Immune system

• 1. Introduction

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  The immune system can tell the difference between a wood splinter and the living tissue around it. That single act of recognition sits at the heart of how an organism defends itself against viruses, bacteria, parasites, cancer cells, and foreign objects. Nearly every living thing has some version of this defense. Even bacteria carry enzymes that protect them against viral attack. In jawed vertebrates, including humans, the machinery becomes far more sophisticated, capable of learning a threat and remembering it for a lifetime. How does a body decide what belongs to it and what does not? Why does the same system that keeps us alive sometimes turn against us, attacking healthy tissue or failing when we need it most? And how did a defense this intricate evolve from something as simple as a bacterium swallowing a particle for food? The answers run from the plague of Athens in 430 BC to vaccines, transplant medicine, and the study of cancer.

• 2. Layered Defense

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  Physical barriers come first. The waxy cuticle of most leaves, the exoskeleton of insects, the shells of externally deposited eggs, and human skin all stand as mechanical walls against infection. No organism can seal itself off completely, so the body openings get their own guards. Coughing and sneezing eject pathogens from the respiratory tract. Tears and urine flush invaders away. Mucus in the respiratory and gastrointestinal tract traps and entangles microorganisms. Chemical defenses follow close behind. Skin and the respiratory tract secrete antimicrobial peptides called beta-defensins. Lysozyme and phospholipase A2 in saliva, tears, and breast milk attack bacteria directly. After menarche, vaginal secretions turn slightly acidic, while semen carries defensins and zinc to kill pathogens. In the stomach, gastric acid handles whatever is swallowed. Living things defend us too. Within the genitourinary and gastrointestinal tracts, commensal flora act as biological barriers, competing with pathogenic bacteria for food and space. Sometimes they change the local pH or available iron. The effect is to lower the odds that any pathogen reaches the numbers it needs to cause illness. When these outer layers fail, the response inside divides into two systems with very different rules: one immediate and generic, the other slow but tailored, and able to remember.

• 3. The Innate Response

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  Pattern recognition receptors are the trigger. These proteins, expressed mainly by dendritic cells, macrophages, monocytes, neutrophils, and epithelial cells, detect molecular structures shared across broad groups of microbes. They flag two classes of signal: pathogen-associated molecular patterns, tied to the invaders themselves, and damage-associated molecular patterns, released when host cells are injured or dying. Toll-like receptors handle threats outside the cell or in its compartments, and they were first discovered in the fruit fly Drosophila. Ten of them have been described in humans. Neutrophils make up 50% to 60% of circulating white blood cells, the most abundant phagocyte in the body. During the acute phase of inflammation they migrate toward the trouble through a process called chemotaxis, usually the first cells to reach an infection. Once a phagocyte swallows a pathogen, the invader is trapped in a vesicle called a phagosome, which fuses with a lysosome to form a phagolysosome. There, digestive enzymes or a respiratory burst of free radicals finishes it off. Phagocytosis likely represents the oldest form of host defense, found in both vertebrate and invertebrate animals. Natural killer cells work by a different logic. Rather than attacking microbes directly, they destroy compromised host cells, such as tumor cells or virus-infected cells, recognizing them by a condition called missing self. A cell showing low levels of the surface marker MHC class I marks itself as a target. Normal cells, displaying intact self MHC, are spared. Inflammation announces all of this with redness, swelling, heat, and pain, the result of increased blood flow into tissue. Eicosanoids and cytokines drive it, released by injured or infected cells. Prostaglandins produce fever and widen blood vessels. Interferons shut down protein synthesis in host cells to block viruses. Among these machines is the inflammasome, a multiprotein complex built from an NLR, the adaptor protein ASC, and pro-caspase-1, which generates the active inflammatory cytokines IL-1 beta and IL-18.

• 4. Complement And The Cascade

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  More than 20 different proteins make up the complement system, named for its ability to complement the killing of pathogens by antibodies. It is the major humoral component of the innate response, and it is widespread, found in plants, fish, and some invertebrates as well as humans. The system switches on when complement binds to antibodies already attached to microbes, or when complement proteins latch onto carbohydrates on a microbe's surface. Speed is its signature. Complement proteins are also proteases, so once the first one binds and activates, it activates the next, and so on, in a catalytic cascade amplified by controlled positive feedback. The chain reaction produces peptides that attract immune cells, increase vascular permeability, and opsonize the pathogen, coating it for destruction. The cascade can also kill directly by assembling a membrane attack complex that ruptures the target's plasma membrane.

• 5. Learning And Remembering

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  B cells and T cells, the lymphocytes that drive adaptive immunity, both come from hematopoietic stem cells in the bone marrow. B cells carry out the humoral response, while T cells handle cell-mediated immunity. Each lineage of B cell expresses a different antibody, so the full set of B cell receptors represents every antibody the body can make. When a B or T cell meets its matching antigen, it multiplies into many clones that all target the same threat, a process called clonal selection. Killer T cells hunt down infected or dysfunctional cells. A killer T cell activates when its receptor binds a specific antigen presented on another cell's MHC class I, aided by the co-receptor CD8. It then releases cytotoxins such as perforin, which punch pores in the target membrane, and granulysin, which pushes the cell into apoptosis. A single MHC-antigen molecule can be enough to set a killer T cell off. Helper T cells command rather than kill. They recognize antigen bound to MHC class II through their CD4 co-receptor, and their grip is weaker, requiring around 200 to 300 receptors to be bound before they activate. Once switched on, they release cytokines that sharpen macrophages and killer T cells, and they raise CD40 ligand, the extra signal that activates antibody-producing B cells. When B and T cells replicate, some offspring become long-lived memory cells. These remember each pathogen and mount a faster, stronger response the next time it appears. This is the foundation of vaccination, which introduces an antigen to build specific immunity without causing the disease. A newborn borrows protection rather than building it. IgG crosses the placenta during pregnancy, and breast milk and colostrum carry antibodies to the infant's gut, a passive immunity that lasts from a few days up to several months.

