Allergies, autoimmunity and the lymphoid organs
Allergy is an exaggerated immune response to harmless allergens; autoimmunity is the body attacking its own cells. Lymphoid organs make, mature and house the lymphocytes.
How an allergy happens
Modern lifestyles seem to have lowered immunity: more children in Indian metro cities now have allergies and asthma, possibly because of a very **protected environment** early in life.
- 1A person meets an allergen, such as dust mites, pollen or animal dander.
- 2★The immune system over-reacts and makes antibodies of the IgE type.
- 3Mast cells release chemicals such as histamine and serotonin.
- 4Symptoms appear: sneezing, watery eyes, running nose and difficulty in breathing.
- 5To find the cause, doctors expose the patient to, or inject, very small doses of possible allergens and watch the reactions.
- 6Drugs such as anti-histamines, adrenalin and steroids quickly reduce the symptoms.
★ key step
Autoimmunity
Acquired immunity works because higher vertebrates can tell foreign molecules and organisms (non-self) from their own cells (self). Sometimes, for genetic and other unknown reasons, the body attacks self-cells. This damages the body and is called an auto-immune disease. Rheumatoid arthritis, a painful disease of the joints that affects many people, is an example.
Primary vs secondary lymphoid organs
The immune system is made of lymphoid organs, tissues, cells and soluble molecules such as antibodies.
Lymphoid organs and their jobs
| Organ | Where | Job |
|---|---|---|
| Bone marrow | Inside bones | Main lymphoid organ; makes all blood cells, including lymphocytes |
| Thymus | Near the heart, beneath the breastbone; lobed | Large at birth, shrinks to a very small size by puberty; with bone marrow, gives the setting for T-lymphocytes to develop and mature |
| Spleen | Upper left abdomen; large and bean-shaped | Contains lymphocytes and phagocytes; filters blood and traps blood-borne microbes; a large reservoir of erythrocytes |
| Lymph nodes | Small solid bodies along the lymphatic system | Trap microbes and antigens in lymph and tissue fluid; trapped antigens activate lymphocytes |
| MALT | Lining of the respiratory, digestive and urogenital tracts | Mucosa-associated lymphoid tissue; about 50% of all lymphoid tissue in the body |
In the pollen season or in dusty rooms, many children in Indian cities get sneezing and wheezing: an IgE-driven allergy. Doctors may advise anti-histamines, and keeping rooms free of dust mites by sunning bedding helps.
2 marks: “What is an allergy? Name the antibody and the two chemicals involved.” An exaggerated immune response to certain environmental antigens (allergens) such as pollen or dust mites. Antibody: IgE. Chemicals: histamine and serotonin from mast cells.
3 marks: “Differentiate between primary and secondary lymphoid organs with examples. What is MALT?” Primary (bone marrow, thymus): immature lymphocytes become antigen-sensitive. Secondary (spleen, lymph nodes, tonsils, Peyer's patches, appendix): lymphocytes meet antigens and proliferate. MALT: lymphoid tissue in lining of major tracts, about 50% of lymphoid tissue.
2 marks: “What is an auto-immune disease? Give one example.” A disease in which the body attacks its own cells, due to genetic and other unknown reasons, damaging the body. Example: rheumatoid arthritis.
Allergy chain: allergen, IgE, mast cell, histamine. Primary lymphoid organs are BT (Bone marrow, Thymus); the rest are secondary.