Difference Between NK Cells and NKT Cells

Main Difference – NK Cells vs NKT Cells

Natural killer (NK) cells and natural killer T (NKT) cells are two types of important cells in innate immunity. Innate immunity triggers a non-specific immune response against infectious agents prior to the activation of specific, adaptive immune responses. Neutrophils, macrophages, mast cells, and dendritic cells are the other cells involved in innate immunity. Both NK and NKT cells are cytotoxic cells, which induce cell death of pathogenic cells as well as tumor cells. The main difference between NK cells and NKT cells is that NK cells are large granular lymphocytes while NKT cells are a type of T cells.

Key Areas Covered

1. What are NK Cells
      – Definition, Type of Immunity, Role
2. What are NKT Cells
      – Definition, Type of Immunity, Role
3. What are the Similarities Between NK Cells and NKT Cells
      – Outline of Common Features
4. What is the Difference Between NK Cells and NKT Cells
      – Comparison of Key Differences

Key Terms: Adaptive Immunity, Innate Immunity, Lymphocytes, Natural Killer (NK) Cells, Natural Killer T (NKT) Cells, T Cells

What are NK Cells

NK cells refer to a type of smaller killer cells that destroy infected cells and tumor cells without prior sensitization. T cells and B cells are the other two types of cells related to the NK cells. But, these two cell types trigger immune responses when they are sensitized. NK cells trigger innate immune responses which are nonspecific to the pathogen. Ideally, NK cells protect the body against pathogens such as bacteria, virus, and tumor cells. Thereby, NK cells are considered as the first group of cells that destroy tumor cells. The action of NK cells on tumor cells is shown in figure 1.

Difference Between NK Cells and NKT Cells

Figure 1: Action of NK Cells on Tumor Cells

NK cells also secrete cytokines such as TNFa and INFg to initiate adaptive immune responses. Self-antigens of the body are recognized by the inhibitory receptors of the NK cells, preventing autoimmune responses.

What are NKT Cells

NKT cells refer to a type of T cells that destroy infected cells and tumor cells without prior sensitization. The development of NKT cells occurs in the thymus. Same as T cells, the NKT cells possess T cell receptors (TCR). Most NKT cells express a semi-invariant TCR. Therefore, they are called type I NKT cells. The semi-invariant TCR reacts with the CD1d, which is an MHC class 1 related protein. Following the stimulation of TCR cells, NKT cells rapidly produce cytokines. The CD1d protein structure is shown in figure 2.

Figure 2: CD1d

NKT cells provide protection against a wide variety of pathogens such as bacteria, virus, parasites, and protozoa. Some microorganisms possess lipid or glycolipid antigens which directly stimulate NKT cells.

Similarities Between NK Cells and NKT Cells

Both NK cells and NKT cells are two types of cytotoxic cells, which trigger innate immune responses.
Both NK cells and NKT cells consist of a lymphoid origin.
Both NK cells and NKT cells provide the first level of defense against infected cells and tumor cells.
Both NK cells and NKT cells produce cytokines.
Both NK cells and NKT cells trigger immune responses without a prior sensitization by the immune system.
Both NK cells and NKT cells enhance the antigen presentation by cytotoxic T cells.

Definition

NK Cells: NK cells refer to a type of smaller killer cells that destroy infected cells and tumor cells without prior sensitization.

NKT Cells: NKT cells refer to a type of T cells that destroy infected cells and tumor cells without prior sensitization.

Cell Type

NK Cells: NK cells are large granular lymphocytes.

NKT Cells: NKT cells are a type of T cells.

Maturation

NK Cells: The maturation of NK cells occurs in the circulation.

NKT Cells: The maturation of NKT cells occurs in the thymus.

Receptors

NK Cells: NK cells possess Fc receptors and inhibitory receptors.

NKT Cells: NKT cells possess TCR.

Granulocytes

NK Cells: NK cells contain cytoplasmic granules.

NKT Cells: NKT cells lack cytoplasmic granules.

Conclusion

NK cells and NKT cells are two types of immune cells that possess a cytotoxic function on pathogen-infected cells and tumor cells. NK cells are large granulocytes whereas NKT cells are a type of T cells. This is the main difference between NK cells and NKT cells.

Reference:

1. “Natural Killer Cells.” British Society for Immunology, Available here.
2. Wu, Lan, and Luc Van Kaer. “Natural killer T cells in health and disease.” Frontiers in bioscience (Scholar edition), U.S. National Library of Medicine, 1 Jan. 2011, Available here.

Image Courtesy:

1. “Antibody-dependent Cellular Cytotoxicity” By Satchmo2000 – Own work (CC BY-SA 3.0) via Commons Wikimedia
2. “Protein CD1D PDB 1zt4” By Emw – Own work (CC BY-SA 3.0) via Commons Wikimedia