Review of Adaptive Immunity: Study with Video Lessons, Practice Problems & Examples

Review of Adaptive Immunity

In this video, we're going to do a review of adaptive immunity by outlining and discussing this image that you see down below, which is an image that we've discussed briefly before in some of our previous lesson videos. And so notice that towards the top of this image in this little box at the top, we have the primary lymphoid organs, which recall includes the thymus, which is where the T cells fully develop and mature, and the bone marrow, which is where the B cells fully develop and mature. And so notice on the left we have a T cell, and on the right we have a B cell. Now notice that after the B cells and T cells have fully matured in the primary lymphoid organs, they can then migrate to the secondary lymphoid organs, and that is what this green box here represents. And so notice that initially the T cells and the B cells are going to exist in their naive states. And so here we have a naive cytotoxic T cell or Tc cell, a naive helper T cell or Th cell, and a naive B cell. And recall that the naive term just refers to the inactive forms of these T cells and B cells. And so what that means is that the naive forms need to become activated before they can generate an immune response. And the way that they become activated is by encountering their very specific antigen that they respond to. Now recall for the T cells that they need to be presented an antigen, and that's because the T cell receptors or TCRs can only recognize antigens that are presented to them on MHCs. And so an antigen presenting cell such as a dendritic cell, which we're showing you over here, so we can go ahead and label this as our dendritic cell. This dendritic cell can present antigens to the naive T cells, and therefore activate the naive T cells. And so recall that the dendritic cell is capable of taking samples from its environment and then presenting those antigens on its surface on MHCs. And the dendritic cell has both MHC class 1 and MHC class 2. Recall that the MHC class 1 is used to present to cytotoxic T cells or TC cells, and the MHC class 2 is used to present to the naive helper T cells. And so here in the center of our image, notice we're showing you our dendritic cell, presenting antigens on its surface, to the cytotoxic T cell and the helper T cell. And also recall from our previous lesson videos that, these co-stimulatory molecules that are produced by the dendritic cell are going to be important for also activating the T cells as well. And so notice that once the T cells become activated, they're capable of dividing and proliferating to create an army of identical clones. And they're also capable of differentiating into different cell types, and those different cell types include effector cells and memory cells. And so notice that the cytotoxic T cell that is activated here, not only does it divide and proliferate, it is also going to differentiate into different cell types. It could differentiate into memory cytotoxic T cells, which are going to remain in the secondary lymphoid organs and be important for generating a response to a future exposure to the same exact antigen, creating an even stronger response when we are encountering the same antigen in the future. Or the T cell could differentiate into an effector cytotoxic T cell, which is going to respond to the primary exposure to the antigen. And so these effector cytotoxic T cells, what they do is they induce apoptosis of cells that are infected with, an intracellular pathogen. And so notice that here we have an infected host cell that is infected with a virus, and so the effector T cell can come and induce apoptosis in this infected cell, and by inducing apoptosis and killing that infected host cell, it helps to limit the spread of the pathogen. Now for the helper T cells, the helper T cells have somewhat of a different function than the cytotoxic T cells. And so instead of inducing apoptosis, the helper T cells are more about activating other immune cell components. And so, when the helper T cell becomes activated by the dendritic cell presenting antigens to it, the helper T cell again can proliferate or divide to create an army of identical clones, and it can also differentiate into either memory helper T cells, which again are going to be important for generating an immune response to future exposures to the same exact antigen. Or it could differentiate into an effector helper T cell. And again, the effector helper T cell is important for activating other types of cells. Here in this image, you can see right here, this effector helper T cell is important for activating the B cell. But also the effector helper T cell can also be important for activating other immune cells such as macrophages, for example. And activating macrophages have increased killing power, increased destructive abilities to eliminate pathogens. And so notice here, we're saying that this helper T cell is capable of activating macrophages and B cells as well, by releasing cytokines, chemicals that allow for the macrophage to become activated. And also, again here the cytokines are not being shown, but the effector T cell can release cytokines to activate the B cell. And so if we turn our attention to the B cells, again, initially, the B cells are going to be in their naive form and inactive form before they have encountered the antigen. But as soon as they are, in as soon as they are exposed to their very specific antigen, their B cell receptors can bind to that antigen, internalize that antigen, process it, and then express that antigen on MHCs on its surface, specifically MHC class 2. And so the effector helper T cell can, recognize that antigen presented on the MHC class 2, and, it can then activate the naive B cell, and make it an activated B cell. And the activated B cell, of course, it's going to divide and proliferate to create an army of B cells, and it can also differentiate into plasma cells or memory B cells. Again, the memory cells are really really important for generating a response to a future exposure to the same antigen, so it creates a faster response upon future infections. And the plasma cells are all about secreting antibodies, and the antibodies can lead to a variety of different immune responses. And so notice that here we have an, a plasma cell secreting hundreds or thousands of antibodies per second in response to, an antigen. And so here we're saying that the antibodies are being released by the plasma cells, and again they can have many different immune responses, for example, tagging pathogens for removal. And so notice here we have the antibodies binding to a pathogen in order to eliminate that pathogen. And so, one of the most important components of adaptive immunity is the memory component. The fact that it is able to generate a bunch of memory cells that are going to be able to respond even better to a future infection. And another important component about the adaptive immune system is that it is very very specific. Each individual T cell and each individual B cell can only respond to one very specific antigen. And so it's important for us to have a wide, diversity of many different types of T cells and many different types of B cells so that we can respond to a wide variety of different antigens. And so this here concludes our brief review of adaptive immunity, and we'll be able to apply some of these concepts as we move forward in our course. And so I'll see you all in our next video.

