TRIM5-alpha is a protein that is found in the cells of many mammals and fends of various retrovirus infections. It protects monkeys from infection with HIV-1, and humans from infection with some other viruses. If a retrovirus has entered a cell, it needs to shed its capsid in order to reversely transcribe its genes, so that they can be expressed by the host cell. It is believed that TRIM5 alpha, which is present in the cytoplasm, somehow recognizes the capsid and blocks its shedding, thereby stopping the virus in its tracks. It thus represents an intracellular defense completely separate from the rest of the body's immune system.