How are T cells activated?

T cells are activated through a series of steps when they encounter specific antigens. 

Antigen-presenting cells (APCs), such as dendritic cells, macrophages, and B cells, capture and conduct constant surveillance, looking for any invaders that don’t belong. When they detect a foreign pathogen, APCs engulf the pathogen as well as its specific antigen by the process of phagocytosis. The antigen is digested and broken up into peptide fragments, which are displayed on the plasma membrane of the T cell using a protein called Major Histocompatibility Complex (MHC). After presenting the peptide antigen on its surface, the APC migrates from the site of infection to the lymph nodes.

Within the lymph nodes, the APC presents to the circulating naïve helper T cells.

Just like a key fits into only one lock, each T cell receptor (TCR) present on the surface of T cells can recognize and bind to only one specific antigen MHC complex on the APC. This binding of the TCR on a T cell and the MHC-antigen complex triggers the initial activation of T cells. 

At this stage, the T cells are only partially activated. Both helper T cells and cytotoxic T cells still need a number of secondary or co-stimulatory signals to become fully activated and capable of responding to the threat. This secondary activation process is known as co-stimulation.

In the co-stimulation or secondary activation stage, co-stimulatory signals are provided by CD28, a receptor expressed on the T cell surface. CD28 binds to the B7 protein, a very important molecule on the APC cell surface, resulting in the T cell getting fully activated. The fully activated T cells undergo rapid proliferation and differentiation, generating a population of effector T cells and memory T cells.