• 6. When Defense Goes Wrong

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  Severe combined immunodeficiency is a rare genetic disorder in which functional T cells and B cells fail to develop, the product of numerous mutations. It marks one face of immune failure: a system too quiet to protect. Immune responses begin declining around 50 years of age through immunosenescence. In developed countries, obesity, alcoholism, and drug use commonly weaken defenses, while malnutrition is the leading cause of immunodeficiency in the developing world. AIDS and some cancers cause acquired immunodeficiency. Autoimmunity is the opposite failure, a system too aggressive. The immune system stops distinguishing self from non-self and attacks the body. Specialized cells in the thymus and bone marrow normally screen young lymphocytes against self antigens and eliminate those that react, preventing this. Common autoimmune diseases include Hashimoto's thyroiditis, rheumatoid arthritis, diabetes mellitus type 1, and systemic lupus erythematosus. Hypersensitivity damages the body's own tissues through the immune response, split into four classes by mechanism and timing. Type I is the immediate or anaphylactic reaction tied to allergy, mediated by IgE, which triggers degranulation of mast cells and basophils. Type II runs on IgG and IgM antibodies that bind a person's own cells and mark them for destruction. Type III comes from immune complexes deposited in tissues. Type IV, the delayed kind, usually takes between two and three days and is carried out by T cells, monocytes, and macrophages, appearing in autoimmune and infectious diseases and in contact dermatitis.

• 7. Tumors And Manipulation

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  Immune surveillance is the system's watch over the body's own cells. Transformed tumor cells display antigens absent from normal cells, and to the immune system these look foreign. Some come from oncogenic viruses like human papillomavirus, which causes cancer of the cervix, vulva, vagina, penis, anus, mouth, and throat. Others are the body's own proteins overproduced, such as tyrosinase, which at high levels turns melanocytes into melanomas. Killer T cells lead the attack, recognizing tumor antigens on MHC class I, with NK cells finishing cells that carry too few MHC molecules. Tumors fight back. Many reduce their MHC class I to dodge killer T cells. Some secrete the cytokine TGF-beta, which suppresses macrophages and lymphocytes. In a darker twist, macrophages can be turned to the tumor's side. Cancer cells attract them with cytokines, and the macrophages then produce tumor-necrosis factor alpha that nurtures growth. Recruited early as anti-tumor M1 cells, they shift progressively to a pro-tumor M2 state under the tumor microenvironment, driven by IL-4 and IL-10. Medicine bends the system both ways. Glucocorticoids are the most powerful anti-inflammatory drugs, with side effects like central obesity, hyperglycemia, and osteoporosis that keep their use tightly controlled. Cytotoxic drugs such as methotrexate or azathioprine kill dividing cells, while cyclosporin stops T cells from reading signals correctly. Against drugs built from larger peptides and proteins, typically above 6000 daltons, the body may raise neutralizing antibodies, a problem that gave rise to computational fields like immunoinformatics to predict immunogenicity.

• 8. Ancient Origins, Modern Science

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  CRISPR sequences let bacteria and archaea keep fragments of the phage genomes they have met before, blocking viral replication through a form of RNA interference. This acquired immunity in prokaryotes sits alongside the restriction modification system, a bacterial defense against viral pathogens called bacteriophages. Defensins, the antimicrobial peptides conserved across all animals and plants, form the main systemic immunity of invertebrates. Plants lack phagocytic cells entirely, relying instead on systemic chemical signals; an infected region can undergo a hypersensitive response, with cells dying rapidly to wall off the spread. The adaptive system arose in an ancestor of the jawed vertebrates. Immunoglobulins and T-cell receptors exist only in jawed vertebrates, yet the jawless lamprey and hagfish solved the same problem differently, building variable lymphocyte receptors from just one or two genes that bind antigens with comparable specificity. The science behind all this began with observation. During the plague of Athens in 430 BC, Thucydides noticed that survivors could nurse the sick without falling ill a second time. In the 10th century, the Persian physician al-Razi, known as Rhazes, wrote the first recorded theory of acquired immunity, observing that surviving smallpox protected against later infection. Louis Pasteur later turned such observations into vaccination and the germ theory of disease. Robert Koch's proofs of 1891 confirmed microorganisms as the cause of infectious disease, earning him a Nobel Prize in 1905, and Walter Reed identified the yellow fever virus in 1901. The honors continued into the modern era. Niels Kaj Jerne developed the immune network theory in 1974, and shared a Nobel Prize in 1984 with Georges J. F. Köhler and César Milstein for their work on the immune system.