3rd Line of Defense: Specific Resistance (Acquired Immunity)
The third line of defense is specific resistance. This system relies on antibodies, which are produced by specific immune cells (called B cells) in response to the antigens on the surface of the invading pathogens.
Immune System: 3 Lines of Defense
When an antigen is detected by a macrophage, this causes the T cells to become activated. The activation of T cells by a specific antigen is called cell-mediated immunity. The body contains millions of different T cells, each able to respond to one specific antigen.
The T cells secrete interleukin 2, which causes the proliferation of certain cytotoxic T cells and B cells. T cells stimulate B cells to divide, forming plasma cells that are able to produce antibodies and memory B cells.
If the same antigen enters the body later, the memory B cells divide to make more plasma cells and memory cells that can protect against future attacks by the same antigen.
When the T cells activate (stimulate) the B cells to divide into plasma cells, this is called antibody-mediated immunity.
Antibodies (also called immunoglobulins) are Y-shaped proteins that circulate through the blood stream and bind to specific antigens, thereby attacking microbes. The antibodies are transported through the blood and the lymph to the pathogen invasion site.
The body contains millions of different B cells, each able to respond to one specific antigen.
Antibodies bind to an antigen, preventing its normal function or making it easier for phagocytic cells to ingest them; or, they activate a complement protein that kills the pathogen or signals other white blood cells; or they binds to the surface of macrophages to further facilitate phagocytosis.