A human's microbiota, the other microorganisms living within and on the body, compete with pathogenic organisms and play a large role in pathogenic control. Lastly, a semi-permeable membrane known as the blood-brain-barrier is a lining of endothelial cells separating the blood from the tissues and organs. Without this lining, viruses and bacteria could easily infect vital human organs such as the brain, lungs, and placenta. Inflammation is one of the first immune responses to pathogenic infection that many host organisms possess. Inflammation involves an elevated temperature surrounding the site of infection, accumulation of CO2 and organic acids, and a decrease in the infected tissue's oxygen tension in response to pathogen-induced cell damage. Coagulation of blood (clotting) also occurs in an inflamed area, providing a physical barrier against pathogenic infection. These changes ultimately create unfavorable living conditions for the pathogen (i.e. pH changes, decrease in ATP, and changes in cellular metabolism) and prevent further replication and growth. Once a bacterium or virus overcomes the body's innate immune system, the host organism's acquired immune system takes over. This immune response is highly specific to pathogens and provides the host with long-lasting immunity against future infection by that specific pathogen. When lymphocytes recognize antigens on a pathogen's surface, they secrete antibodies that bind to the pathogen and alert macrophages and natural killer cells. These cells target the pathogen itself, killing it or rendering it inactive. This process further produces memory B cell and memory T cells that allow long-lasting immunity to occur. In conclusion, if a pathogen is capable of overcoming various host defenses, recognizing a host cell for infection, and successfully replicating within a host tissue, then the pathogen is likely to possess tropism for that specific host. Viral host tropism is determined by a combination of susceptibility and permissiveness: a host cell must be both permissive (allow viral replication) and susceptible (possess the receptor complement needed for viral entry) for a virus to establish infection. Once a virus binds to a host cell, the host cell must then provide the necessary transcription factors needed for viral replication to occur. When the virus is able to use the cell to replicate its genetic information, the virus can spread infection throughout the body. The human immunodeficiency virus exhibits host tropism for CD4 related immune cells (e.g. T helper cells, macrophages or dendritic cells).