CD4 and infection: For example, several groups have reported that human immunodeficiency virus (HIV) exploits Sialoadhesin/Siglec-1 to enhance infection of CD4+ T cells (the primary target cells) by trans-infection (i.e., the virus particle is captured by macrophages with Sialoadhesin/Siglec-1, which transfers the virus to CD4+ T cells and facilitates the infection) (104–107).