Every cell in your body has the same genes, or genome. What distinguishes one cell type from another is the pattern of gene expression and, consequently, the proteins each cell makes. A muscle cell makes a different set of proteins than a B cell, a type of immune-system cell. Muscle cells, for example, express large amounts of actin and myosin proteins, which help muscles contract, whereas B cells express high levels of antibody proteins, which help the body fight infections.