MHCs PDF

. Major Histocompatibility Complex (MHC): The Key to Antigen Presentation MHC molecules are like cellular "billboards" that display fragments of proteins (antigens) to immune cells. There are two main types of MHC molecules: MHC Class I:  Found on all nucleated cells in the body.  Pu...

. Major Histocompatibility Complex (MHC): The Key to Antigen Presentation MHC molecules are like cellular "billboards" that display fragments of proteins (antigens) to immune cells. There are two main types of MHC molecules: MHC Class I:  Found on all nucleated cells in the body.  Purpose: Displays protein fragments (peptides) from inside the cell, including fragments from viruses if the cell is infected.  Function: o If a cell is infected with a virus or has become cancerous, it will display abnormal or foreign peptides via MHC Class I. o This "red flag" signals Cytotoxic T-cells (CD8+ T-cells), which then destroy the infected or abnormal cell. Analogy: MHC Class I is like a security guard checking IDs at the door. If the ID looks suspicious (a foreign peptide), the guard (T-cell) takes action to remove the threat. MHC Class II:  Found only on professional antigen-presenting cells (APCs), like macrophages, dendritic cells, and B-cells.  Purpose: Displays peptides from pathogens engulfed and digested by the APC.  Function: o MHC Class II presents these peptides to Helper T-cells (CD4+ T-cells). o Helper T-cells then coordinate the immune response by activating other immune cells, such as B-cells to produce antibodies or cytotoxic T-cells to kill infected cells. Analogy: MHC Class II is like a teacher sharing important information with students (Helper T-cells) so they can respond e ectively. 2. Cellular Defense Mechanisms Now that we understand the role of MHC molecules, let’s look at how immune cells work together to defend the body. A. Antigen-Presenting Cells (APCs) These cells "show" antigens to T-cells to initiate a response. The key APCs are: 1. Dendritic Cells: The most potent APCs; they patrol tissues and present antigens to naïve T-cells in lymph nodes. 2. Macrophages: Engulf and digest pathogens, then present antigens via MHC Class II. 3. B-Cells: Present antigens to Helper T-cells and get activated to produce antibodies. B. T-Cells T-cells are specialized lymphocytes that recognize antigens presented by MHC molecules. 1. Helper T-Cells (CD4+ T-Cells): o Activated by antigens presented on MHC Class II molecules. o Once activated, they:  Release cytokines (chemical signals) to recruit and activate other immune cells.  Stimulate B-cells to produce antibodies.  Help Cytotoxic T-cells function more e ectively. 2. Cytotoxic T-Cells (CD8+ T-Cells): o Activated by antigens presented on MHC Class I molecules. o They kill infected or cancerous cells by:  Releasing perforin, which punches holes in the target cell’s membrane.  Releasing granzymes, which enter the target cell and trigger apoptosis (programmed cell death). 3. Regulatory T-Cells (Tregs): o Suppress overactive immune responses to prevent damage to the body’s own tissues. C. Natural Killer (NK) Cells  Unlike T-cells, NK cells do not rely on MHC for activation.   They target cells that have downregulated MHC Class I to evade detection (a tactic often used by viruses and cancer cells).  NK cells kill by releasing perforin and granzymes, similar to cytotoxic T-cells. 3. Cytokines: The Immune System’s Communication Network Cytokines are proteins that act as messengers between immune cells, ensuring a coordinated defense. Key cytokines include:  Interleukins (ILs): Stimulate growth and activation of immune cells.  Interferons (IFNs): Released by infected cells to warn nearby cells and inhibit viral replication.  Tumor Necrosis Factor (TNF): Induces inflammation and can help destroy tumor cells. 4. Complement System: Assisting Cellular Defenses The complement system is a group of proteins in the blood that:  Tag pathogens for destruction (opsonization).  Form a Membrane Attack Complex (MAC) that punctures the pathogen’s membrane, killing it.  Enhance inflammation to attract more immune cells to the site of infection. 5. How Does the Immune Response Work Together? Here’s how it all comes together: 1. Detection: o Dendritic cells or macrophages engulf a pathogen and break it down into antigens. o These antigens are displayed on MHC Class II molecules. 2. Activation: o Helper T-cells recognize the antigens via their T-cell receptor (TCR) and become activated. o Simultaneously, infected cells display antigens on MHC Class I to alert Cytotoxic T-cells.  3. Response: o Helper T-cells release cytokines to recruit and activate B-cells, Cytotoxic T-cells, and macrophages. o B-cells produce antibodies, while Cytotoxic T-cells and NK cells kill infected or abnormal cells. 4. Memory Formation: o After the infection is cleared, memory T-cells and B-cells remain in the body, ready to respond faster if the same pathogen appears again. 6. Importance of MHC in Transplants  MHC molecules are also known as Human Leukocyte Antigens (HLAs) in humans.  In organ transplantation, mismatched MHC molecules can trigger rejection because the immune system recognizes the transplanted tissue as "non-self." Summary The cellular defense mechanisms of the immune system are like a well-coordinated military operation:  MHC molecules display antigens to alert T-cells.  APCs, T-cells, and NK cells work together to eliminate threats.  Cytokines and the complement system ensure communication and reinforcement. This intricate system keeps you safe from infections while balancing the need to avoid attacking your own cells.