When the Immune System Overreacts or Under-Reacts

Immune problems do not fit one simple strong versus weak scale. Allergy involves a response to an allergen. Autoimmune disease involves loss of immune tolerance to selected body targets. Immunodeficiency means that one or more defence pathways are missing or impaired.

Immune function has several controls A healthy immune system must recognise useful targets, limit unnecessary inflammation and provide enough defence against infection. Different cells, antibodies and signalling pathways handle different parts of this work. Problems therefore do not affect one single immune strength setting. One pathway may be overactive, misdirected or impaired while other pathways still work normally. Allergy, autoimmune disease and immunodeficiency are useful categories. They can sometimes overlap in the same person or condition. Allergy is a response to an allergen An allergy is an immune response to an allergen that is usually harmless to most people in that context. Common allergens include selected foods, pollen, medicines, insect venom and latex. An allergen does not cause an allergic reaction in everyone. The reaction depends on the person's immune response and previous sensitisation. Sensitisation means the immune system has developed recognition of an allergen. Sensitisation can exist without symptoms, so a positive allergy test does not prove clinical allergy by itself. Many immediate allergies involve immunoglobulin E, shortened to IgE. IgE can sit on mast cells in tissues. When the matching allergen activates these mast cells, they release histamine and other chemical messengers. These substances can cause itching, swelling, hives, wheeze, vomiting or other symptoms. Not every allergy is IgE mediated or immediate. Food intolerance, predictable medicine side effects and toxic reactions are not automatically allergies. Anaphylaxis is a severe allergic reaction Anaphylaxis is a serious allergic reaction that develops rapidly and can affect the airway, breathing or circulation. A person may also develop hives or swelling, but skin signs are not required. Sudden throat or tongue swelling, breathing difficulty, faintness, confusion or collapse needs emergency action. The Think Like Your Doctor section explains the immediate steps. Autoimmune disease involves loss of immune tolerance Immune tolerance is the set of controls that usually prevents damaging responses against the body's own tissues. Autoimmune disease develops when selected parts of this control fail. Immune cells or autoantibodies may then contribute to inflammation and tissue damage. An autoantibody is an antibody that recognises one of the body's own molecular targets. Some autoimmune diseases mainly affect one organ. Others are systemic and can affect several tissues. The whole immune system is not simply switched into a uniformly overactive state. Autoimmune symptoms depend on the tissue involved and the type of immune response. Joint pain, fatigue or a rash can occur for many other reasons, so the pattern does not diagnose autoimmunity by itself. Immunodeficiency affects selected defence pathways Immunodeficiency means that one or more immune pathways are absent, reduced or not working normally. It can affect antibodies, immune cells, complement proteins or other defence mechanisms. A primary immunodeficiency usually arises from an inherited or genetic problem affecting immune development or function. A secondary immunodeficiency develops because of another condition, infection, poor nutrition or medicine. Immunosuppression means reduced immune activity caused by a condition or treatment. Medicines used after transplantation, for cancer or for inflammatory disease can produce different degrees of immunosuppression. Repeated, severe, persistent or unusual infections can raise concern about immunodeficiency. However, common infections are frequent in otherwise healthy people, especially young children. The infection pattern, age, growth, health problems in the family, medicines and other health problems all matter. Some immunodeficiencies can also cause inflammation or autoimmune problems rather than infection alone. These patterns are not simple opposites Allergy and autoimmune disease are not merely an immune system that is too strong. They involve selected responses occurring against an unhelpful target or in an unhelpful way. Immunodeficiency is not simply the opposite condition. A person can have weak defence against some infections while also developing allergy, inflammation or autoimmunity. Treatment can also change the pattern. A medicine that controls damaging autoimmune inflammation may increase susceptibility to selected infections. Tests answer specific questions Allergy assessment begins with the details of the reaction and timing. A skin prick test or specific IgE blood test can show sensitisation to a selected allergen. The result must match a convincing clinical reaction. A positive result alone does not prove allergy, and a negative result does not exclude every type of allergic reaction. Autoantibody tests can support assessment of selected autoimmune diseases. The same autoantibody can occur in more than one condition or in some healthy people. A negative autoantibody result also does not exclude every autoimmune disease. Clinicians combine symptoms, examination findings, inflammation tests, organ tests and imaging when needed. Immunodeficiency assessment may include blood cell counts, immunoglobulin levels and lymphocyte groups. Selected patients need tests of vaccine responses, immune cell function or genes. One normal count cannot prove that every immune pathway works normally. One abnormal result does not identify the cause by itself. Treatment targets the affected pathway Allergy care may include avoiding a confirmed allergen, symptom medicines and an adrenaline auto injector for people at risk of anaphylaxis. Selected allergies may be treated with specialist immunotherapy. Autoimmune treatment may reduce harmful inflammation or block a selected immune pathway. Some conditions also need treatment that replaces a function lost through tissue damage. Immunodeficiency care may include treating and preventing infections. Immunoglobulin replacement supplies antibodies for selected antibody deficiencies. Some severe immunodeficiencies may need a stem cell transplant or another pathway specific treatment. Clinicians may also adjust a medicine causing secondary immunodeficiency. These approaches are more precise than simply calming or strengthening the whole immune system. Know when urgent help is needed Call 999 for sudden throat or tongue swelling, serious breathing difficulty, faintness, confusion or collapse after a possible allergen exposure. Use an adrenaline auto injector immediately if one has been prescribed. If you have a diagnosed immunodeficiency or significant immunosuppression, follow your personal infection plan when you develop fever or feel unwell. Contact the named clinical team promptly rather than waiting for symptoms to become severe.

Allergy, autoimmune disease and immunodeficiency affect particular targets and pathways. Tests and treatments must match the suspected mechanism because immune dysfunction is not one simple strong versus weak scale.