Fig. 1 |. Dynamics of a CD4⁺ T cell response.

1. Naive CD4⁺ T cells quiescently recirculate through blood (dark red) and lymphoid (light green) tissues. Upon infection, for example by a respiratory pathogen, APCs migrate from infected barrier sites (magenta) to the draining lymph nodes through afferent lymphatics and present peptides from the pathogen on MHC-II molecules. 2. Recognition of the peptide–MHC-II complex through TCR in combination with co-stimulation and cytokine signals lead to the activation, differentiation and expansion of naive CD4⁺ lymph node T cells. 3. CD4⁺ T cells proliferate and differentiate into various effector subsets that each become poised to make specialized contributions to immunity. 4. Many proliferated T cells leave the lymph nodes and migrate to the infected tissue through blood to assist in pathogen control at sites of infection. 5. Once the infection is cleared, most pathogen-specific CD4⁺ T cells die resulting in contraction of the population. 6. However, a few survive to establish long-lived memory and stay widely distributed across the body. 7. Upon reinfection, memory CD4⁺ T cells can mount anamnestic responses that are quicker and of higher magnitude than a primary response.