughout the body and many different chemical mediators some of which are involved directly in defence while others have a regulatory role. Babies are born with an immature immune system that fully develops in the first few years of life. Immune competence can decline with ageing. The sub-optimal immune competence that occurs early and late in life increases susceptibility to infection. Undernutrition decreases immune defences, making an individual more susceptible to infection. However, the immune response to an infection can itself impair nutritional status and alter body composition. Practically all forms of immunity are affected by protein-energy malnutrition, but non-specific defences and cell-mediated immunity are most severely affected. Micronutrient deficiencies impair immune function. Here, vitamins A, D and E, and Zn, Fe and Se are discussed. The gut-associated lymphoid tissue is especially important in health and well-being because of its close proximity to a large and diverse population of organisms in the gastrointestinal tract and its exposure to food constituents. Certain probiotic bacteria which modify the gut microbiota enhance immune function in laboratory animals and may do so in human subjects.

Effects of host diet and thermal state on feeding performance of the flea Xenopsylla ramesis

We examined feeding performance of the flea Xenopsylla ramesis on three different hosts: its natural, granivorous, rodent host, Sundevall’s jird (Meriones crassus); the frugivorous Egyptian fruit bat (Rousettus aegyptiacus); and an insectivorous bat, Kuhl’s pipistrelle (Pipistrellus kuhlii). Because these fleas are not known to occur on bats, we hypothesized that the fleas’ feeding performance (i.e. feeding and digestion rates) would be higher when feeding on their natural host than on either of the bats that they do not naturally parasitize. We found that mass-specific blood-meal size of both male and female fleas was significantly lower when feeding on Kuhl’s pipistrelles than on the other two species, but was not different in female fleas feeding on fruit bats or on jirds at all stages of digestion. However, more male fleas achieved higher levels of engorgement if they fed on Sundevall’s jirds than if they fed on Egyptian fruit bats. The fleas digested blood of fruit bats and jirds significantly faster than blood of Kuhl’s pipistrelle. In addition, after a single blood meal, the survival time of fleas fed on normothermic Kuhl’s pipistrelles was significantly shorter than that of fleas fed on Sundevall’s jirds and even lower when male fleas fed on Egyptian fruit bats. Thus, our prediction was partially supported: normothermic Kuhl’s pipistrelles were inferior hosts for fleas compared with Sandevall’s jirds and Egyptian fruit bats. Interestingly, the proportion of engorged fleas that fed on torpid Kuhl’s pipistrelles was significantly higher than the proportion of the fleas that fed on normothermic individuals, indicating that becoming torpid might be a liability, rather than an effective defense against parasites.

Diminished primary CD8 T cell response to viral infection during protein energy malnutrition in mice is due to changes in microenvironment and low numbers of viral-specific CD8 T cell precursors

Protein energy malnutrition (PEM) increases the incidence and severity of infection, causing morbidity and mortality in malnourished populations. Viral-specific cells are an important component of protective immunity. We hypothesized that reduction in the expansion of viral-specific cells and the microenvironment of the PEM host leads to increased incidence and severity of infections. We tested this hypothesis using a mouse model of lymphocytic choriomeningitis virus (LCMV) infection and an adoptive transfer system using P14 transgenic mice cells bearing T cell receptors specific for the D(b)-restricted LCMV glycoprotein 33-41 epitope. We transferred equal numbers of P14 cells from mice fed either an adequate, 18% protein or low, 0.6% protein diet into C57BL/6 mice that had been fed adequate-protein (AP) or low-protein (LP) diets for 2 wk, infected them with LCMV, and followed them 1 wk postinfection. During PEM, the expansion of primary viral-specific CD8 T cells diminished; in LP diet-fed mice, it was only 2-3% of that in the AP diet-fed mice. Furthermore, the diminished primary CD8 T cell response during PEM may in part have been due to low numbers of viral-specific CD8 T cells and an altered microenvironment.

Re-feeding rapidly restores protection againstHeligmosomoides bakeri(Nematoda) in protein-deficient mice

This study determined whether the timing of re-feeding of protein-deficient mice restored functional protection against the gastrointestinal nematode,Heligmosomoides bakeri. Balb/c mice were fed a 3% protein-deficient (PD) diet and then transferred to 24% protein-sufficient (PS) diet either on the day of primary infection, 10 days after the primary infection, on the day of challenge infection, or 7 days after the challenge infection. Control mice were fed either the PD or PS diet. Onset of challenge, but not primary, infection caused short-term body weight loss, anorexia and reduced feed efficiency. Weight gain was delayed in mice when re-feeding commenced on the day of challenge infection; alkaline phosphatase (ALP) was also elevated in these mice on day 28 post-challenge. In contrast, other re-feeding groups attained similar body weights to PS mice within 4 days and had similar ALP at day 28. Serum leptin was higher in PD than PS mice and positively associated with food intake. As expected, worm survival was prolonged in mice fed the PD diet. However, egg production and worm burdens were similar in all re-feeding groups to the PS mice, indicating that protein re-feeding during either the primary or challenge infection rapidly restored normal parasite clearance.

