Early Dietary Influence on Later Immunocompetence

The more evidence that accumulates, the more complex the immune system appears. Only a simple outline will be presented here as background to the discussion. The antigen-presenting cells (APCs) are crucial because they are required for initiation of immune responses to all protein antigens. In the gut the active APCs are dendritic cells and macrophages. It has been debated whether gut epithelial cells can function as APCs; the evidence is still incomplete. APCs degrade the protein and present peptides in their inherited human leukocyte antigens (HLA) class I1 surface molecules to CD4+ T helper (TH) lymphocytes (Figure 1). They can only present peptides that fit the grooves in their HLA class I1 molecules, and the T cell is HLA restricted in recognizing this complex. Thus, immune responses are HLA dependent. The immune response will primarily develop into cell-mediated immunity if the TH cell binding the HLA-peptide complex with its T-cell receptor is exposed to the cytokine interleukin (1L)-12. In that case, the T H cell will turn into a TH type 1 (TH1) cell, which primarily produces the cytokines interferon (1FN)-y, IL-2, and tumor necrosis factor (TNF)-a. The IFN-y will activate macrophages able to kill intracellular parasites such as viruses, fungi, and certain bacteria like Listeria and Salmonella typhi. Cytotoxic CD8+ T cells (T,) also develop, using IL-2 for activation. The TH1 thus stimulates cellmediated immunity, which can also bring about antitumor activity. If, on the other hand, the TH cell is mainly exposed to IL-10, a TH2 cell, which chiefly produces IL-4, IL-5, IL-6, and IL-10, will develop. These cy-