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CD4+ cells: Lymphocytes that express CD4+ on its surface. CD4 is a molecule that participates in the maturation of thymus and the transmission of signals during the activation of HLA II. On the other hand, lymphocytes T CD8 are restricted to the HLA I. Heterodimer: Two different subunits that form part of a protein. Incidence: Number of new cases of an illness in a defi ned population during a defi ned period of time. Linkage disequilibrium: Terms to defi ne a non-random combination of alleles. Microchimerism: Small number of cells that originally belonged to another subject and persist in a new individual laying a distinct genotype. Prevalence: Number of cases of a determinate illness in a specifi c population in a determinate time. Relative risk: It is the risk to develop a disease in relation to the exposure. It is defined as the ratio between the probability of occurring a disease in a non exposed group, divided to the probability of an exposed group. Rheumatoid factor: Serological test that measures the presence of the immunoglobulin G (IgG), produced by the lymphocytes of synovial membrane. Salting out method: Technique of DNA isolation. It is based on the principle of precipitation of salts. When using blood, the fi rst lysis of red blood cells is achieved through hypotonic solutions and the addition of detergents; then nucleic acids are precipitated with salts and extracted with organic solvents like ethanol. Synovial membrane: Soft tissue located within the non-cartilaginous surfaces and joints with cavities. It can be also known as synovium. System of complement: Combination of plasmatic molecules implied in biochemical cascades as part of innate immunity. Fundamental component of immune response against a strange agent. T-cell: Type of cell that belong to leukocyte group that detect antigens from the major histocompatibility complex (MCH) and segregate proteins and cytokines, coordinating the immune response.