Imunologi Pangan 2009 Arif Hartoyo ITP Fateta IPB

 Nonspecific or “innate” immunity is the front line of host defense against microorganisms in the gut and other sites  A fundamental mechanism used by a host to avoid microbial infection is performed by the epithelial barriers, which exclude most (>99%) proteins in the intestinal tract  Secretions provide another major form of nonspecific immunity in the GI tract. For example: ◦ the low pH of the stomach-- facilitates destruction of pathogens, as well as of their toxins and immunogenic macromolecules. ◦ Bile acids and pancreatic secretions that contain proteases such as trypsin and carboxypeptidase also can function as protectants against microbial pathogens. ◦ mucus provides a vehicle for antibacterial substances (secretory immunoglobulin A, lysozyme, lactoferrin) and prevents passage of large molecular weight materials into enterocytes.

 When a microbial invader overcomes nonspecific immune defenses, a mammalian host can activate a system that recognizes and then inactivates a foreign material or “antigen” that is then removed or destroyed  Fundamental to this specific or “acquired” immune system are capacities (1) to recognize minute differences in the chemical structure of an antigen and (2) to “remember” these structures for long periods of time.  Antigens are typically high molecular weight (> 10,000) proteins or polysaccharides. Components of bacteria, fungi, and viruses such as cell wall, flagella, capsule, and toxins are excellent antigens and are multivalent, that is, have more than one antigenic determinant or recognition site.

 Abbas et af. (1991) suggested that specific responses could be divided into (1) a cognitive phase, (2) an activation phase, and (3) an effector phase  The cognitive phase refers to the binding of foreign antigens to specific receptors on lymphocytes that are present prior to antigen stimulation.  The activation phase refers to the series of events that is induced in lympho-cytes as a consequence of specific antigen recognition. Activation events include proliferation, which leads to expansion of antigen-specific lymphocytes and differentiation from cognitive to effector functions. Activation requires antigen as well as “helper” or “accessory” signals from another cell for a complete signal.  Finally, the effector phase represents the active functional manifestations of antigen recognition and activation.

 The movement of high molecular weight antigens from the gut lumen to the blood circulation has been demonstrated experimentally in humans and animals  Prior to uptake, the antigens must resist proteolytic activity in the lumen and penetrate the mucus/IgA layer to interact with the various absorptive cell types.  Factors that disrupt mucosal barrier function and cause extensive uptake include immature gastrointestinal function, malnutrition, inflammation, and immunoglobulin deficiencies  Macromolecules may be taken up by the gut by at least two distinct mechanisms

 Many food- and waterborne microbial agents can colonize the GI tract and induce gastroenteritis. Endogenous components or secreted products of bacteria can cause a variety of toxic effects.  Staphylococcal enterotoxin and botulinum toxin must survive ingestion and evade GI immunity in the gut prior to inducing their specific toxic effects.  Mycotoxins are low molecular weight secondary metabolites that are produced by various fungi and frequently are found in foods. Because they resist digestion and processing, mycotoxins can enter into the intestinal tract; because of their size, they evade normal GI immune mechanisms of protection. Their toxic effects are variable and can include cancer, impaired reproduction, and gastroenteritis.

 Although foods contain a multitude of proteins, very few of these can trigger IgE- mediated food allergies.  Typical food allergens are naturally occurring water-soluble proteins that are heat and acid stable and resist digestion.  Taylor (1992) has noted that, with the exception of cow’s milk proteins and egg proteins, most allergenic proteins in foods are of plant or marine origin.

 When exogenous chemicals interact with lymphoid tissue, immune homeostasis might be disrupted and induce undesirable “immunotoxic” effects such as: ◦ (1) immunosuppression, ◦ (2) uncontrolled proliferation, ◦ (3) impaired host resistance, ◦ (4) allergy, ◦ (5) autoimmunity  Chemicals that are potentially immunotoxic in the gut might be found among natural components, additives, growth promoters, animal drugs, and various contaminants

 Infeksi saluran pencernakan sering terjadi pada individu yang kekurangan gizi  Kekurangan Energi Protein merupakan penyebab utama imunodefisiensi.  Specific immunologic effects include thymic atrophy, decreased spleen weight, decreased T cell counts, and impaired production of cytokines, thymic hormones, and antibodies. Thus, both cellular and humoral immu- nity are affected.

 Innate and specific defense mechanisms must be well developed for the epithelium to function as an impermeable barrier to proteins and their fragments. However, such defense mechanisms are not fully developed in the infant during the postpartum period, particularly when born prematurely  Newborn infants are especially susceptible to pathologic penetration by deleterious intestinal contents because of this delayed maturation of the mucosal barrier.  Repercussions of this immaturity include enhanced susceptibility to infection, potential for hypersensitivity reactions, and formation of immune complexes. In some situations, these conditions can be fatal.  Clinical conditions that have been associated with an immature mucosal barrier include allergy, necrotizing enterocolitis, dermatitis, malabsorption, sudden infant death syndrome, and toxigenic diarrhea in early childhood, as well as inflammatory bowel disease, nephritis, and autoimmunity in adulthood  Human milk is a natural means of passively assisting a sensitive neonate against the hazards of a deficient GI immune system. Ingestion of colostrum decreases antigen penetration. Intake of colostrum apparently enhances the maturation of mucosal epithelial cells and accelerates the development of an intact mucosal barrier.

 Reduction or Elimination of Microbial Pathogens  Ingestion of Probiotic Cultures  Immunostimulatory Properties of Ingested Bacteria