PATHOLOGY

Detailed explanation-1: -An APC, such as a macrophage, engulfs and digests a foreign bacterium. An antigen from the bacterium is presented on the cell surface in conjunction with an MHC II molecule Lymphocytes of the adaptive immune response interact with antigen-embedded MHC II molecules to mature into functional immune cells.

Detailed explanation-2: -Macrophages interact with T cells in order to bring about T cell activation in target organs, and are themselves activated by inflammatory messenger molecules (cytokines) produced by the T cells. Macrophages produce toxic chemicals, such as nitric oxide, that can kill surrounding cells.

Detailed explanation-3: -Once activated, the effector helper T cell can then activate a B cell that specifically displays the same complex of foreign peptide and class II MHC protein on its surface (see Figure 24-66).

Detailed explanation-4: -Armed helper T cells that recognize the peptide:MHC complex then deliver activating signals to the B cell. Thus, protein antigens binding to B cells both provide a specific signal to the B cell by cross-linking its antigen receptors and allow the B cell to attract antigenspecific T-cell help.