T-lymphocytes are not able to recognize free antigen molecules. In order for an immune response to be initiated in T cells, the T cell must first be presented with the foreign antigen along with a self antigen known as a major histocompatibilty protein. This is the function of an antigen-presenting cell or an APC.
The APC recognizes the foreign antigen and moves towards it.
The APC surrounds the antigen and internalizes it by endocytosis.
Once the antigen has been ingested, lysosomes containing digestive enzymes fuse with the phagosome containing the antigen. The antigen is degraded into molecular fragments. Molecular fragments of the anitgen are combined with MHC class two proteins. This complex is sent to the cell membrane.
The T cell can now recognize the antigen. The T cell antigen receptor binds to the MHC class 2 protein and antigen fragment complex. An immune response will be initiated.