Imiquimod: mode of action and therapeutic potential.

The immune system’s main function is to protect the host against infection and can be divided into two main parts, innate (non-specific) and acquired (specific) immunity. Innate immunity provides the first line of defense against pathogens, and includes mechanisms already present that can be activated immediately, as in the skin and mucous membranes, e.g. the interferon alpha (IFNa) response, the cytokine response, and the neutrophil and macrophage response. In contrast, acquired immunity is specific for each pathogen and consists of humoral and cellular responses. Humoral immunity involves the production of immunoglobins (antigens) by B lymphocytes, which bind specifically to the antigen that induced them. A humoral response is initiated when an antigen activates a specific B-cell with the support of CD4 T cells. The B-cell proliferates and differentiates to form plasma cells, which then produce the antibodies with high affinity against the target antigen. Cell-mediated immunity depends on direct interactions between T-cell lymphocytes and cells primed by professional antigen presenting cells lymphocytes and cells; the cells bearing the antigen the T cells recognize in the HLA class I/II complex. A cellular response is initiated when an antigen on the surface of an abnormal cell is identified by and activates T-helper and cytotoxic T cells. Immunomodulators orchestrate the immune response, either up-regulating (immunostimulation) or down-regulating (immunosuppression) the immune response. Imiquimod belongs to the family of immunostimulators and is a novel synthetic molecule which enhances both the innate and acquired immune response, in particular the cell-mediated pathways.