Antigen-Presenting Cells (APCs) are specialized immune cells central to the body’s defense system. They capture fragments of foreign invaders, known as antigens, and display them to other immune cells. This display informs the adaptive immune system about a threat, initiating a targeted response against pathogens like viruses or bacteria. This process helps the immune system distinguish between the body’s own cells and foreign substances.
Key Types of APCs
The immune system employs several types of professional APCs, each with distinct roles in recognizing and presenting antigens. Dendritic cells are potent APCs, found in tissues that frequently encounter the external environment, such as the skin, nose, lungs, and intestines. They capture antigens and migrate to lymph nodes to activate T cells.
Macrophages are another type of APC, found throughout nearly all tissues in the body where they engulf and digest pathogens and cellular debris. While macrophages are active in innate immunity, they also present antigens to T cells, contributing to specific immune responses. B cells, primarily known for producing antibodies, also function as APCs. They capture antigens using their surface-bound receptors and present them to T cells, which supports the development of high-affinity antibodies.
The Process of Antigen Presentation
Antigen presentation begins with APCs capturing foreign or abnormal substances. Dendritic cells and macrophages commonly take up antigens through phagocytosis, engulfing entire pathogens or large particles. B cells, in contrast, often use receptor-mediated endocytosis, where antigens bind to specific receptors on their surface for internalization. After uptake, the antigens are processed within the APC.
Inside the APC, enzymes degrade the captured antigens into smaller peptide fragments. These fragments are then loaded onto specialized Major Histocompatibility Complex (MHC) proteins. Two main types exist: MHC class I molecules present peptides derived from inside the cell, such as viral proteins, to cytotoxic T cells, while MHC class II molecules display peptides from external sources, like bacteria, to helper T cells. Once loaded, these MHC-peptide complexes are transported to the APC’s cell surface for recognition by T cells.
APCs in Immune Activation
Antigen presentation by APCs activates specific T cells, orchestrating the adaptive immune response. When T cells encounter their specific antigen presented on an APC’s surface via MHC molecules, it triggers the first signal for T cell activation. The T cell receptor recognizes the antigen-MHC complex.
A second signal, provided by co-stimulatory molecules on the APC interacting with receptors on the T cell, is also needed for full T cell activation. For helper T cells, this involves molecules like B7-1 (CD80) or B7-2 (CD86) on the APC binding to CD28 on the T cell, leading to T cell proliferation. This two-signal activation ensures that T cells only respond effectively when a true threat is present, linking the initial innate immune response with the adaptive immune response.
APC Function and Health Conditions
The function of APCs is connected to various health conditions, and their dysfunction can lead to immune system imbalances. In autoimmune diseases, APCs can mistakenly present “self” antigens, leading the immune system to attack the body’s own healthy tissues. For instance, in conditions like multiple sclerosis, APCs may contribute to the activation of T cells that target components of the nervous system.
APCs also play a role in cancer immunity by recognizing and presenting tumor antigens to T cells, which can then mount an anti-cancer response. However, cancer cells can evade this process, for example, by reducing the expression of proteins that APCs recognize. Researchers are exploring therapies that use artificial APCs to enhance the immune system’s attack on malignant cells. In the context of infectious diseases, effective APC function helps the immune system identify and clear pathogens. Some pathogens, like HIV, can impair APC function, hindering the immune response and allowing the infection to persist.