Adaptive Immunity
Mechanisms of Innate Immunity
Innate immunity constitutes the initial defense against pathogens and functions through several primary methods: direct elimination of microbes and the elimination of already infected cells. Direct elimination is largely achieved through phagocytosis, performed by phagocytes that attach to, ingest, and destroy pathogens. Additionally, innate responses involve the release of granules containing perforins and granzymes to destroy targets.
The process of phagocytosis involves six distinct steps: 1) Attachment of the phagocyte to the pathogen; 2) Ingestion of the pathogen; 3) Formation of a phagosome; 4) Formation of a phagolysosome via the fusion of the phagosome with a lysosome containing digestive enzymes; 5) Destruction of the pathogen and the formation of a residual body containing indigestible material; and 6) Elimination of waste materials. For infected cells, Natural Killer (NK) cells utilize a signaling balance between activating receptors and inhibitory receptors. If an activating ligand is detected on a target cell, the NK cell releases lytic granules. Perforin forms pores in the target cell membrane, allowing granzymes to enter and induce lysis.
Inflammation is a critical component of the innate response, triggered by chemical signals and the action of mast cells. This process involves the recruitment of macrophages and other phagocytic cells to the site of infection (e.g., a splinter), characterized by increased fluid and red blood cell presence in the capillaries near the damaged tissue.
Fundamental Properties of Adaptive Immunity
Adaptive immunity is activated when innate immunity fails to prevent an infection, and the two systems work in tandem to eliminate pathogens. Unlike the innate system, adaptive immunity is highly specific to the particular pathogen that induced the response. A hallmark of this system is memory, which provides long-lasting protection against reinfection. Furthermore, adaptive immunity exhibits non-reactivity against self-antigens, meaning it can distinguish between "self" and "non-self" (pathogens).
The responses of the adaptive system are carried out by specialized lymphocytes known as T cells and B cells. Adaptive immunity is categorized into two broad classes: cell-mediated immune responses and antibody responses. These responses depend on the action of B and T cells which possess specific antigen receptors. Each individual lymphocyte has only one type of receptor, and the interaction between a receptor and an antigen is compared to a "lock and key" mechanism due to its high specificity.
Lymphocytes and T-Cell Differentiation
T-cells serve as the primary mediators for cell-mediated immune responses. These cells originate in the bone marrow but migrate to the thymus for maturation. T-cells interact with Antigen Presenting Cells (APCs) through the T-cell Receptor (TCR). When a naïve T cell is activated, it undergoes differentiation and multiplication to form three specific types of cells: Helper T-cells (), which interact with APCs; Cytotoxic T-cells (), which recognize and destroy infected cells; and Memory T-cells, which possess a long life span and are responsible for immunologic memory.
Antigen-Presenting Cells (APCs) and Their Functions
Antigen Presenting Cells (APCs) are responsible for capturing antigens, processing them, and presenting fragments on their cell surface via Major Histocompatibility Complex (MHC) molecules. It is important to note that a fragment of only one type of microbe is presented on a particular MHC molecule at any given time.
There are three main types of APCs. Dendritic Cells are found in epithelial tissues (such as skin layers and underneath mucosal tissues) and secondary lymphoid tissues like the lymph nodes and spleen. They present antigens from bacteria, viruses, and soluble antigens like toxins. Macrophages are found in various tissues and present antigens from bacteria, viruses, soluble antigens, and large particulate material such as dead cells. B cells circulate in the peripheral blood and reside in secondary lymphoid tissues, primarily presenting soluble antigens and toxins.
Cell-Mediated Immunity: The Process
In cell-mediated immunity, T-cells act as mediators and requires the presence of APCs. The TCR on T cells recognizes the antigen fragment presented by an APC. Cytokines, which are chemical messengers, are secreted by cells to act on other cell types during this process. Specifically, only Dendritic cells play a major role in activating naïve helper T cells.
Helper T-cells interact with the APCs through their TCR, leading to the activation of both Helper T-cells and Cytotoxic T-cells. Once activated, Cytotoxic T-cells kill infected cells by releasing perforins and granzymes, which induce lysis of the target cell. This pathway ensures that pathogens residing within host cells are identified and destroyed.
Antibody-Mediated (Humoral) Immunity
Antibody-mediated immunity, also known as humoral immunity, involves the activation of B cells which produce antibodies that bind to free-floating antigens in body fluids. Some B cells differentiate into memory cells, providing future immunity against the same antigen. B cells can be activated through two distinct pathways: T Cell-Independent and T Cell-Dependent activation.
Antibodies are -shaped proteins consisting of two arms. They feature variable regions that form an antigen-binding site specific to a particular antigen. The binding follows a lock-and-key model. Antibodies can exist as single -shaped molecules (monomers), pairs (dimers), or clusters of five molecules (pentamers). Their primary functions include neutralization—where antibodies coat viruses or toxins to remove their threat—and the formation of immune complexes, which are clumps of antigens and antibodies that are subsequently removed from the system.
Mechanisms of B-Cell Activation
T-cell Independent activation occurs when B-cell receptors (BCRs) interact directly with an antigen. Naïve B cells have membrane-bound BCRs. When an antigen binds to the BCR of a specific B cell, that cell produces clones of itself. These clones become plasma cells that secrete antibodies specific to that antigen. This response is typically short-lived and does not result in the production of memory B cells.
T-cell Dependent activation involves a more complex six-step process. 1) The interaction between the BCR and an antigen stimulates the internalization of the antigen. 2) The internalized antigen is processed and presented on the cell surface with MHC II. 3) The presented antigen is recognized by a helper T-cell specific to that antigen. 4) The helper T-cell secretes cytokines that activate the B cell. 5) The B cell undergoes clonal proliferation to become plasma cells that secrete antibodies. 6) The B cell clones also differentiate into memory B cells, which allow for a rapid response to subsequent exposures to the same protein epitope.
Major Histocompatibility Complex (MHC) Classification
MHC molecules are essential cell surface markers that enable immune cells to signal each other. MHC Class I molecules are expressed by all nucleated cells in the body. They are responsible for presenting intracellular or endogenous antigens, such as viral proteins synthesized inside the cell, to cytotoxic T cells.
MHC Class II molecules have restricted expression and are only found on APCs, including B-cells, dendritic cells, and macrophages. These molecules present extracellular or exogenous antigens—those taken up via endocytosis or phagocytosis—to helper T cells. The type of T-cell activated is determined by the source of the antigen (intracellular vs. extracellular), rather than the type of pathogen itself. A single pathogen may activate only cells or both and cells depending on how the antigens are processed.
Synergistic Interaction Between Innate and Adaptive Immunity
Innate and adaptive immunity work together to provide a comprehensive defense. Innate immunity offers a rapid, initial response through Pattern Recognition Receptor (PRR)-mediated recognition and inflammation. APCs, particularly dendritic cells, serve as the bridge between the two systems by processing antigens for MHC presentation.
CD4+ helper T cells coordinate the overall response by releasing cytokines that support the activation of both CD8+ cytotoxic T cells and B cells. B cells recognize native antigens directly via BCRs and differentiate into antibody-secreting plasma cells and long-term memory B cells. Ultimately, the coordination of these systems ensures rapid defense, high specificity, and the establishment of long-term immune memory.