o Virus enters body→adheres to lectin on DC
§ Virus uses the DC as a “Trojan Horse” to sneak into lymph node
· Th cells congregate in the lymph nodes
o Virus binds by gp120 envelope to the CD₄ molecule on Th cell
§ Gp120 has a high affinity for Th cells
o Binding to the Th cell via gp120 allows binding to co-receptors CCR₅ and CXCR₄
§ Binding of the co-receptors causes the Th cell membrane to melt away, virus + cell fuse
o Virus injects core into cell, activates reverse transcriptase and copying begins
§ RNA→DNA→protein
§ Virus’s DNA is now incoroporated into the host Th cell’s DNA
§ All new copies of cells now infected
o Virus buds from infected cell, tearing holes in the membranes
o As new copies made, gp120 protein is replicated and allows fusion to more Th cells
o When the Th cells fuse, a huge SYNCYTIUM forms
§ Syncytium is a multinucleated mass of cytoplasm created by fusion of many cells
§ This is how virus infects multiple Th cells at once
o Syncytium prevents receptor availability for Ab binding
§ Antibodies are useless
o At first CD₄ cells are replenished as virus is cleared, but over time CD₄ cells are lost
§ Seen as a falling CD₄/CD8 ratio
§ Normal is ~1.5-3
§ An accelerating fall in the CD₄/CD8 ratio is poor prognostic sign | 
