Natural Killer Cells: Study with Video Lessons, Practice Problems & Examples

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

This video, we're going to begin our lesson on natural killer cells. And so first, we need to recall from some of our previous lesson videos that natural killer cells are commonly abbreviated as NKs. And these natural killer cells are lymphocytes, just like the T cells and the B cells that we've been talking about so far. However, unlike the B cells and T cells that we've talked about so far, which are part of adaptive immunity, these natural killer cells are not considered part of adaptive immunity. And this is because these natural killer cells lack specificity in antigen recognition. And so these natural killer cells are considered part of innate immunity, and they're referred to as innate lymphoid cells. Now the role of these natural killer cells is to destroy infected host cells by recognizing irregular patterns on the host cell surface. Now these natural killer cells when they do recognize these irregular patterns on infected host cells, they release these death packages that include molecules such as perforin, which creates pores in the membrane of the infected host cell, and it also releases other protease-containing granules. And those proteases are basically enzymes that destroy proteins. And so collectively, these death packages released by the natural killer cells can induce apoptosis in the infected host cell. Now, the natural killer cell can actually recognize one of two irregular patterns on the infected host cell. The first irregular pattern that it can recognize is the lack of MHC class I on the surface of the infected host cell. And so the lack of MHC class I can actually be a result of some viruses. So some viruses have evolved complex mechanisms that allow them to, basically interfere with antigen presentation. And so when it does that, these infected cells will not have MHC on their surface. And so these natural killer cells are able to respond to cells that lack MHC class I on their surface. And so notice that in this image down below we're showing you how natural killer cells can induce apoptosis in cells that lack MHC molecules. And so notice over here on the left-hand side, we're showing you a virus infecting the host cell. And this particular virus, again, has evolved mechanisms to interfere with antigen presentation. And so notice that whereas the original cell had MHC I on its surface, the infected cell no longer has MHC I. So the virus is preventing the cell from displaying MHC class I molecule. So notice it does not have those molecules on its surface anymore. Now, what this means is without the MHC, the T cells will not be able to respond. However, the natural killer cell is able to respond and essentially eliminate cells that do not have MHC on their surface. So the natural killer cell, which is this pink cell over here on the right-hand side comes along and it recognizes the lack of MHC and it will release death packages that include perforin to basically, create pores in the membrane and also protease-containing granules. And of course, the protease is going to include enzymes that break down proteins, and, that is going to lead to the apoptosis of the infected host cell. And so the infected host cell here is going to undergo apoptosis, getting rid of this infected host cell because it lacked those MHC molecules. Now the second irregular pattern that these natural killer cells can recognize on infected host cells is that these natural killer cells can respond to the Fc region or the constant region of IgG antibodies that are bound to the surface of an infected host cell. And that will lead to the processed antibody-dependent cellular cytotoxicity, or ADCC, which we got to talk a little bit about in some of our previous lesson videos. And so if we take a look at our image down below, notice that the natural killer cells can induce apoptosis via ADCC, antibody-dependent cellular cytotoxicity. And so over here on the far left-hand side, notice that we have a host cell, that is infected, and this host cell, that is infected is going to have specific proteins on its surface. And IgG antibodies can recognize and bind to those antigens on the target cell surface. And so these antibodies are somewhat marking this infected host cell. And so the natural killer cell, the NK cell can come along and recognize the Fc region of these antibodies. And the Fc region is the constant region, basically the bottom of the Y shape of the antibody. And so the natural killer cell binds the Fc region of the IgG antibodies that are bound to the target cell. And when it does that, it's going to initiate the release of death packages that includes perforin and those protease-containing granules to ultimately, induce apoptosis in the target host cell. And so ultimately what we're saying here is that these natural killer cells, they lack specificity. And so, they're part of innate immunity. However, they can work alongside adaptive immunity because again, it's recognizing the Fc region of the antibodies. And they also can kill target cells that lack MHC molecules. And so this here concludes our brief introduction to these natural killer cells, and we'll be able to get some practice applying these concepts as we move forward. So I'll see you all in our next video.

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.

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.

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.

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.