- Null cells are lymphocytes with cytotoxic properties.
- They are neither B-cells not T cell. They are intermediate between T and B cells.
- They form less than 3%.
- There are two types of Null cells namely, Natural Killer Cells (NK cells) and Killer Cells.
Natural Killer Cells:
- NK cells are a group of null cells.
- They form the third population of lymphocytes.
- The NK cells have 2 or 3 large granules in the cytoplasm. Hence they are also Large Granular Lymphocytes (LGL).
- They have a kidney shaped nucleus.
- The NK cells kill the target cells without the aid of antibody or complement. So they are antibody independent.
- They destroy the cancer cells and cells infected with herpes and mumps virus.
- They are activated by Interferons and Interleukins-2.
Killer Cells (K cells):
- Killer cells are antibody dependent.
- These cells possess Fc receptors for binding with IgG antibodies. Hence they can bind with cells coated with IgG antibodies and can kill them.
- These cells can combine with specific antibody with it is in complex with antigen. If the complex is on the surface of a target cell, these lymphocytes become activated destroying the target cell.
- This cell can kill a variety of cells such as tumor cells, bacteria, viruses, fungi and parasites.