Natural Killer and Mast Cells

These are two cells that I should have mentioned in the last essay but I should also include in the next. Instead, I’ll write about them here and leave them in limbo between the two articles.

Natural Killer Cells

Natural Killer (NK) cells are involved in hunting down and killing the host’s own cells that are either infected by a virus or are cancerous. Like T-cells and B-cells which we shall discuss in specific immunity, NK cells are lymphocytes, and all three of these cells arise due to the differentiation on a cell called a lymphoblast. While many of the cues other cells previously discussed used to find pathogens are chemical, NK cells look at differences in the surface of host cells to determine which are infected or tumorous. A cell that is infected by a viral pathogen release stress molecules, called cytokines, and the NK cells will zero in on these via chemotaxis. Both cytokines and interferons trigger NK cells into activation. Tumour cells signal the NK cells in a different manner. The tumour cells produce less Class 1 MHC markers than regular cells (we’ll get to MHC when we look at Specific Immunity) and in response to these low levels of MHC markers the cell is attacked.

So how do NK cells destroy virally infected and tumorous cells, you ask? They do this chemically in a two-step process. NK cells are granular and contain several types of chemicals in these vesicles. When they come in close contact to one of these dodgy cells they first release perforin. This creates pores in the cell membrane by which the second chemical, protease, enters the cell. Protease is an enzyme that causes protein catabolism (breaks down proteins). We call these protease enzymes collectively granzymes. As opposed to most of the other cell death discussed in this blog so far they do not cause cell lysis. Lysis would spread the virus further. Instead they trigger apoptosis, cell death. The cell and all the virus particles within die.

Mast Cells

Mast cells are large cells found in connective tissue and are responsible for histamine release in response to injury and allergic reactions. They are similar to basophils in role and function. While basophils leave the bone marrow completely differentiated, mast cells leave the marrow and migrate to tissues where they mature. There are two types of mast cells, connective tissue mast cells and mucosal mast cells which I’ll discuss in specific immunity. In response to injury and chemical signals produced by and because of pathogens mast cells release chemicals, primarily histamine, which trigger the inflammatory response. Mast cells can take on the antibodies released by lymphocytes and place them on their membrane. Then when they come across an antigen act a little like a landmine.

Both mast and NK cells have roles in both specific and non-specific immunity.