Major Histocompatibility Complex Molecules: Videos & Practice Problems

Major histocompatibility complexes (MHCs) are crucial surface proteins on host cells that present antigens to T cells, enabling the immune system to distinguish between infected and healthy cells. There are two classes: MHC class I, found on all nucleated cells, presents endogenous antigens to cytotoxic T cells, while MHC class II, present only on antigen-presenting cells (APCs) like dendritic cells and macrophages, presents exogenous antigens to helper T cells. This interaction is vital for generating appropriate immune responses, highlighting the importance of MHCs in adaptive immunity.

Major Histocompatibility Complex Molecules

The immune system plays a crucial role in identifying and responding to threats, such as infected cells and pathogens, while avoiding harm to healthy cells. A key component in this process is the interaction between T cells and major histocompatibility complexes (MHCs). MHCs are surface proteins found on host cells that present antigens—substances that can provoke an immune response—to T cells. This interaction is essential for T cells to recognize and respond appropriately to threats.

For instance, macrophages, a type of immune cell, display MHCs on their surface. These MHCs present antigens to T cells, specifically helper T cells (also known as CD4 cells). The T cell receptor (TCR) on the helper T cell binds to the MHC-antigen complex, enabling the T cell to initiate a suitable immune response. This process is vital for distinguishing between harmful and harmless cells, ensuring that the immune system targets only those that pose a threat.

There are two major types of MHCs, which will be explored further in subsequent discussions. Understanding the function and types of MHCs is fundamental to grasping how the immune system operates and how it can be harnessed in medical applications, such as immunotherapy and vaccine development.

Classes of MHC Molecules

The immune system utilizes Major Histocompatibility Complex (MHC) molecules to present antigens, which are crucial for the activation of T cells. There are two primary classes of MHC molecules: MHC class I and MHC class II, each serving distinct roles in immune response.

MHC class I molecules are present on all nucleated cells and are responsible for presenting endogenous antigens, which originate from within the cell. These antigens can include viral proteins produced by an infected cell. The recognition of these antigens is specific to cytotoxic T cells (TC cells), which express CD8 markers. When a TC cell identifies an antigen presented by MHC class I, it can initiate an immune response to eliminate the infected cell.

In contrast, MHC class II molecules are found only on specialized antigen-presenting cells (APCs), such as dendritic cells, macrophages, and B cells. These molecules present exogenous antigens, which are derived from outside the cell, such as pathogens that have been engulfed and processed by the APCs. Helper T cells (TH cells), marked by CD4, recognize these antigens. Upon recognition, TH cells can activate other immune cells, orchestrating a broader immune response.

It is important to note that while APCs possess both MHC class I and class II molecules, the latter is exclusive to them. This distinction is crucial for the immune system's ability to differentiate between self and non-self antigens, ensuring appropriate responses to infections while maintaining tolerance to the body’s own cells.

In summary, MHC class I molecules present endogenous antigens to cytotoxic T cells, while MHC class II molecules present exogenous antigens to helper T cells. This division of labor is essential for a coordinated immune response, allowing the body to effectively combat infections and maintain homeostasis.

Which markers are found on all nucleated cells?

• A. MHC class I.
• B. MHC class II.
• C. CD4.
• D. CD8.

Answer: (A)

MHC class II molecules are found on:

• A. Basophils & macrophages.
• B. B cells & Neutrophils.
• C. Macrophages & Dendritic cells.
• D. T helper cells & Macrophages.
• E. All antigen presenting cells (APCs).

Answer: (C)

Once an MHC II molecule on an APC presents an antigen:

• A. Effector cells with CD8 markers activate the APC.
• B. Effector cells with CD4 kill the APC.
• C. Effector cells with CD8 kill the APC.
• D. Effector cells with CD4 activate the APC.

Answer: (D)

If an effector CD4 cell encounters an antigen presented on a MHC Class II molecule, how would it respond?

• A. Send signals to the APC trigger apoptosis in the APC.  
• B. Engulf the APC and degrade the infected cell and the intracellular pathogen.
• C. Send signals to the APC, activating the APC and triggering an immune response.
• D. All of the following are possible responses of the effector CD4 cell.

Answer: (C)

Here’s what students ask on this topic:

What are major histocompatibility complex (MHC) molecules?

Major histocompatibility complex (MHC) molecules are surface proteins found on host cells that play a crucial role in the immune system. They present antigens to T cells, enabling the immune system to distinguish between infected and healthy cells. There are two main classes of MHC molecules: MHC class I and MHC class II. MHC class I molecules are found on all nucleated cells and present endogenous antigens to cytotoxic T cells. MHC class II molecules are found only on antigen-presenting cells (APCs) like dendritic cells and macrophages and present exogenous antigens to helper T cells. This interaction is vital for generating appropriate immune responses, highlighting the importance of MHCs in adaptive immunity.

What is the difference between MHC class I and MHC class II molecules?

MHC class I and MHC class II molecules differ in their distribution, the type of antigens they present, and the T cells they interact with. MHC class I molecules are found on all nucleated cells and present endogenous antigens, which originate from inside the cell, to cytotoxic T cells (CD8+). In contrast, MHC class II molecules are found only on antigen-presenting cells (APCs) such as dendritic cells, macrophages, and B cells. They present exogenous antigens, which originate from outside the cell, to helper T cells (CD4+). This distinction is crucial for the immune system to mount appropriate responses to different types of pathogens.

How do MHC molecules present antigens to T cells?

MHC molecules present antigens to T cells by displaying peptide fragments on the surface of host cells. MHC class I molecules present endogenous antigens, derived from proteins within the cell, to cytotoxic T cells (CD8+). These antigens are typically from intracellular pathogens like viruses. MHC class II molecules present exogenous antigens, derived from extracellular sources, to helper T cells (CD4+). These antigens are usually from extracellular pathogens like bacteria. The T cell receptors (TCRs) on T cells recognize these antigen-MHC complexes, leading to the activation of the T cells and the initiation of an immune response.

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

Antigen-presenting cells (APCs) play a crucial role in the immune response by capturing, processing, and presenting antigens to T cells. APCs include dendritic cells, macrophages, and B cells. They express MHC class II molecules, which present exogenous antigens to helper T cells (CD4+). This interaction is essential for the activation of helper T cells, which then coordinate the immune response by activating other immune cells, such as B cells and cytotoxic T cells. APCs also express MHC class I molecules, allowing them to present endogenous antigens to cytotoxic T cells (CD8+), further contributing to the immune defense.

Why are MHC molecules important for adaptive immunity?

MHC molecules are essential for adaptive immunity because they enable the immune system to recognize and respond to specific pathogens. By presenting antigens to T cells, MHC molecules help the immune system distinguish between infected and healthy cells. MHC class I molecules present endogenous antigens to cytotoxic T cells (CD8+), which then target and destroy infected cells. MHC class II molecules present exogenous antigens to helper T cells (CD4+), which activate other immune cells to mount a coordinated response. This antigen presentation is crucial for the specificity and memory of the adaptive immune response, allowing the body to effectively combat infections and remember past encounters with pathogens.