Developmental immunotoxicity of lead in the rat: influence of maternal diet

The effect of maternal dietary protein intake on lead-induced developmental immunotoxicity was studied in female Fischer 344 rats receiving lead acetate (250 ppm) or sodium acetate (control) in the drinking water during breeding and pregnancy until parturition. Dams were fed isocaloric diets (either 20% casein or 10% casein) from 2 wk prior to mating until the end of lactation. After weaning, dams and female offspring were given the 20% casein diet and regular water. Immune function was assessed in dams at 8 wk postpartum and in offspring at 13 wk of age. Dams showed no marked difference in any of the immune endpoints examined, regardless of diet or lead treatment. In contrast, lead exposure during early development produced a subsequent significant reduction of both the delayed-type hypersensitivity response and interferon gamma production in adult offspring independent of maternal diet. Lead-exposed offspring from the high-dietary-protein group had significantly elevated production of both interleukin-4 and tumor necrosis factor alpha(TNF-alpha) with increased relative spleen weight and a decreased body weight compared to offspring in the lead control group. In contrast, lead-exposed offspring from dams receiving the low-protein diet had no marked change in TNF-alpha levels, relative spleen weight, or body weight, while interleukin-4 levels were significantly reduced compared with the lead control group. In conclusion, maternal dietary protein intake can modulate the immunotoxic effects of lead exposure during early development. This occurred at levels of protein intake and doses of lead exposure that produced no detectable effect on the maternal immune system.

Chronic consumption of a moderately low protein diet does not alter hematopoietic processes in young adult mice

The current studies examined whether hematopoiesis in the bone marrow and T-cell development in the thymus were attenuated in young adult A/J mice fed a moderately low protein diet (MPD, 50 g protein/kg) for 15 wk compared with mice fed a control protein diet (CPD, 180 g protein/kg). Flow cytometric analyses using antibodies against CD31 and Ly-6C as well as CD4 and CD8 were performed to identify stem, mixed progenitor, erythroid, lymphoid, granuloid and monocytic compartments in the bone marrow and four thymocyte subsets, respectively. Chronic restriction of young adult mice to MPD neither decreased the cellularity nor altered the distribution of subpopulations in either primary tissue. Subsequently, a new set of mice were provided with CPD and a low protein diet (LPD, 25 g protein/kg). After 5 wk, body and thymus weights in LPD group were reduced 26 and 30%, respectively, which was accompanied by a 505% increase in serum corticosterone. Surprisingly, LPD did not alter the number or distribution of cells in the bone marrow and the percentages of thymocyte subsets, supporting the findings from the MPD group. We conclude that chronic consumption of a marginal protein diet by young adult mice does not disrupt hematopoietic processes.

The capacity of noninflammatory (steady-state) dendritic cells to present antigen in the primary response is preserved in acutely protein- or energy-deficient weanling mice

The objective of this investigation was to determine the influence of wasting protein and/or energy deficits on the capacity of dendritic cells to initiate primary responses. Weanling male and female C57BL/6J mice were permitted free access to a complete purified diet, free access to an isocaloric low protein purified diet (combined deficiencies of protein and energy) or restricted intake of the complete diet (energy deficiency) for up to 14 d; a 19-d-old zero-time control group was also included. Malnourished mice lost 1.5-2% of initial body weight daily. Antigen presentation by dendritic cells from spleen and lymph nodes was assessed in vitro by the primary one-way allogeneic mixed lymphocyte reaction using CBA/J mononuclear or CD4(+) T cells as responders. This function was sustained despite advanced weight loss and, remarkably, was increased in cell suspensions from 14-d energy-deficient mice. Antigen presentation by dendritic cells in mononuclear suspensions was examined in vivo using the host-vs.-graft response in CBA/J recipients, and an ontogeny-related increase was sustained in both malnourished groups through 14 d of weight loss. Neither wasting protocol influenced the proportion of mononuclear cells (1-2%) exhibiting dendritic cell phenotype (CD11c(+)F4/80(-/low)) in the cellular suspensions used to study antigen-presenting activity. Consequently, these functional studies are interpretable on a per dendritic cell basis. In the absence of infectious or inflammatory pressure, the dendritic cell retains antigen-presenting capacity despite acute (wasting) deficiencies of protein and/or energy. The results are relevant to presentation of both foreign (adjuvant role) and self (tolerizing role) antigens by the dendritic cell.

