Antigens: Videos & Practice Problems

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 different immune cells in responding to pathogens.

Antigens

Antigens play a crucial role in the adaptive immune system, which encompasses both cell-mediated immunity and humoral immunity. The term "antigen" refers to any molecule that can specifically interact with T cells, B cells, or antibodies, and is derived from the phrase "antibody generator." This highlights the function of antigens in stimulating immune responses.

Not all antigens are equally effective in provoking an immune response. Some, particularly very small antigens, are considered non-immunogenic, meaning they do not elicit an immune reaction. In contrast, immunogenic antigens are those that successfully trigger a strong immune response. The distinction between these types of antigens is essential for understanding how the immune system recognizes and responds to foreign substances.

A key concept related to antigens is the term "epitope." Epitopes are specific regions on antigens that can be recognized and bound by B cell receptors (BCRs), T cell receptors, or antibodies. Each antigen can have multiple epitopes, which are the precise sites where immune cells interact with the antigen. For example, an antigen may have three distinct epitopes, allowing for various immune responses depending on which epitopes are recognized by the immune cells.

In summary, antigens are broad categories of molecules that provoke immune responses, while epitopes are the specific sites on these antigens that are targeted by immune cells. Understanding these concepts is fundamental as we delve deeper into the mechanisms of the immune system and its responses to pathogens.

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.

Answer: (B)

Free Antigens vs. Presented Antigens on APCs

In the immune system, understanding the distinction between free antigens and presented antigens is crucial for grasping how B cells and T cells function. Free antigens are those that exist independently in the environment, not bound to any host cells. B cells are specifically designed to respond to these free-floating antigens. They possess B cell receptors (BCRs) on their surface, which allow them to bind to these antigens, leading to their activation and subsequent immune response.

On the other hand, T cells require a different mechanism for activation. They only respond to presented antigens, which are antigens that have been processed and displayed by antigen presenting cells (APCs). APCs, such as dendritic cells, macrophages, and even B cells, play a vital role in the immune response by presenting these antigens to T cells. This presentation is essential for T cell activation, enabling them to initiate a targeted immune response against pathogens.

To illustrate, when a dendritic cell (an example of an APC) processes an antigen, it presents this antigen on its surface using specific molecules. T cells can then recognize these presented antigens, which is a critical step in their activation. In summary, B cells are activated by free antigens, while T cells require the presence of antigens that are presented by APCs. This fundamental difference highlights the specialized roles of B cells and T cells in the immune system's response to infections.

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.

Answer: (A)

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.

Answer: (C)

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 can be proteins, polysaccharides, lipids, or nucleic acids. The immune system recognizes these antigens as foreign and mounts a response to neutralize or eliminate them. Antigens can be categorized into free antigens, which float freely and activate B cells, and presented antigens, which are processed by antigen-presenting cells (APCs) like dendritic cells and activate T cells. Epitopes are specific regions on antigens where immune receptors bind, making them crucial for the specificity of the immune response.

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 presented 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 function in adaptive immunity.

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 are the precise sites of interaction between antigens and the immune system.

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 present them 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 the initiation of an immune response. APCs are essential for bridging innate and adaptive immunity, ensuring a targeted and effective immune response.

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 float freely in the environment by binding to them with their B cell receptors (BCRs). This binding can lead to B cell activation and the production of antibodies. T cells, however, only respond to antigens that are presented by antigen-presenting cells (APCs). These presented antigens are processed and displayed on the surface of APCs, and T cells recognize them through their T cell receptors (TCRs), leading to T cell activation and subsequent immune responses.