Macrophage tropic strains of HIV use ?
**Core Concept**
Macrophage tropic strains of HIV are a subset of the virus that preferentially infect macrophages, a type of immune cell involved in phagocytosis and antigen presentation. This tropism is mediated by specific interactions between the viral envelope glycoprotein gp120 and host cell receptors.
**Why the Correct Answer is Right**
Macrophage tropic strains of HIV primarily use CCR5 as their co-receptor to gain entry into macrophages. CCR5 is a chemokine receptor that is highly expressed on the surface of macrophages and is involved in the recruitment of these cells to sites of inflammation. The interaction between gp120 and CCR5 allows the virus to fuse with the host cell membrane and initiate the replication cycle. This is in contrast to other strains of HIV that use CXCR4 as their co-receptor, which are often associated with more rapid disease progression.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as it refers to the co-receptor used by T-cell tropic strains of HIV, not macrophage tropic strains.
**Option B:** This option is incorrect as it refers to the co-receptor used by some strains of HIV, but not specifically macrophage tropic strains.
**Option D:** This option is incorrect as it is not a known co-receptor used by HIV.
**Clinical Pearl / High-Yield Fact**
It's worth noting that macrophage tropic strains of HIV are often associated with a slower disease progression compared to T-cell tropic strains, which is thought to be due to the more gradual activation of the immune system in response to macrophage infection.
**Correct Answer:** C. CCR5