Malnutrition alters the innate immune response and increases early visceralization following Leishmania donovani infection

Malnutrition is a risk factor for the development of visceral leishmaniasis. However, the immunological basis for this susceptibility is unknown. We have developed a mouse model to study the effect of malnutrition on innate immunity and early visceralization following Leishmania donovani infection. Three deficient diets were studied, including 6, 3, or 1% protein; these diets were also deficient in iron, zinc, and calories. The control diet contained 17% protein, was zinc and iron sufficient, and was provided ab libitum. Three days after infection with L. donovani promastigotes, the total extradermal (lymph nodes, liver, and spleen) and skin parasite burdens were equivalent in the malnourished (3% protein) and control mice, but in the malnourished group, a greater percentage (39.8 and 4.0%, respectively; P = 0.009) of the extradermal parasite burden was contained in the spleen and liver. The comparable levels of parasites in the footpads in the two diet groups and the higher lymph node parasite burdens in the well-nourished mice indicated that the higher visceral parasite burdens in the malnourished mice were not due to a deficit in local parasite killing but to a failure of lymph node barrier function. Lymph node cells from the malnourished, infected mice produced increased levels of prostaglandin E(2) (PGE(2)) and decreased levels of interleukin-10. Inducible nitric oxide synthase activity was significantly lower in the spleen and liver of the malnourished mice. Thus, malnutrition causes a failure of lymph node barrier function after L. donovani infection, which may be related to excessive production of PGE(2) and decreased levels of IL-10 and nitric oxide.

T cells with a quiescent phenotype (CD45RA+) are overabundant in the blood and involuted lymphoid tissues in wasting protein and energy deficiencies

The objective of this investigation was to determine the influence of distinct forms of acute weight loss on the expression of the quiescence marker, CD45RA, by T cells in several lymphoid compartments including the blood. Male and female weanling mice, CBA/J and C57BL/6J strains, were allocated to the following groups: ad libitum intake of a complete purified diet; restricted intake of the complete diet; and ad libitum intake of an isocaloric low-protein diet. The restricted intake protocol produced weight loss through energy deficiency (marasmic-type malnutrition), whereas the low-protein diet caused wasting through inadequate protein nitrogen and induced a condition mimicking incipient kwashiorkor. In one experiment, weanling mice of both strains were maintained for 14 days according to each of these dietary protocols and, in a supplementary study, C57BL/6J weanlings consumed either the complete diet or the low-protein diet ad libitum for 21 days. Zero-time control groups (19-days old and 23-days old in C57BL/6J and CBA/J strains, respectively) were included in the first experiment to control for ontogeny-related change. Expression of CD45RA was assessed by two-colour flow cytometry in CD4+ and CD8+ T cells from the spleen, mesenteric lymph nodes and blood. Within 14 days, energy-restricted mice exhibited a high percentage of CD4+ T cells expressing CD45RA in all three lymphoid compartments in both mouse strains (an average of 50% CD45RA+ versus 9% in well-nourished controls), and a similar outcome was apparent in the CD8+ subset (93% CD45RA+ versus 63%). Mice fed the low-protein diet required up to 21 days to exhibit the same imbalance within the CD4+ T-cell subset (33% CD45RA+ versus 4% in well-nourished controls). A shift toward a quiescent phenotype occurs throughout the peripheral T-cell system in acute wasting disease. Consequently, the quiescence-activation phenotype of blood T cells reflects the same index in secondary lymphoid organs in such pathologies. Naive-type quiescence among T cells is implicated as a component of depressed adaptive immunocompetence in the advanced stages of diverse forms of acute weight loss.