Why would a person who has their tonsils removed be more susceptible to certain types of infections of the 

throat and respiratory tract?

• A. Tonsils produce high levels of lactoferrin, a strong natural antibacterial compound.
• B. Tonsils produce large amounts of interferons, natural antiviral compounds.
• C. Tonsils are secondary lymphoid organs where immune cells aggregate providing easy immune response to oral microbes.
• D. Tonsils are the location where T cells develop and mature. Without tonsils, the T cells would not be able to fully mature and the individual’s immune system functions would suffer.

How are T cell receptors similar in function to B cell receptors?

• A. Both receptors are composed of two chains, a heavy and a light chain.
• B. Both receptors bind epitopes (small unique sections of antigen molecules).
• C. Both bind structures directly on the surface of microbes.
• D. Both can be secreted from lymphocytes to bind to extracellular pathogens.

Which of the following is not typical of an immunogenic antigen?

• A. Viral protein.
• B. Foreign nucleic acid.
• C. Molecules not made by the host organism.
• D. Cytoplasmic proteins commonly found in host skin cells.

In opsonization with IgG, why would it be important that IgG react with the antigen BEFORE a phagocytic cell recognizes the antibody molecule?

• A. If the IgG is bound to the phagocyte before opsonization, it would most likely be ingested by the phagocyte before it could bind to a pathogen.
• B. Binding of IgG by phagocytes would block the antigen binding sites on the IgG molecules, preventing them from binding to the microbes.
• C. Binding of IgG by phagocytes changes the conformation of the antibody's antigen binding site making it less effective at binding the correct foreign antigen.
• D. Binding of an antibody by phagocytes results in immediate release of damaging hydrolytic enzymes to the outside of the cell which would destroy all remaining antibodies.

Here’s what students ask on this topic:

What are the primary lymphoid organs and their roles in adaptive immunity?

The primary lymphoid organs are the thymus and bone marrow. In adaptive immunity, these organs are crucial for the maturation of T cells and B cells. T cells fully develop and mature in the thymus, while B cells mature in the bone marrow. Once matured, these cells migrate to secondary lymphoid organs where they can encounter antigens and become activated. This maturation process ensures that T and B cells are prepared to recognize and respond to specific antigens, forming the basis of the adaptive immune response.

How do naive T cells become activated in adaptive immunity?

Naive T cells become activated through the presentation of specific antigens by antigen-presenting cells (APCs) like dendritic cells. The T cell receptors (TCRs) on naive T cells can only recognize antigens presented on Major Histocompatibility Complex (MHC) molecules. Dendritic cells present antigens on MHC class I to cytotoxic T cells and on MHC class II to helper T cells. Additionally, co-stimulatory molecules produced by dendritic cells are essential for full T cell activation. Once activated, T cells proliferate and differentiate into effector and memory cells, enabling a targeted immune response.

What is the role of helper T cells in adaptive immunity?

Helper T cells play a pivotal role in adaptive immunity by activating other immune cells. Upon activation by antigen-presenting cells, helper T cells proliferate and differentiate into effector and memory cells. Effector helper T cells release cytokines that activate B cells and macrophages. Activated B cells can then differentiate into plasma cells that secrete antibodies, while activated macrophages enhance their pathogen-killing abilities. This coordination amplifies the immune response, making helper T cells essential for a robust and effective adaptive immune system.

How do B cells contribute to adaptive immunity?

B cells contribute to adaptive immunity by producing antibodies. Initially, naive B cells encounter their specific antigen, bind to it via B cell receptors, internalize, process, and present it on MHC class II molecules. Effector helper T cells recognize this presentation and activate the B cells. Activated B cells proliferate and differentiate into plasma cells and memory B cells. Plasma cells secrete large quantities of antibodies that tag pathogens for elimination, while memory B cells ensure a faster and stronger response upon future exposures to the same antigen.

What is the significance of memory cells in adaptive immunity?

Memory cells are crucial in adaptive immunity because they provide long-lasting protection against previously encountered antigens. Both memory T cells and memory B cells are generated during the initial immune response. These cells remain in the body and can quickly respond to subsequent exposures to the same antigen, leading to a faster and more robust immune response. This memory component is the basis for the effectiveness of vaccines, which aim to create memory cells without causing disease.