Human Health and Disease · Lesson 6 of 11
Allergy, Autoimmunity and Lymphoid Organs
“The IgE–mast-cell pathway, self-tolerance, primary and secondary lymphoid organs, spleen, lymph nodes and MALT.”
• Explain allergen sensitisation, IgE binding and the response of mast cells. • Distinguish ordinary allergy from severe systemic allergic reactions. • Explain autoimmunity as a breakdown of tolerance to self. • Identify the locations and roles of primary lymphoid organs. • Compare lymph nodes, spleen and mucosal lymphoid tissue.
Allergies are inappropriate responses to particular antigens
An allergy is an exaggerated immune response to an environmental substance that is ordinarily harmless to most individuals. Pollen, dust-mite proteins and animal-associated proteins are common examples of allergens. Sensitisation may cause a person's B cells to produce allergen-specific IgE. IgE binds receptors on mast cells and related cells. At a later exposure, molecules of the allergen can cross-link IgE already bound to these cells and trigger rapid mediator release. The immune system is responding to a real molecular structure, but the response is disproportionate to the biological threat presented by that structure.
Mast-cell mediators include histamine and other inflammatory compounds. Histamine can alter small blood vessels, stimulate mucus secretion and influence smooth muscles, helping explain sneezing, watery eyes, itching, hives or wheezing. Bronchoconstriction may contribute to allergic asthma in susceptible people. Allergy must not be diagnosed from one nonspecific symptom: many infections and irritants cause similar effects. Tests measuring sensitisation require interpretation alongside actual exposure-related symptoms, because recognition of an antigen does not always establish clinically significant allergy.
Allergic reactions vary from mild rhinitis to severe anaphylaxis, which can affect breathing and blood pressure. Antihistamines may relieve some mild histamine-related symptoms; broader anti-inflammatory treatments and allergen-specific approaches depend on clinical assessment. Anaphylaxis is a medical emergency in which epinephrine is the first-line treatment. The purpose of a biological explanation is to link mediator release to tissue effects and to avoid treating all allergic reactions as identical in severity.
Autoimmune disease results from a failure of self-tolerance
The immune system must discriminate between threatening foreign molecules and the body's own tissues. During lymphocyte development, some strongly self-reactive cells are removed or regulated; peripheral mechanisms continue to control autoreactivity throughout life. When these safeguards fail, immune responses can target normal tissues, producing autoimmune diseases. Rheumatoid arthritis is one example in which immune processes contribute to long-term inflammation and injury in joints. Autoimmune disease is not acquired by ordinary contact with another affected person and is not simply the body's correct response to a dangerous pathogen.
Different autoimmune disorders involve different tissues and combinations of antibodies, T cells and other immune components. Genetic susceptibility contributes to several conditions, but environmental and regulatory factors can also matter. Allergy and autoimmunity can both involve harmful immune activity, yet the targets differ: an allergy responds to an external allergen, while autoimmunity reacts against self components. The distinction is essential for explaining why a highly active immune response can sometimes cause illness instead of protecting the host.
Primary lymphoid organs support maturation of immune cells
The lymphoid system includes organs, tissues, circulating cells and soluble proteins. Bone marrow is a primary lymphoid organ where blood-cell precursors arise and B lymphocytes develop. The thymus, another primary lymphoid organ located in the upper chest, supplies the specialised environment in which immature T cells mature and undergo selection. The thymus is generally more prominent early in life and undergoes age-related involution. At this stage lymphocytes acquire the capacity to recognise antigens but have not necessarily encountered the foreign antigen for which their receptor is specific.
Secondary lymphoid organs bring antigens and lymphocytes together
Mature lymphocytes circulate through blood and lymph to secondary lymphoid organs, which concentrate encounters with antigens and antigen-presenting cells. Lymph nodes filter lymph draining from tissues. Microbial material or activated antigen-presenting cells can enter nodes, where matching lymphocytes may become activated and undergo clonal expansion. The spleen filters blood and is particularly relevant for immune surveillance of antigens circulating within it. The distinction between blood and lymph explains why these organs have different, though overlapping, immunological functions.
Tonsils, Peyer's patches in the intestine and lymphoid tissue in the appendix are examples of immune structures near interfaces where external antigens may enter. Mucosa-associated lymphoid tissue, or MALT, consists of organised and diffuse immune tissue within or near the linings of respiratory, digestive and other mucosal tracts. This helps provide local responses without requiring all antigens to travel to the spleen. The source describes mucosal tissues as representing a substantial fraction of all lymphoid tissue; a single numerical estimate depends on what tissue is included.
Immune recognition can protect or injure
Allergy, autoimmunity and transplant rejection demonstrate the importance of immune regulation. In allergy, the target is generally a harmless external substance and IgE may recruit mast-cell inflammation. In autoimmune disease, tissue belonging to the host is the target. In a transplanted organ, donor tissue markers may be recognised as foreign by recipient lymphocytes. A healthy immune response therefore requires more than maximum strength: it requires an appropriate target, suitable intensity and a timely resolution after danger has been controlled.
| Site | Type | Main purpose |
|---|---|---|
| Bone marrow | Primary | Blood-cell production and B-cell maturation |
| Thymus | Primary | T-cell maturation and selection |
| Lymph nodes | Secondary | Respond to antigens arriving in lymph |
| Spleen | Secondary | Survey blood-borne antigens |
| MALT, tonsils and Peyer's patches | Secondary and mucosal | Immune responses near common entry surfaces |
Quick Quiz
Quiz
Which antibody class is most closely associated with many immediate allergic reactions?
Which mast-cell mediator contributes to watery eyes and itching?
What makes autoimmune disease distinct from allergy?
Which pair consists of primary lymphoid organs?
Which organ monitors antigens in circulating blood?
What does MALT stand for?
Which condition is an example of autoimmunity?
Practice Problems
- Explain the sequence from exposure to pollen to symptoms of immediate allergy.
- Compare allergy and autoimmunity according to their antigenic targets, mechanisms and outcomes.
- Distinguish bone marrow, thymus, spleen, lymph nodes and MALT by immune-cell activity.
- Why can the body react differently to a pathogen entering through a wound versus the digestive tract?
After sensitisation to pollen proteins, an atopic person may have allergen-specific IgE bound to mast-cell receptors. Repeated exposure can cross-link IgE and activate the mast cells. Mediators including histamine increase mucosal secretion, alter vessel permeability and can affect airway smooth muscle, producing sneezing, watery eyes, itch or wheeze. The response targets an external substance that is usually harmless. Mild allergic rhinitis differs from systemic anaphylaxis, which requires emergency intervention.
Key Takeaways
• Allergy often involves IgE-mediated mast-cell activation and histamine release. • The severity of allergic reactions varies; anaphylaxis requires urgent medical care. • Autoimmune disease reflects harmful failure of self-tolerance. • Bone marrow and thymus support lymphocyte maturation. • Spleen and lymph nodes are secondary sites for antigen recognition. • MALT supports local immune surveillance along mucosal surfaces. • Immune recognition must be controlled to avoid tissue injury.