Hormone
A hormone is a chemical signalling molecule produced by endocrine glands or specialised cells and secreted directly into the bloodstream to regulate the physiology and behaviour of distant target organs. Hormones act at very low concentrations by binding to specific receptors on or inside target cells, triggering a cascade of biochemical responses. They coordinate processes such as growth, metabolism, reproduction, mood, and homeostasis across the entire organism.
Major Human Hormones and Their Functions
| Hormone | Gland | Target Organ | Primary Function |
|---|---|---|---|
| Insulin | Pancreas (β cells) | Liver, muscle, fat | Lowers blood glucose |
| Thyroxine (T4) | Thyroid | Most body cells | Regulates metabolic rate |
| Adrenaline | Adrenal medulla | Heart, liver, muscles | Fight-or-flight response |
| Testosterone | Testes | Reproductive organs | Male secondary sex characteristics |
| Oestrogen | Ovaries | Uterus, breast | Female reproductive cycle |
| Cortisol | Adrenal cortex | Liver, immune cells | Stress response, anti-inflammation |
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Related Terms
Immune System
The immune system is a complex network of cells, tissues, organs, and soluble molecules that defends the body against pathogens, abnormal cells, and foreign substances. It comprises two principal arms: the innate immune system, which provides rapid, non-specific responses such as inflammation and phagocytosis, and the adaptive immune system, which mounts pathogen-specific responses mediated by B lymphocytes (antibody production) and T lymphocytes (cell-mediated immunity). Immunological memory generated by the adaptive arm enables faster and stronger responses upon re-exposure to the same pathogen, forming the basis of vaccination.
Cellular Respiration
Cellular respiration is the set of metabolic reactions by which cells break down organic molecules — primarily glucose — in the presence or absence of oxygen to generate ATP, the universal energy currency of life. The complete aerobic pathway yields approximately 30–32 ATP molecules per glucose molecule through three sequential stages: glycolysis (cytoplasm), the citric acid (Krebs) cycle (mitochondrial matrix), and oxidative phosphorylation via the electron transport chain (inner mitochondrial membrane). Understanding cellular respiration is fundamental to nutrition science, exercise physiology, and the treatment of metabolic diseases.
Antigen
An antigen is any molecule — typically a protein, polysaccharide, or glycolipid — that is recognised by the adaptive immune system and capable of inducing a specific immune response. The term encompasses both immunogens (which trigger antibody or T-cell responses) and haptens (small molecules that are antigenic only when conjugated to a carrier protein). Antigens can be exogenous (from pathogens, toxins, or transplanted tissue) or endogenous (e.g., tumour-associated antigens or self-antigens involved in autoimmunity), and they are the molecular basis of diagnostic tests, vaccine design, and blood-group typing.
From Greek "hormon" (ὁρμών), present participle of "hormao" (ὁρμάω) meaning "to set in motion" or "to urge on". The term was coined by British physiologist Ernest Starling in 1905 to describe secretin, the first identified chemical messenger.