Linked from
The 21 pages that link to Antigenic variation, each with the reason it gives.
PathogenRelated: Changing recognizable antigens can help pathogens evade existing immunity.
PlasmodiumRelated: Plasmodium falciparum changes infected-cell surface proteins during infection.
MycoplasmaRelated: Surface-protein variation in some mycoplasmas can complicate immune clearance.
NeisseriaRelated: Changing surface structures helps Neisseria persist despite antibody responses.