Antigen processing is a biological process where the body breaks down foreign or abnormal substances, known as antigens, into smaller pieces. This mechanism is central to the immune system’s ability to recognize and respond to various threats, including viruses, bacteria, and cancerous cells. By processing antigens, the body effectively distinguishes between its own healthy components and potentially harmful invaders, ensuring a targeted immune response.
Understanding Antigen Processing
Antigen processing is necessary because pathogens and abnormal cells often keep their unique identifiers, or antigens, hidden inside cells. For the immune system to detect these hidden threats, the antigens must be processed and then displayed on the cell surface. This display function is carried out by specialized cells called antigen-presenting cells (APCs), which include macrophages, dendritic cells, and B cells. The interaction between these presented antigens and other immune cells is how the body initiates a specific and effective immune response.
How Cells Process External Threats
The body handles external threats through a pathway often referred to as the “exogenous” or “MHC Class II pathway.” Antigens originating from outside the cell, such as bacteria or viral particles, are initially taken up by APCs. This uptake commonly occurs through processes like phagocytosis, where large particles are engulfed, or endocytosis, which involves the internalization of smaller molecules.
Once inside the APC, these external antigens are directed into a series of compartments called endosomes and lysosomes. Within these acidic environments, powerful enzymes break down the antigens into smaller peptide fragments.
Simultaneously, Major Histocompatibility Complex (MHC) Class II molecules are synthesized in the endoplasmic reticulum. These MHC Class II molecules, along with an associated invariant chain, are then transported to these endosomal/lysosomal compartments. There, the invariant chain is removed, allowing the processed antigenic peptides to bind to the binding groove of the MHC Class II molecules. The stable MHC Class II-peptide complex is then transported to the cell surface.
How Cells Process Internal Threats
Internal threats are processed through a distinct mechanism known as the “endogenous” or “MHC Class I pathway.” Antigens that originate from within the cell, such as viral proteins or abnormal proteins from a cancerous cell, are handled differently. These intracellular proteins are first marked for degradation.
Subsequently, they are broken down into smaller peptides by a large protein complex in the cytoplasm called the proteasome. These newly generated peptides are then transported from the cytoplasm into the endoplasmic reticulum (ER) by specialized transporters known as Transporters associated with Antigen Processing (TAP). Within the ER, these peptides bind to newly synthesized Major Histocompatibility Complex (MHC) Class I molecules. The assembly of the MHC Class I-peptide complex is assisted by various chaperone proteins, forming a stable structure. This MHC Class I-peptide complex then exits the ER and is transported through the Golgi apparatus to the cell surface.
Presenting Antigens to the Immune System
The display of processed antigen-MHC complexes on the cell surface is a crucial final step, directly leading to the activation of the immune response. T-cells, immune cells, recognize these complexes. Specifically, MHC Class II-peptide complexes are recognized by CD4+ helper T-cells. This recognition provides a primary signal that activates helper T-cells, enabling them to orchestrate broader immune responses, including the activation of B cells and the production of antibodies.
In contrast, MHC Class I-peptide complexes are recognized by CD8+ cytotoxic T-cells. Upon recognizing these complexes, cytotoxic T-cells become activated and are responsible for directly identifying and eliminating infected or cancerous cells. This precise recognition ensures that the immune response is targeted, preventing the immune system from attacking healthy cells. Accurate processing and presentation of antigens are therefore fundamental for adaptive immunity, enabling a targeted and effective defense.