Table 1 CD4 and CD8 T-cell immunophenotyping and their biological functions

HLA-DR

Critical for efficient antigen presentation to CD4+ T cells.

HLA DR is expressed primarily by antigen presenting cells and, together with CD38, is a useful marker of T-cell activation following viral infection.

CD38

Multifunctional ecto-enzyme involved in signal transduction, cell adhesion, and calcium signaling.

Used to study the processes of B- and T-cell differentiation and activation. Increased expression of CD38 on both CD4+ and CD8+ T-cells in HIV-infected patients is associated with disease progression.

CD27

Important for the generation and long-term maintenance of immune response. Memory marker.

CD45RO

Regulator of T-cell antigen signaling.

Memory marker.

CD27⁺CD45RO^-

Naïve cells recognize cognate antigen and initiate an immune response.

CD27⁺CD45RO⁺

Central memory (CM) T-cells are activated in secondary lymphoid organs following recognition of antigen on DCs and generate large numbers of effector cells.

CD27⁻CD45RO⁺

Effector memory (EM) cells exhibit effector function immediately upon recognition of antigen presented on non-professional APCs and limit the early spread of infection.