Immune Response Overview
Sunir J. Garg
BASIC CONCEPTS
• The immune response is a complex system that protects the body from harm. Ideally, the immune response recognizes pathologic material and then eliminates it with minimal collateral damage. Autoimmune diseases occur when these responses malfunction.
• The eye has no lymphatic drainage, so it protects itself in other ways.
The first way is physical protection using the skin, eyelids, eyelashes, blink reflex, tears, and tear pH.
If the physical defenses do not work, soluble inhibitors, such as immunoglobulin A (IgA) and lysozymes, can block the effects of an antigen.
If that does not work, the body can attempt to kill or neutralize the antigen through both innate and adaptive immunity.
If the body still cannot get rid of a particular pathogen, it can tolerate it through anterior chamber-associated immune deviation (ACAID).
Innate Immunity
• Innate immunity is an inborn immune response.
• Innate immunity targets a variety of common infections, foreign toxins, and damaged host “self” material.
• Although rapid, this type of reaction does not improve with subsequent exposures and has a limited repertoire.
Adaptive Immunity
• In contrast, adaptive immunity is antigen specific. When challenged by a novel organism or virus, the naive immune system recognizes the material as foreign, processes the material, and then responds with an antigen-specific immune reaction.
• On exposure to a new antigen, the naive adaptive immune system typically responds slowly, requiring several days to mount an appropriate response.
• In contrast, subsequent (secondary) exposures lead to a more potent and rapid response to the same antigen; an antigen (a protein, carbohydrate, lipid, etc.) triggers a response specific to that antigen by activating an antibody or T-cell receptor specific to that antigen.
BUILDING BLOCKS OF THE IMMUNE RESPONSE
• A variety of mechanisms work together to generate an immune response: leukocytes, antibodies, T-cell receptors, major histocompatibility complex (MHC) molecules, complement, cytokines, various enzyme systems, vasoactive amines, and lipid mediators.
• Hypersensitivity reactions also play a role.
Leukocytes
• A common stem cell gives rise to the following cells: neutrophils, monocytes/macrophages, eosinophils, lymphocytes (B cells and T cells), mast cells, basophils, megakaryocytes (which give rise to platelets), and red blood cells.
 FIGURE 1-1. A. Polymorphonuclear leukocytes “neutrophils” are the most common phagocytes and can rapidly leave the bloodstream to engulf bacteria and damaged tissue. They have a multilobed nucleus. B. Macrophages play a critical role in eradicating microbes and damaged and/or dysplastic cells, in antigen presentation, and in inflammatory regulation. They are called histiocytes when they enter tissue. In this example, the cytoplasm is filled with lens material in a patient with phacolytic glaucoma. C. Eosinophils play an important role in fighting parasites and bacteria; they also play a role in the allergic response and in asthma. These cells characteristically have multiple, red cytoplasmic granules that surround a large nucleus. D. Plasma cells are white blood cells that are antigen specific. They can rapidly produce a large quantity of antibodies. (Note the characteristic “cartwheel” appearance.) Plasma cell dysregulation can lead to diseases such as multiple myeloma and Waldenström macroglobulinemia. (Courtesy of Ralph Eagle, MD.) |
Phagocytes
Polymorphonuclear Neutrophils
• Polymorphonuclear neutrophils (PMNs or “polys”) are cells that travel in the bloodstream. When recruited toward inflamed tissues, they adhere to the blood vessel wall and exit via diapedesis through gaps between cells that make up the vessel wall.
• Once in the inflamed site, they engulf (phagocytize) and destroy the targeted antigens.
• These cells have a brief life span (Fig. 1-1).
Mononuclear Phagocytes
• These are a second group of cells that phagocytize abnormal material. Examples include monocytes in the blood and Kupffer cells in the liver. These cells live much longer than PMNs. Blood-borne monocytes migrate into tissues and become tissue macrophages (histiocytes).
• Macrophages serve three main functions:
They play a scavenging role.
They present antigens to T cells (in conjunction with their MHC molecules).
They serve an important inflammatory effector role.
Eosinophils
• Eosinophils constitute 2% to 5% of blood leukocytes.
• They are seen in skin infiltrates (late phase reaction), atopic reactions, asthma, and around parasites.
• Certain immune stimuli cause these cells to degranulate and fuse their granules to the plasma membrane.
• In theory, large pathogens, such as parasitic worms, are too large to be engulfed. Eosinophils surround them and release their granules into the extracellular space to destroy the parasite.
Auxiliary Cells
Basophils and Mast Cells
Stay updated, free articles. Join our Telegram channel
Full access? Get Clinical Tree