Tumoricidal activity of lymphokine-activated killer cells during acute protein restriction in the cotton rat (Sigmodon hispidus)

Habitat-induced alterations of immune system function have been implicated in the regulation of survival rates in wild herbivore populations. Protein availability in the diet has been shown to fluctuate with density and influence immunity in hispid cotton rats (Sigmodon hispidus), a common herbivorous rodent of the southeastern United States. In this study, we examined the impact of short-term, moderate restrictions in dietary protein on the tumoricidal activity of lymphokine-activated killer (LAK) cells in the spleen of subadult male cotton rats in captivity. Animals were fed complete, isocaloric diets containing either 20% casein (high quality diet), or one of three moderate levels of protein (10, 8, or 5% casein) for two weeks prior to assessing LAK cell activity in vitro in the presence of YAC-1 tumor cells. Moderate restrictions in protein resulted in depressed body growth, although all animals gained mass during the second week of the trial, without significant increases in food intake. Immune organ development and cellularity were suppressed in moderately restricted cotton rats when compared to those on a high quality diet. Tumoricidal activity of LAK cells against YAC-1 targets were significantly altered by diet treatments, being elevated in the group fed a diet containing 10% casein. There was a general tendency for increased LAK cell activity among those fed one of the three moderate quality diets, but observed suppressions in splenic cellularity tended to result in a slight decline in total lytic capacity of spleens.

Immunocompetence in relation to a heat-processed diet (Maillard reaction) in weanling rats

Diets containing unheated casein (CD; control) or a casein-glucose mixture (CGD) previously heated at 140 degrees for 2 h were fed to two groups of young rats for 21 d. Differences in body weight, feed consumption, thymus, and spleen growth, protein metabolism and in vivo immune response were then determined. For this last experiment, animals were inoculated with sheep erythrocytes (SRBC) on day 15 to provide an immunological challenge. No changes were seen in body weight, feed consumption or feed conversion ratios. Neither were significant differences found in spleen weight, protein content, DNA content, DNase (EC 3.1.4.6) activity or lymphocyte count, suggesting that spleen cell growth remained similar in all the animals studied. The CGD induced marked increases in thymus DNA content whilst the protein:DNA ratio became lower. Spleen RNA content was similar in all rats, but thymus RNA content was 29% lower in the CGD group, although this difference did not reach statistical significance. This fact might be a consequence of the low RNase (EC 2.7.7.16) activity and RNase:RNA ratios in the thymus glands of CGD-fed animals. Further, the number of splenic plasma cells secreting anti-SRBC antibodies (direct plaque-forming cells) was significantly decreased in the same group. It might be concluded that both diets are adequate for rat growth and that the differences observed in the thymus of CGD-fed rats may be directed towards preserving tissue function. Nevertheless, the CGD did cause immunological disturbances affecting the humoral immune response.

Splenocyte glutathione and CD3-mediated cell proliferation are reduced in mice fed a protein-deficient diet

Protein-energy malnutrition (PEM) is associated with decreased host immune defense. Glutathione (GSH) status is reported to be decreased in PEM, and GSH is important for lymphocyte function. The objective of the present study was to investigate the effects of PEM and dietary repletion (RP) on GSH status in various tissues and splenocytes and on CD3-mediated calcium mobilization and cell proliferation of splenic T-lymphocytes. For the PEM model, mice were fed a 0.5% protein diet (LP group) for 4 or 6 wk, and control mice were fed a 15% protein diet (CP group). In the RP study, LP mice were fed the 15% protein diet for 3 d, 1 wk, 2 wk or 3 wk (RP groups). Glutathione concentrations were significantly lower in liver, lung, heart and spleen of LP mice compared with CP mice at 4 and 6 wk. Splenocytes from LP mice were significantly lower in number and had a lower intracellular GSH concentration, depressed CD3-stimulated T-lymphocyte proliferation in culture media without thiol supplementation (2-mercaptoethanol), and enhanced CD3-stimulated proliferation in thiol-supplemented culture media compared with splenocytes from CP mice. CD3-stimulated calcium mobilization was significantly lower in CD8+, but not CD4+, splenocytes from LP mice. Within 1 wk of dietary repletion, splenocyte GSH concentration was normal and splenocyte numbers were greater, and in vitro sensitivity of CD3-stimulated T-lymphocyte proliferation to thiol was lower, compared with LP mice. Glutathione status in vivo and thiol supplementation in vitro seem to modulate the signal transduction pathway for T-lymphocyte proliferation in mice with PEM.

