Acquired Immunity

Acquired responses consist of the Ag-specific humoral and cell-mediated immune responses. They express by synthesis of antibodies (Abs) and cellular responses, respectively. They involve three kinds of cells: Ag-presenting cells (APC) (mainly macrophages and DCs), T cells, and B cells. For cellular responses only APC and mainly CD8 + T cells are involved, while another population of T cells, CD4 + T cells, and B cells are needed for Ab synthesis. Antibodies can belong to several kinds of immunoglobulin isotypes: IgD, IgM, IgG, IgE, and IgA, and different subclasses including IgG1, IgG2, IgG3, and IgG4 or IgA1, and IgA2 in humans. After an initial contact with the Ag, acquired responses are slowly established (7-10 days) but are endowed with memory enabling a very rapid response after a further contact with the same Ag (within one day). The first step in the induction of the immune response is the presentation by APC, and recognition of the epitope (small part of an Ag), associated with major histocompatibility complex (MHC) molecules of class I or II, to the epitope-specific T cell receptor. In addition, the binding of co-stimulation molecules and equivalent receptors expressed by both APCs and T cells (CD40 and CD40 ligand, B7.2 and CD28, etc.), leads to the full activation and proliferation of naive T cells. Subsequently, a large proportion of those activated cells will die of apoptosis, the others surviving in the form of memory T cells.

All those "lock-and-key" mechanisms are important and greatly contribute to modulate the immune responses. It has been shown that DCs play a key role in the acquired immune responses. They exist in an immature form in tissues. The mature form is obtained following contact with an Ag and phagocytosis. Mature DCs are able to synthesize cytokines and migrate into the neighboring draining lymph nodes in order to supply Ag information to the T cells. DC populations are heterogeneous (26) and, as will be further described, subsets of intestinal DCs display specific properties in terms of Ag presentation and cytokine secretion.

Pregnancy And Childbirth

Pregnancy And Childbirth

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