What are Null Cells?

• In the world of immunology, there exists a fascinating group of cells known as “null cells.” These lymphocytes possess cytotoxic properties, and while they are not classified as B cells or T cells, they play a crucial role in our immune defense.

What are Null Cells?

• Null cells are a unique group of lymphocytes that possess cytotoxic properties.
• Unlike B cells and T cells, null cells are considered intermediate between the two and form the third population of cells in the immune system, known as the third population cells (TPC).
• They constitute less than 3% of the total lymphocyte population.

Types of Null Cells

Natural Killer Cells (NK Cells)

• NK cells are a remarkable subset of null cells, making up an essential part of the lymphocyte population.
• These cells are non-phagocytic and distinguish themselves through large granules, earning them the name “large granular lymphocytes.”
• One of the most intriguing aspects of NK cells is their ability to exhibit “natural cytotoxicity.”
• They can identify and destroy a wide range of tumor cells and virus-infected cells without the need for antigen stimulation.
• Originating from lymphoid progenitors alongside B and T lymphocytes, NK cells undergo differentiation and maturation in various organs, including the bone marrow, lymph nodes, spleen, tonsils, and thymus, before entering the circulation. They serve as key components of both innate and adaptive immunity.

Functions of NK Cells

1. Apoptosis – Killing of Virus Hidden inside the Cell: NK cells play a crucial role in targeting and killing viruses that hide within host cells.
2. Lysis of Virus-Infected Cells: When viruses infect our cells, they can release cytokines, which activate NK cells. In turn, activated NK cells secrete gamma interferons (IFN) that activate macrophages, leading to the phagocytosis and lysis of virus-infected cells.
3. Lysis of Tumor Cells: NK cells have the unique ability to recognize and eliminate tumor cells without the assistance of other cells.
4. Activation and Cytokine Secretion: Upon activation, NK cells release certain chemical substances, such as perforin or cytolysin, which lyses virus-infected cells, resulting in their destruction.
5. Recognition of Low MHC Cells and Tumor Cells: NK cells can identify and kill body cells with low levels of class I MHC molecules, as well as malignant tumor cells.

Antibody-Dependent Killer Cells (K Cells)

• Killer cells form another subgroup of null cells and are antibody dependent.
• These cells possess Fc receptors, allowing them to bind with IgG antibodies.
• Through this mechanism, killer cells can bind to cells coated with IgG antibodies and eliminate them, a process termed as antibody-dependent cell-mediated cytotoxic reaction (ADCC).

Mechanism of Action of Killer Cells

• When killer cells encounter a target cell, they can combine with specific antibodies that are complexed with antigens on the target cell’s surface.
• This interaction activates the killer cells, leading to the destruction of the target cell.

The Versatility of Killer Cells

• Killer cells demonstrate an impressive range of cytotoxic properties.
• They can target and destroy not only tumor cells but also bacteria, viruses, fungi, and parasites.
• In addition, killer cells play a crucial role in killing cancer cells and foreign graft tissue, ensuring the body’s defense against foreign invaders and abnormal cells.

The Role of Killer Cells in Disease Control

• Killer cells’ ability to recognize and eliminate altered body cells is vital for controlling various diseases.
• By detecting and neutralizing infected and malignant cells, killer cells contribute significantly to maintaining overall health and preventing the spread of diseases.

Conclusion

• Null cells, especially the Natural Killer Cells and Killer Cells, represent a fascinating aspect of the immune system. These lymphocytes with cytotoxic properties play a crucial role in our body’s defense against infections, viruses, and even cancerous cells. Understanding the functions and mechanisms of these cells sheds light on the complexity and effectiveness of our immune system.

FAQs

1. Q: Are null cells part of the innate or adaptive immune system?

• A: Null cells, including Natural Killer Cells and Killer Cells, are components of both innate and adaptive immunity.

2. Q: Can NK cells kill virus-infected cells without prior antigen stimulation?

• A: Yes, NK cells can kill a range of virus-infected cells without the need for antigen stimulation, exhibiting “natural cytotoxicity.”

3. Q: What activates NK cells to release chemical substances for cell destruction?

• A: NK cells become activated by cytokines secreted by virus-infected cells, leading to the release of chemical substances like perforin.

4. Q: Can killer cells target and destroy tumor cells?

• A: Yes, killer cells have the ability to recognize and eliminate tumor cells, contributing to the body’s defense against cancer.

5. Q: Do killer cells play a role in organ transplant rejections?

• A: Yes, killer cells can target and eliminate foreign graft tissue, contributing to organ transplant rejections.