Disruptions in the Immune System

Core Concepts

Immunodeficiency: the system that is too weak

Immunodeficiency is a failure of one or more parts of the immune system, making a person unusually susceptible to infections. It comes in two forms. Primary immunodeficiencies are present from birth, usually due to genetic defects — for example, severe combined immunodeficiency (SCID), in which both B and T cell function is missing and even mild infections can be fatal without treatment. Secondary (acquired) immunodeficiencies develop later, caused by something external: malnutrition, certain medications (such as chemotherapy or long-term immunosuppressants), or infections. The most important secondary cause is HIV.

HIV (human immunodeficiency virus) infects helper T cells (CD4+) — the coordinators of adaptive immunity — as well as macrophages and dendritic cells. Over years, untreated HIV progressively destroys CD4+ cells, and the immune system loses its ability to orchestrate responses. When the CD4+ count falls low enough and characteristic opportunistic infections or cancers appear, the person is said to have AIDS (acquired immunodeficiency syndrome). People with AIDS typically die not from HIV itself but from infections (such as certain pneumonias or fungal infections) or cancers that a healthy immune system would control. HIV is transmitted through blood, sexual contact, and from mother to child during pregnancy, birth, or breastfeeding; it is not transmitted by casual contact. Prevention (safer practices, testing, and antiretroviral treatment that suppresses the virus) and research continue to evolve — study the general biology, and treat any specific statistics or guidelines as things to verify against current sources.

Hypersensitivity: the system that overreacts

Hypersensitivity is an excessive or inappropriate immune response to a substance that is not itself harmful. The best-known form is allergy (type I hypersensitivity). The sequence is: first exposure to an allergen (such as peanut protein or pollen) triggers production of IgE antibodies that attach to mast cells; on later exposure, the allergen cross-links the IgE, causing mast cells to release histamine and other mediators within minutes. The result — sneezing, itchy eyes, hives, swelling, bronchial constriction — depends on where the response happens. Most allergic reactions are localized and uncomfortable. In anaphylaxis, however, the response is massive and body-wide: widespread vasodilation, a dangerous drop in blood pressure, and airway swelling. Anaphylaxis is a medical emergency treated with epinephrine, which rapidly reverses vasodilation and airway constriction. Type II hypersensitivity involves antibodies attacking cells (as in some transfusion reactions), type III involves antibody–antigen immune complexes depositing in tissues (as in some forms of serum sickness or autoimmune kidney disease), and type IV is a delayed, cell-mediated response (as in contact dermatitis from poison ivy or some drug rashes). The take-home pattern: hypersensitivity is a normal immune mechanism aimed at the wrong target or scaled up out of proportion.

Autoimmune disease: the system that attacks self

Autoimmune disease occurs when the immune system loses self-tolerance — the ability to leave the body's own molecules alone — and attacks self-antigens as if they were foreign. Examples include type 1 diabetes (immune destruction of the insulin-producing cells of the pancreas), rheumatoid arthritis (chronic inflammation of the joints), multiple sclerosis (immune attack on the myelin that insulates nerve fibers), and systemic lupus erythematosus (lupus), in which antibodies and immune complexes damage many tissues. Why tolerance fails is not fully understood, but genetics and environmental triggers (such as infections) are thought to contribute. Autoimmune diseases are typically chronic, with flares and remissions, and treatment usually involves dampening the immune response (immunosuppressive drugs) rather than "curing" the underlying mistake. The person-first framing matters: we say "a person with type 1 diabetes," not "a diabetic," and the same principle applies to all these conditions.

Transplant rejection: the immune system meets modern medicine

Transplantation (organs, tissues, or blood) confronts the immune system with a difficult problem: donor cells display MHC molecules (HLAs in humans) that differ from the recipient's, and the recipient's T cells recognize those differences as foreign — the same mechanism used to detect infected cells. The result is graft rejection: cytotoxic T cells and antibodies attack the transplanted tissue. Clinically, rejection is managed by matching donor and recipient MHC types as closely as possible and by giving immunosuppressive drugs that dampen the recipient's response for as long as the transplant is needed. This is why transplant recipients must take medication indefinitely and are at increased risk of infections: suppressing rejection means suppressing the immune system generally. Blood transfusions are a special, well-managed case: matching blood-group antigens (such as the ABO and Rh systems) prevents antibody-mediated destruction of transfused cells.

A unifying way to think about disruptions

All four categories reduce to the same question: against what is the immune response directed, and is its strength appropriate? Too little response to a real pathogen = immunodeficiency. Too much response to a harmless substance = hypersensitivity. Response directed at self = autoimmunity. Response directed at transplanted tissue = rejection. The immune system must balance attack and restraint, and each disruption is a failure of that balance in a different direction.

Common Confusions