Natural Killer Cells
NK cells possess nearly all of the features of adaptive immunity, including memory. ~ American immunologists Joseph Sun & Lewis Lanier
Viruses must hijack cells to reproduce. As part of the adaptive immune system, killer T cells recognize viruses by their memorized patterns. Natural killer (NK) cells use a more basic technique to determine a foreign invader: what isn’t displayed.
Natural killer cells are lymphocytes that recognize glycoprotein structures (MHC) on the surface of virally infected cells or tumor cells, such as cancer. These recognizable structures occur because the inhibitory receptors that prevent killer cell sanction are lost when a virus disturbs normal operation of nucleated cells.
All cells have protein-processed marker molecules called major histocompatibility complex (MHC). Class 1 MHC (MHC-1) molecules display on the surface of all body cells except mature red blood cells.
When a host cell has been virally infected, class 1 MHC molecules are modified to include viral peptides, which are a recognizable protein pattern to the acquired immunity detectives that know the distinctive mark of a virus.
To evade the wiles of acquired immunity, some viruses have learned to inhibit expression of MHC-1 on the host cell surface. This keeps telltale virus peptides from showing up on the surface. That maneuver blinds killer T cells from knowing what is going on inside a virally hijacked host cell. In the instance of viruses that take up long-term residence inside a cell, such as HIV, the result of successful concealment can be catastrophic.
NK cells are ready to kill any cell and are stopped only by one sign: an MHC-1 ID identifier. Any cell with that badge gets past a natural killer cell.
Any cell without MHC-1 display, such as tumor or virally infected cells that suppress MHC-1 expression, have a death mark by omission. As all other immune responses rely on a sign, rather than an absence of one, that is a rather ingenious mechanism.
To make the kill, NK cells attach and activate a polarization of granules that causes the target cell to spill vital contents into the space between the target and the NK cell, causing the death of the infested cell.
Natural killer cells not only lyse virally infected and tumor cells, they also play on the team fighting inflammation, as well as stimulating other responses, and modulating acquired immune response.
While NK cells do not have antigen-specific receptors, they can recognize virally infected cells that have their surfaces slathered with antibodies. NK cells recognize a specific molecular portion of an antibody.
It was long thought that only the lymphocytes of the acquired immune system retained memory of previous infections; that the innate immune system lacked the intelligence for adaptive response. Not so. Despite seemingly lacking the traditional receptors for memory, NK cells remember viral infections and more effectively respond to subsequent attacks.