The immune system is a complex network of cells, tissues, and organs that work together to defend the body against infections, remove harmful substances, and maintain overall health. It operates through two interconnected branches: the innate and adaptive immune systems, each with its own effector cells and mechanisms of action. Together, these systems defend our bodies against pathogens while maintaining tolerance to self-tissues.
Innate Immune System
The innate immune system serves as the body’s first line of defense, providing rapid, non-specific responses to invading pathogens. Its primary components include physical barriers, such as the skin and mucous membranes, and certain defense cells. The cells of the innate immune system include:
Phagocytes
These cells, including macrophages, neutrophils, and dendritic cells, function by identifying and eliminating pathogens. Phagocytes have special receptors called pattern recognition receptors (PRRs), (an example of a PRR is Toll-like receptors, or TLR). These receptors attach to pathogen-associated molecular patterns (PAMPs) on foreign substances, like a harmful bacterium. Upon activation, phagocytes engulf and degrade pathogens through phagocytosis and release pro-inflammatory cytokines to recruit additional immune cells to the site of infection.
Granulocytes
These cells are called such because they contain many granules, which are essentially packets of chemicals housed in the cells, waiting to be released when the cell is activated by a foreign substance. Types of granulocytes include neutrophils (note that neutrophils are phagocytes and granulocytes – cells can fit multiple descriptions!), eosinophils, basophils, and mast cells. These cells have PRRs just like phagocytes, and when these PRRs are activated, granulocytes release their toxic contents to destroy the foreign substance.
Natural Killer (NK) Cells
NK cells target and destroy infected or abnormal host cells, such as cancer cells or virally infected cells. They identify their targets through a balance of activating and inhibitory signals on the surface of potential target cells. For example, when a host cell is infected with a virus, it will release a certain type of cytokine called interferons to signal to NK cells that they are infected and should be destroyed. NK cells release cytotoxic granules containing perforin and granzymes, leading to apoptosis in the target cell. Despite this, NK cells are no commonly considered granulocytes.
Complement System
This group of plasma proteins enhances the immune response by promoting opsonization (coating pathogens to enhance phagocytosis), inflammation, and the formation of the membrane attack complex (MAC), which destroys pathogens by creating a hole in their cell membranes.
In summary, the main effectors of the innate immune system are physical barriers like the skin, and quick acting but nonspecific cells/proteins including phagocytes, granulocytes, NK cells, and the complement system.
Adaptive Immune System
The adaptive immune system provides a more specific and long-lasting response to pathogens. It relies on the activation and expansion of lymphocytes, which include B cells and T cells.
B-cells and Antibody-Mediated Immunity
B-cells are responsible for humoral immunity, which means immunity mediated by antibodies. Upon encountering antigens, B-cells differentiate into plasma cells (which produce antibodies specific for the antigen) or memory B-cells (which will store information on how to fight the pathogen in the case of re-infection). Antibodies, also called immunoglobulins, neutralize pathogens by binding to them, preventing their interaction with host cells. They also mediate opsonization and activate the complement system.
T-cells and Cell-Mediated Immunity
T cells are subdivided into cytotoxic T cells (CD8+) and helper T cells (CD4+). Cytotoxic T cells act in a manner very similar to NK cells. They target and kill infected or cancerous cells by recognizing specific antigens on abnormal host cells. For example, cancer cells might display certain proteins attached to Major Histocompatibility Complex 1 (MHC-1), indicating that they need to be destroyed. Cytotxic T-cells will then release perforin and granzymes to induce apoptosis of target cells.
Helper T cells recognize antigens presented on MHC class II molecules (note the association between MHC-1 and CD8+ cells, and the separate association between MHC-2 and CD4+ cells!). They will then secrete cytokines that activate other immune cells, such as B-cells, macrophages, and cytotoxic T cells. Subtypes of helper T cells (e.g., Th1, Th2, and Th17) help tailor the immune response to different types of pathogens.
Regulatory T cells (Tregs) play a critical role in maintaining immune tolerance and preventing autoimmune reactions by suppressing excessive immune activity. They do this by releasing cytokines that suppress the immune cells discussed previously. This is very a very important process that prevents our immune cells from acting on our own tissues!
The immune system's arsenal of effector cells and mechanisms provides multiple layers of defense against pathogens.
Innate immunity acts swiftly to contain infections and recruit adaptive responses, while the adaptive immune system offers specificity and long-term protection. Together, these systems maintain a delicate balance, protecting the body from external threats while preserving its own tissues.
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