PEP is recommended because it has been observed that systemic infection does not occur immediately after an exposure and there is a brief window period of opportunity available to contain or eradicate HIV from body before wide spread cellular infection occur. After a needle-stick injury, immune cells (dendritic) in skin present the HIV to T-lymphocytes and natural killer cells. It is seen that in monkey models, Simian immunodeficiency virus (SIV) when injected into vagina, virus is engulfed by immune cells in the vaginal lining. Within 2 days SIV is found in inguinal lymph nodes. Within 5 days SIV infected T-cells and free SIV can be detected in blood stream 9. Migration of SIV after rectal or penile tip exposure follows a similar time frame. PEP can stop viral replication during this early period before it gains a 'foothold' in the body.