Expansion of the humoral effector cell compartment of both systemic and mucosal immune systems in a weanling murine model which duplicates critical features of human protein-energy malnutrition

A direct comparison of systemic (spleen) and mucosal (intestine) antibody-producing systems was made in weanling male C57BL/6J mice subjected to wasting protein-energy malnutrition (PEM) by means of a low-protein protocol known to duplicate immunological and physiological features of human malnutrition. ELISA revealed low concentrations of biliary and gut lumen immunoglobulin (Ig) A in malnourished mice concomitantly with a high concentration of blood IgA. The low-protein model, therefore, exhibited fidelity to human protein-energy malnutrition in its influence on the concentrations of the mucosal Ig, IgA, in critical biological fluids. The number of IgA-, IgM- and IgG-containing cells was estimated morphometrically on a per organ basis. The low-protein protocol supported expansion in numbers of mucosal IgA-containing cells (18 x relative to a zero-time control group) and of splenic IgG-containing cells (135x), albeit an attenuated expansion in comparison with that of well-nourished control animals (132x and 571x respectively relative to zero-time controls). Up to terminal differentiation of Ig-containing cells, systemic and mucosal antibody-producing systems exhibited similarly remarkable resistance to wasting malnutrition. Epithelial transport of IgA may be an aspect of the mucosal antibody response which is particularly sensitive to PEM.

Immune depression induced by protein calorie malnutrition can be suppressed by lesioning central noradrenaline systems

Depressed immune function is well documented in protein calorie malnutrition (PCM). Also, central noradrenergic hyperactivity has recently been reported in malnourished animals. Increase in central noradrenaline activity could be responsible for cell-mediated immune depression. The present study is designed to address this hypothesis by testing whether neurotoxic lesion of central noradrenergic systems by 6-hydroxydopamine (6-OHDA) could improve lymphoproliferative response to mitogens and interleukin (IL)-1 production in PCM rats. A significant enhancement of lymphoproliferative response to concanavalin A (ConA) and in IL-1 production was observed in spleen mononuclear cells of PCM rats injected intracerebroventricularly with 120 micrograms of 6-OHDA, as compared with solvent injected and untreated PCM animals. A significant decrease in brain noradrenaline levels was produced in the drug-injected animals. These results suggest that central noradrenergic hyperactivity is one of the mechanisms involved in the immunodepression produced by malnutrition.

Effects of triiodothyronine supplements on splenic natural killer cells in malnourished weanling mice

The main purpose of this investigation was to determine whether exogenous triiodothyronine (T3) administered according to a protocol known to prevent depression in acquired immunity in weanling murine protein-energy malnutrition (PEM) would, likewise, influence the splenic natural killer (NK) cell in this disease. Weanling mice of disparate inbred strains, C57BL/6J and CBA/J, were subjected to wasting PEM produced by means of two low-protein diets (0.5% crude protein) identical in every respect except that one diet contained supplemental T3 (0.2 micrograms/g diet). NK cell lytic activity toward YAC-1 targets was assessed in vitro using suspensions containing 0.5 x 10(6) mononuclear spleen cells. Lytic activity in this assay was low in mice fed the unsupplemented low-protein diet, but was not depressed in malnourished animals given exogenous T3. Surface marker analysis using the NK cell-specific antigen, NK 1.1 (PK 136), revealed no effect of the low-protein diet or of exogenous T3 on the proportion of splenic mononuclear cells exhibiting NK 1.1+ phenotype. Previous investigations have shown that acquired immune competence in PEM can be manipulated, by means of endocrine hormonal intervention, independently of continued wasting disease. The present results extend this fundamental new concept to include an innate immune function, namely NK cell lytic activity. In this system of experimental PEM, exogenous T3 prevented depression in NK cell lytic activity expressed on a per cell basis. The malnourished weanling rodent is a particularly powerful experimental system with which to investigate the mechanisms whereby thyroid hormones influence NK cells.

Effects of a protein-free diet or food restriction on the immune system of Wistar and Buffalo rats at different ages

The effects of a protein-free diet or food restriction on the immune system were examined in two rat strains, Wistar and Buffalo, in different age-groups. Unlike Wistar rats, Buffalo rats have an unusually hyperplastic thymus and a large number of peripheral T cells. The protein-free diet (PFD) in rats resulted in marked thymic involution together with a reduction of splenic T cells, both in number and in antibody response to sheep red blood cells. The depressive effect of the PFD on the immune system was more serious in young immature rats than in older rats, but less serious in Buffalo rats having enhanced T cell functions regardless of age. Thymic involution was also accelerated in both strains of rats by feeding them a restricted amount of the control diet containing well-balanced nutrients (food restriction, FR). In the FR experiment, no significant change was observed in immune functions of Wistar rats. A slight reduction was observed in the immune functions of Buffalo rats with FR, but absolute levels were distinctly higher in Buffalo rats than in Wistar rats even after FR. These results suggested (1) that the thymic function is sensitive to protein deficiency; (2) that a well-balanced dietary condition is necessary for immunological maturation in the early stage of life and preservation of immune functions at older age; (3) that animals having higher immune functions are more resistant to malnutrition than ordinary ones.