Natural Killer Cells: Videos & Practice Problems

Natural killer (NK) cells are innate lymphocytes crucial for immune defense, distinguishing them from T and B cells, which are part of adaptive immunity. NK cells recognize infected host cells by identifying irregular patterns, such as the absence of MHC class 1 molecules or binding to IgG antibodies. They release death packages containing perforin and proteases, inducing apoptosis in targeted cells. This mechanism allows NK cells to eliminate cells that evade adaptive immune responses, highlighting their role in both innate and adaptive immunity.

Natural Killer Cells

Natural killer (NK) cells are a type of lymphocyte that play a crucial role in the immune system, specifically within innate immunity. Unlike T cells and B cells, which are part of adaptive immunity and exhibit specificity in antigen recognition, NK cells do not have this specificity. Instead, they are classified as innate lymphoid cells and are essential for identifying and destroying infected host cells.

The primary function of NK cells is to recognize irregular patterns on the surface of infected cells. They achieve this by detecting two main irregularities: the absence of MHC class I molecules and the presence of IgG antibodies bound to the infected cell's surface. The lack of MHC class I can occur when certain viruses interfere with antigen presentation, preventing the infected cell from displaying these molecules. In such cases, NK cells can identify these cells and respond by releasing cytotoxic substances known as death packages.

These death packages contain perforin, which forms pores in the membrane of the infected cell, and protease-containing granules, which are enzymes that degrade proteins. This process ultimately leads to apoptosis, or programmed cell death, of the infected host cell. The ability of NK cells to induce apoptosis in cells lacking MHC class I is a critical mechanism for controlling viral infections.

In addition to recognizing cells without MHC class I, NK cells can also target infected cells that have IgG antibodies bound to them. This process is known as antibody-dependent cellular cytotoxicity (ADCC). In this scenario, NK cells bind to the Fc region of the IgG antibodies, which marks the infected cell for destruction. Upon binding, NK cells release their death packages, leading to the apoptosis of the antibody-coated target cell.

In summary, natural killer cells are vital components of the immune response, bridging innate and adaptive immunity. They effectively eliminate infected cells through mechanisms that do not rely on specific antigen recognition, making them a first line of defense against infections.

What is the main difference between natural killer cells & T cytotoxic cells?

• A. Natural killer cells do not need to be activated to target & kill infected host cells, but T cytotoxic cells do.
• B. T cytotoxic cells do not need to be activated to target & kill infected host cells, but natural killer cells do.
• C. T cytotoxic cells bind to antibodies on the surface of host cells & natural killer cells do not.
• D. Natural killer cells are a part of the adaptive immune response & T cytotoxic cells are innate lymphocytes.

Answer: (A)

Certain viruses have evolved to be able to avoid detection by cytotoxic T cells. These viruses remove the MHC class 1 molecules from the surface of the cell that they are infecting. This ensures that the cytotoxic T cells do not bind to the infected cell and kill the cell and the virus within. Which immune cell is specifically designed to recognize these types of viral threats?

• A. CD4 effector cells.
• B. T helper cells.
• C. Regulatory T cells.
• D. Natural killer cells.
• E. CD8 effector cells.

Answer: (D)

How are natural killer cells able to kill infected host cells?

• A. NK cells bind to IgG antibodies on the infected cell’s surface and send apoptosis signals to the infected cell.
• B. NK cells send perforin and protease granules to the infected cell triggering apoptosis of the infected cell.
• C. NK cells bind to antigens on the infected cell’s surface and send signals triggering apoptosis of the infected cell.
• D. NK cells mark infected cells for destruction and the infected cell will later be destroyed by cytotoxic T cells.
• E. A and B.
• F. C and D.
• G. D only.
• H. All of the above.

Answer: (E)

Here’s what students ask on this topic:

What are natural killer (NK) cells and how do they function in the immune system?

Natural killer (NK) cells are a type of lymphocyte that play a crucial role in the innate immune system. Unlike T and B cells, which are part of adaptive immunity, NK cells lack specificity in antigen recognition. They identify infected host cells by recognizing irregular patterns, such as the absence of MHC class 1 molecules or the presence of IgG antibodies. Upon recognition, NK cells release death packages containing perforin and proteases, which induce apoptosis in the targeted cells. This mechanism allows NK cells to eliminate cells that evade adaptive immune responses, highlighting their role in both innate and adaptive immunity.

How do natural killer cells recognize infected host cells?

Natural killer (NK) cells recognize infected host cells by identifying irregular patterns on their surfaces. One key pattern is the absence of MHC class 1 molecules, which some viruses can cause by interfering with antigen presentation. Another pattern is the presence of IgG antibodies bound to the surface of infected cells. NK cells have receptors that can bind to the Fc region of these antibodies. Upon recognizing these patterns, NK cells release death packages containing perforin and proteases, leading to the apoptosis of the infected host cells.

What is the role of perforin and proteases in the function of natural killer cells?

Perforin and proteases are crucial components of the death packages released by natural killer (NK) cells. Perforin creates pores in the membrane of the targeted infected host cell, allowing the entry of proteases. These proteases, which are enzymes that break down proteins, then enter the cell and induce apoptosis. This process effectively eliminates the infected host cell, preventing the spread of infection and contributing to the immune defense.

What is the difference between innate and adaptive immunity, and where do natural killer cells fit in?

Innate immunity is the body's first line of defense and responds to pathogens in a non-specific manner. It includes physical barriers, phagocytic cells, and natural killer (NK) cells. Adaptive immunity, on the other hand, is specific and involves T and B cells that recognize and remember specific antigens. NK cells are part of innate immunity because they lack specificity in antigen recognition. However, they can interact with adaptive immunity by recognizing the Fc region of IgG antibodies, allowing them to target cells marked by the adaptive immune system.

What is antibody-dependent cellular cytotoxicity (ADCC) and how do natural killer cells participate in it?

Antibody-dependent cellular cytotoxicity (ADCC) is a mechanism through which natural killer (NK) cells target and destroy infected host cells. In ADCC, IgG antibodies bind to antigens on the surface of the infected cell. NK cells recognize the Fc region of these bound antibodies through their receptors. Upon binding, NK cells release death packages containing perforin and proteases, which induce apoptosis in the target cell. This process allows NK cells to work alongside the adaptive immune system to eliminate infected cells.