Danger model

Function of T helper cells: Antigen-presenting cells (APCs) present antigens on their Class II MHC molecules (MHC2). Helper T cells recognize these by expressing the CD4 co-receptor. The activation of a resting helper T cell causes it to release cytokines and other signals (green arrows) that stimulate the activity of macrophages, killer T cells, and B cells, the last of which produces antibodies. The proliferation of Helper T cells stimulates B cells and macrophages.

The danger model of the immune system proposes that it differentiates between components that are capable of causing damage, rather than distinguishing between self and non-self.