Comprehensive Study Notes on Adaptive Immunity: Cell-Mediated and Antibody-Mediated Responses
Mechanisms of Innate Immunity and Pathogen Elimination
Innate immunity serves as the initial defense system of the body, functioning to eliminate microbes directly or by destroying infected cells. Direct elimination methods include phagocytosis and the release of granule contents such as perforins and granzymes. In the process of phagocytosis, the phagocyte first attaches to the pathogen. This is followed by the ingestion of the pathogen through the extension of a pseudopodium, leading to the formation of a phagosome within the cytoplasm. The phagosome then fuses with a lysosome to form a phagolysosome, where digestive enzymes destroy the pathogen. Finally, a residual body containing indigestible material is formed, and waste materials are eliminated through the plasma membrane.
To eliminate infected cells, the innate system utilizes processes like inflammation and the signaling balance of Natural Killer (NK) cells. NK cells possess both activating and inhibitory receptors. The activating receptor binds to an activating ligand on a target cell, while the inhibitory receptor looks for an MHC-related inhibitory ligand. If the target cell is compromised, the NK cell releases lytic granules. Perforin forms pores in the target cell membrane, allowing granzymes to enter and initiate lysis. Inflammation further aids this by releasing chemical signals that recruit macrophages and mast cells to the site of injury, such as a splinter, causing fluid and phagocytic cells to migrate from capillaries to the infected tissue.
Properties of Adaptive Immunity
Adaptive immunity is activated when innate immunity fails to prevent an infection. Both systems work in tandem to eliminate pathogens. The adaptive system is characterized by high specificity to the particular pathogen that induced the response. It also features immunologic memory, which provides long-lasting protection against subsequent exposures to the same pathogen. A critical feature of adaptive immunity is its non-reactivity against self-antigens, meaning it can distinguish self-tissues from non-self pathogens. These responses are primarily carried out by specialized lymphocytes known as T cells and B cells.
There are two broad classes of adaptive immune responses: cell-mediated immune responses and antibody-mediated (humoral) responses. Both depend on the actions of B cells 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 highly specific, functioning like a lock and key. This specificity ensures that the immune response is precisely targeted toward the invading pathogen.
Cell-Mediated Immunity and T-Cell Differentiation
T cells are the primary mediators of cell-mediated immune responses. They originate in the bone marrow but migrate to the thymus for maturation. T cells interact with Antigen Presenting Cells (APCs) through their T-cell Receptor (TCR). When a naïve T cell is activated, it undergoes differentiation and multiplication to form effector T cells and memory T cells. The effector T cells include Helper T-cells (), which interact with APCs to coordinate the immune response, and Cytotoxic T-cells (), which recognize and destroy infected cells directly.
Memory T cells are a subset with a long lifespan, and they are responsible for immunologic memory. They ensure that the body can mount a faster and more vigorous response if the same pathogen is encountered again. The activation of T cells is a tightly regulated process that requires the presentation of antigen fragments on the surface of APCs, coupled with chemical messengers known as cytokines, which are secreted by cells to act on other cell types.
Antigen-Presenting Cells (APCs)
Antigen Presenting Cells (APCs) are specialized cells that capture antigens, process them, and present fragments on their cell surface bound to 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, Macrophages, and B cells. Dendritic cells are found in epithelial tissues, such as the skin layers and underneath the mucosal epithelial cell layer, as well as in secondary lymphoid tissues like the lymph nodes and spleen. They are capable of presenting antigens from bacteria, viruses, and soluble toxins, and they play a major role in activating naïve helper T cells.
Macrophages are also located in various tissues and can process bacteria, viruses, soluble antigens, and large particulate material like dead cells. B cells circulate in the peripheral blood and reside in secondary lymphoid tissues. They primarily focus on processing soluble antigens such as toxins. The specific location and type of antigen handled by these cells allow the immune system to monitor both peripheral tissues and the circulatory system for signs of infection.
Antibody-Mediated (Humoral) Immunity
Antibody-mediated immunity, or humoral immunity, involves the activation of B cells and the subsequent production of antibodies that bind to free antigens in body fluids. Activated B cells can differentiate into plasma cells, which secrete antibodies, or into memory B cells. Memory B cells provide long-term immunity by becoming active during future encounters with the same antigen. B cells can be activated through two distinct pathways: T-cell independent activation and T-cell dependent activation.
Antibodies are Y-shaped proteins composed of two heavy chains and two light chains. They contain variable (V) regions and constant (C) regions. The variable regions form the antigen-binding site, the shape of which is specific to a particular antigen. Like the TCR, the antigen and antibody bind in a lock-and-key manner. Antibodies can exist as single Y-shaped monomers, paired dimers, or pentamers, which are clusters of five Y-shaped molecules linked together. Their primary functions include neutralization, where they coat viruses and toxins to remove the threat, and the formation of immune complexes, where clumps of antigens and antibodies are formed to be removed by the system.
B-Cell Activation Pathways
T-cell independent B cell activation occurs when B-cell receptors (BCRs) on naïve B cells interact directly with an antigen. Naïve B cells have membrane-bound BCRs, and when an antigen binds to the BCR of a specific B cell, that cell produces a clone of itself. These clones become plasma cells that produce antibodies specific to that antigen. However, this response is typically short-lived and does not result in the production of memory B cells.
T-cell dependent B cell activation is a more complex process. First, the interaction between the BCR and an antigen stimulates the internalization of the antigen. Once inside the B cell, the antigen is processed and presented on the surface with MHC II molecules. Helper T cells specific to that same antigen recognize the presented fragment. These helper T cells then produce and secrete cytokines that activate the B cell. This leads to B cell proliferation into a clone group, which differentiates into both antibody-secreting plasma cells and long-lived memory B cells. Memory B cells are prepared to quickly respond to subsequent exposures of the same protein epitope.
Major Histocompatibility Complex (MHC) and Antigen Interaction
MHC molecules are essential cell surface markers that enable infected cells to signal T cells. All nucleated cells in the body express MHC I molecules. MHC Class II molecules have a restricted expression, occurring only on APCs like B-cells, dendritic cells, and macrophages. MHC Class I molecules are involved in presenting intracellular or endogenous antigens, such as viral proteins synthesized inside an infected cell, to cytotoxic T cells. These cytotoxic cells then work to destroy the infected cell.
MHC Class II molecules are responsible for presenting exogenous or extracellular antigens that are taken up by APCs through endocytosis or phagocytosis. These fragments are presented to helper T cells. The type of T cell activated depends on the source of the antigen (intracellular or extracellular), not necessarily the pathogen itself. A single pathogen can potentially activate only cells or both and T cells depending on how its antigens are processed and presented.
Integration of Innate and Adaptive Immunity
The innate and adaptive immune systems are highly integrated. Innate immunity provides the first rapid 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. helper T cells coordinate the broader immune response by releasing cytokines that support the activation of both cytotoxic T cells and B cells. Meanwhile, B cells can recognize native antigens directly via their BCRs. Together, these mechanisms ensure a comprehensive defense that includes rapid immediate action, high specificity, and the establishment of long-term immune memory against pathogens.