Antigens - Online Tutor, Practice Problems & Exam Prep

Antigens are molecules that elicit immune responses, interacting specifically with T cells, B cells, or antibodies. They can be categorized as free antigens, which float freely and activate B cells, or presented antigens, processed by antigen-presenting cells (APCs) like dendritic cells, which activate T cells. Epitopes are specific regions on antigens where immune receptors bind. Understanding these distinctions is crucial for grasping adaptive immunity and the roles of various immune cells in responding to pathogens.

Antigens

In this video, we're going to begin our lesson on antigens. It's important to note that adaptive immunity, which includes cell-mediated immunity and humoral immunity, will actually generate immune responses towards foreign antigens in our bodies. The term antigens refers to any molecule that reacts specifically with T cells, B cells, or antibodies. The term itself, antigens, is derived from "antibody generator." Different antigens will differ in their effectiveness to elicit an immune response; some antigens will generate an immune response, but other antigens, like very small antigens, usually are not immunogenic. By saying that they are not immunogenic, we're saying that they do not cause an immune response. Immunogenic antigens are specific types of antigens that will cause a strong immune response.

Another important term is epitopes, which is more specific than antigens. Epitopes are distinct regions of antigens that a B cell, T cell receptor, or antibodies can directly bind to. These epitopes are specific regions found on antigens. If we take a look at our image below, we can get a better understanding of these antigens. In this image, we show two different antigens: a red antigen and a purple antigen. Zooming in on the left-hand side, notice that the one on the left is labeled as antigen A, and the one on the right is labeled as antigen B. These specific regions on the antigens are referred to as epitopes. Antigen A seems to have three distinct epitopes, and antigen B also has three distinct epitopes. These epitopes are the specific regions where the B cell receptors, T cell receptors, or antibodies will directly bind to.

Looking at the image on the right-hand side, notice that we have a B cell receptor in the middle that has its BCRs, its B cell receptors. Notice that the B cell receptors are binding specifically to the epitope on the antigen. We can label these purple structures as the BCRs, B cell receptors. The term antigens broadly refers to any molecules that can react with these T cells, B cells, or antibodies. We'll be using this term a lot as we move forward throughout our course. It's important to know that it's a broad term. Epitopes are also going to be specific regions on the antigens. This concludes our brief introduction on antigens, and we'll be able to get some practice applying these concepts and learn more as we move forward. I'll see you all in our next video.

Epitopes are:

• A. A portion of an antibody molecule.
• B. A portion of antigen recognized by antibody.
• C. T cell receptors.
• D. A molecule that binds to antigens.

Free Antigens vs. Presented Antigens on APCs

In this video, we're going to distinguish between free antigens and presented antigens that are found on what are known as APC's. And so first, it's important to note that B cells only respond to what are known as free antigens. And as their name implies, free antigens are free floating in the environment. And that just means that these free antigens are antigens that are not being presented by another host cell. Now it's also important to note that T cells, on the other hand, are only going to respond to antigens that are presented. Or in other words, T cells only respond to presented antigens. And those presented antigens are going to be presented by antigen presenting cells. Now, antigen presenting cells are commonly abbreviated as APCs for short. And these antigen presenting cells or APCs are really just immune cells that will process and present antigens in order for T cell activation to occur, activating the T cell so that the T cell can carry out an immune response. Now the APCs that will be important for you to know throughout this course are going to be dendritic cells, macrophages, and B cells themselves can actually serve as antigen presenting cells. And so we'll get to talk more about these APCs as we move forward in our course as well. But for now, let's take a look at our image down below to get a better understanding of the difference between free antigens and presented antigens. And so notice on the left-hand side over here, what we're showing you is a B cell. And of course, we know that B cells have B cell receptors or BCRs. And so that's what these receptors are sticking out of the membrane of these B cells. Now notice that B cells only respond to free antigens. And once again, free antigens are free floating in the environment. And so notice this antigen right here is a free antigen because it is just floating around in the environment. And so, when the free antigen binds to the BCR, that can potentially lead to the activation of the B cell and allow the B cell to carry out its immune functions. Now on the right-hand side over here, notice that we're showing you presented antigens. And recall that T cells only respond to presented antigens, and those presented antigens will be presented by antigen presenting cells. And so notice over here in blue, we have an antigen presenting cell, an APC. And we're specifically showing you a dendritic cell here in this image. And so this antigen presenting cell can process an antigen and present that antigen on the surface using one of its molecules on the surface. And so this molecule right here is an antigen, but it is a presented antigen. It is being presented by an antigen presenting cell. And the T cell over here will only be able to recognize antigens that are being presented by antigen presenting cells. And so the T cells don't respond to free antigens. They only respond to presented antigens. And again, the B cells will only respond to free antigens, not presented antigens. And so this here concludes our brief lesson on the difference between free antigens and presented antigens on APCs. And once again, we'll be able to learn more about these concepts as we move forward in our course. And so I'll see you all in our next video.

