Antigen-presenting cells (APCs) are specialized immune cells that form a bridge between the body’s general defenses and its targeted, adaptive immune response. They constantly survey the body, identifying potential threats like pathogens or foreign substances. APCs are responsible for processing these threats and presenting them to other immune cells to initiate a coordinated defense.
The Primary Role of Antigen Presentation
The main job of an antigen-presenting cell is to provide a snapshot of the molecular environment to the immune system. APCs continuously patrol tissues and fluids, engulfing materials from their surroundings, including pathogens, cellular debris, and other foreign substances known as antigens. Once an APC internalizes a potential threat, the foreign material is broken down into smaller fragments called peptides.
The APC then attaches these peptides to specialized display molecules on its cell surface. This act of presentation serves as an alert, signaling to T cells that a foreign entity has been detected. This process ensures that the adaptive immune response is directed against legitimate threats that have been vetted by APCs.
Key Types of Antigen Presenting Cells
The immune system employs several types of professional APCs, each with distinct characteristics. Dendritic cells are the most potent activators of the immune response. They are positioned in tissues that serve as common entry points for pathogens, such as the skin and the linings of the nose, lungs, and gut. Their name comes from their tree-like projections, which maximize their surface area. When a dendritic cell captures an antigen, it migrates to nearby lymph nodes to present it to T cells, initiating a targeted immune reaction.
Macrophages, whose name means “large eaters,” function as the clean-up crew of the immune system. Found in nearly all tissues, their primary job is phagocytosis—the process of engulfing and digesting cellular debris, foreign substances, and pathogens. While this cleaning function is part of the innate immune response, macrophages also process and present antigens from the materials they consume. This helps activate the adaptive immune system by presenting evidence of an invasion to T cells.
B cells, or B lymphocytes, perform a unique role as APCs. Unlike dendritic cells and macrophages that engulf pathogens nonspecifically, B cells are equipped with B-cell receptors (BCRs) on their surface that are highly specific to one antigen. When a B cell encounters its matching antigen, it binds to and internalizes it. The B cell then processes the antigen and presents fragments to helper T cells, a required step for the B cell to become fully activated and produce antibodies.
The Mechanism of Antigen Display
The process of displaying antigen fragments on the cell surface is managed by proteins called the Major Histocompatibility Complex (MHC). Professional APCs primarily use MHC class II molecules to present pieces of external threats. This pathway begins when an APC engulfs a pathogen from outside the cell. The pathogen is contained within a vesicle that fuses with a lysosome containing digestive enzymes, which break the pathogen into peptide fragments.
These fragments are then loaded onto MHC class II molecules. The resulting peptide-MHC complex is transported to the cell surface, where it is displayed for recognition by CD4+ helper T cells. When a helper T cell with a matching receptor recognizes the complex, it becomes activated. This activation signals the helper T cell to orchestrate a broader immune response, including stimulating B cells and directing other immune cells.
A different pathway, involving MHC class I molecules, is used by nearly all nucleated cells to display internal antigens. This system serves as a status report, presenting peptides from proteins made inside the cell. If a cell becomes infected with a virus or turns cancerous, it will produce abnormal proteins. These proteins are broken down, and the fragments are loaded onto MHC class I molecules and sent to the cell surface, alerting CD8+ cytotoxic T cells to destroy the compromised cell.
Role in Health and Disease
The function of antigen-presenting cells is the reason vaccines are effective. Vaccines introduce a harmless version of a pathogen or one of its antigens into the body. APCs capture these harmless antigens, process them, and present them to T cells, initiating an adaptive immune response without causing actual illness. This process creates immunological memory, allowing the body to quickly fight off a future infection by the real pathogen.
When the antigen presentation system malfunctions, it can lead to significant health problems. In autoimmune diseases like type 1 diabetes or multiple sclerosis, APCs mistakenly identify the body’s own healthy cells as foreign. They present these “self-antigens” to T cells, triggering an immune attack against the body’s own tissues. This breakdown in self-tolerance can cause chronic inflammation and progressive damage to organs.
An overreaction to harmless foreign substances is the basis of allergies. In this scenario, APCs process common environmental antigens, such as pollen or certain food proteins, and present them in a way that provokes an aggressive immune response. This leads to the activation of T cells and the production of antibodies that drive the characteristic symptoms of an allergic reaction.