7). B cells can differentiate into antibody-secreting cells upon encounter with a given antigen or pathogen. In most cases, direct activation of B cells by an antigen is observed in response to repetitive antigenic structures, such as carbohydrates found in bacterial walls. These T cell-independent responses are characterised by the secretion of low-affinity antibodies of the IgM type. This Cyclopamine order type of response is often stereotyped

in nature, lacking the typical memory response upon re-exposure to the same antigen (see section titled Immunological memory). In most cases, optimal B-cell activation and differentiation into antibody-secreting plasma cells is only observed when both B and T cells are simultaneously activated by the same pathogen. In these instances, CD4+ T cells differentiate into Tfh cells that are able

to provide a helper signal to B cells. T cell-dependent B cell responses are characterised by the secretion of high-affinity antibodies and a large spectrum of isotypes (in particular IgG), and are typically associated with immunity resulting from natural exposure. Cytokines are small proteins secreted by activated innate and adaptive immune cells (such as DCs, macrophages and T cells), which direct the activity of other cells to coordinate an appropriate immune response. Cytokines R428 in vitro Y-27632 2HCl are a diverse family of molecules which include interleukins, interferons and growth factor

responses (Appendices, Supplementary Table 5). Cytokines may act in an autocrine, paracrine or endocrine fashion, by binding cell-surface receptors and stimulating signalling pathways, ultimately affecting the gene expression of the target cell. Cytokines are referred to as either proinflammatory or anti-inflammatory, depending on their role during the establishment of immune responses. These two types then act together to control and regulate different aspects of the immune response. Immune responses are prevented, down-regulated or terminated by multiple mechanisms. These mechanisms include clonal deletion, the activity of suppressor monocytes and anti-inflammatory cytokines, induction of apoptosis, induction of unresponsiveness by resting APCs, expression of inhibitory cell-surface co-receptors and the activity of regulatory CD4+ T cells. Regulatory T cells (Treg cells) belong to the CD4+ T-cell subset. Their role is to inhibit immune or inflammatory responses by blocking the activity of effector T cells, helper T cells and APCs.