Specific Immunity
The 3rd line of defense involves the activation of the specific or acquired immune system. Once activated, the specific immune system mounts a defense that is specific to the invader, global, and long-lasting. The specific immune response is mediated by B- and T-lymphocytes, which work together but utilize different mechanisms to destroy the microbial invader. Only B- or T-cells that have been exposed to and recognize the specific antigen will become active.
Antigen Presentation & Recognition
T-lymphocytes differentiate into either cytotoxic (killer) T-cells, helper T-cells, regulatory (or suppressor) T-cells, and memory T-cells. Helper T-cells are essential in an efficient immune response. Helper T (CD4+) cells act as the generals of the immune system. Their job is to recognize the invading microbe and coordinate an appropriate immune response by activating cytotoxic (CD8+) T cells or B cells. There are 2 types of CD4+ helper T cells: Th1 and Th2. Th1 cells activate cytotoxic (CD8+) T-cells which are involved in cell-mediated attacks on virally infected or cancerous cells. Th2 cells stimulate B cells to produce antibodies that will be used to neutralize and eliminate the invading microbe. T cells utilize T-cell receptors (TCRs) to recognize specific antigens presented by macrophages or other host cells. Host cells display self or abnormal self antigens to cytotoxic (CD8+) T cells via MHCI. Macrophages and other antigen presenting cells (APC) display foreign antigens to CD4+ T helper cells via MHCII.
A minority of T-lymphocytes will differentiate into memory T-cells which circulate in the blood for weeks, months, years, or decades. These cells respond quickly upon a 2nd exposure to an antigen that stimulated the initial immune response. The presence of memory cells explains the long duration of acquired immunity after exposure to an antigen.
Cellular Immunity
Activated by Th1 cells, cytotoxic (CD8+) T-lymphocytes attack and destroy virally infected or cancerous host cells through the release of perforins and granzymes, which are enzymes that induce cell lysis. Cytotoxic T-cells recognize infected or cancerous host cells because the infected cells display bacterial, viral or mutated proteins on their surface.
Natural killer (NK) cells are a sub-class of T-lymphocytes that lyse virally infected or cancerous host cells through the release of perforin. Unlike cytotoxic (CD8+) T cells, NK cells do NOT take orders from anyone, including the general!