Which of the following cell types are antigen-presenting cells (APCs).

1. Macrophages   2. Neutrophils   3. B cells    4. T cells    5. Plasma Cells

• A. 1 & 3.
• B. 4 & 5.
• C. 1, 2 & 3.
• D. 1 & 2.
• E. 3 & 4.

Many phagocytic immune cells such as macrophages and dendritic cells commonly engulf foreign or dangerous materials in the body. If these cells engulf antigens, the cells will present the antigens on their surface. Which immune cells will respond to these 'presented' antigens and trigger an immune response if necessary?

• A. B cells.
• B. Plasma cells.
• C. T cells.
• D. Monocytes.

Here’s what students ask on this topic:

What are antigens and how do they function in the immune system?

Antigens are molecules that elicit immune responses by interacting specifically with T cells, B cells, or antibodies. They are crucial for adaptive immunity, which includes cell-mediated and humoral immunity. Antigens can be proteins, polysaccharides, lipids, or nucleic acids. When an antigen enters the body, it is recognized as foreign, prompting an immune response. B cells respond to free antigens floating in the environment, while T cells respond to antigens presented by antigen-presenting cells (APCs) like dendritic cells. The specific regions on antigens where immune receptors bind are called epitopes. Understanding antigens is essential for grasping how the immune system identifies and combats pathogens.

What is the difference between free antigens and presented antigens?

Free antigens are antigens that float freely in the environment and are not presented by another host cell. B cells respond to these free antigens by binding to them with their B cell receptors (BCRs). Presented antigens, on the other hand, are processed and displayed by antigen-presenting cells (APCs) such as dendritic cells, macrophages, and B cells. T cells respond to these presented antigens by recognizing them through their T cell receptors (TCRs). This distinction is crucial for understanding how different immune cells are activated and how they contribute to the immune response.

What are epitopes and why are they important in immunology?

Epitopes are specific regions on antigens where immune receptors, such as B cell receptors (BCRs), T cell receptors (TCRs), or antibodies, bind. These distinct regions are crucial because they determine the specificity of the immune response. Each epitope can elicit a unique immune response, allowing the immune system to target specific parts of a pathogen. Understanding epitopes is essential for vaccine development and for designing targeted immunotherapies, as they help in identifying the most effective regions of an antigen to stimulate a strong immune response.

What are antigen-presenting cells (APCs) and what role do they play in the immune response?

Antigen-presenting cells (APCs) are immune cells that process and present antigens to T cells, facilitating their activation. Key APCs include dendritic cells, macrophages, and B cells. These cells capture antigens, process them, and display the antigen fragments on their surface using major histocompatibility complex (MHC) molecules. T cells recognize these presented antigens through their T cell receptors (TCRs), leading to T cell activation and subsequent immune responses. APCs are essential for initiating and regulating adaptive immunity, ensuring that T cells respond to specific pathogens effectively.

How do B cells and T cells differ in their response to antigens?

B cells and T cells differ in their response to antigens based on the form of the antigen they recognize. B cells respond to free antigens that are floating in the environment. They use their B cell receptors (BCRs) to bind directly to these free antigens, leading to B cell activation and antibody production. T cells, however, respond to antigens that are presented by antigen-presenting cells (APCs). These presented antigens are processed and displayed on the surface of APCs using major histocompatibility complex (MHC) molecules. T cells recognize these presented antigens through their T cell receptors (TCRs), leading to T cell activation and subsequent immune responses.