Immunity is the body’s ability to defend itself against foreign agents (microbes, viruses, their toxins, foreign cells and proteins) and to keep its internal environment constant. Countless microbes enter the body every day, yet a healthy person does not fall ill — the immune system sees to that. Knowledge of immunity underlies vaccination, blood transfusion, organ transplants and the fight against infectious disease. Exams often ask about the internal environment, the mechanisms and types of immunity, the difference between a vaccine and a serum, blood groups, the Rhesus factor and transfusion rules.
§1. The internal environment
Blood, tissue fluid and lymph together make up the body’s internal environment. They are linked:
blood plasma seeps through capillary walls into the tissues, forming tissue fluid; cells take food and oxygen from it and release wastes into it;
part of the tissue fluid returns to the blood, and part enters blind-ended lymph capillaries and becomes lymph;
lymph flows along lymph vessels, is cleaned as it passes through lymph nodes (their white cells destroy microbes), and then drains through the thoracic duct into large veins.
Homeostasis is the relative constancy of the composition and properties of the internal environment. For example: body temperature about 36.6 °C, blood pH about 7.4, blood glucose about 3.3–5.5 mmol/l, salt concentration about 0.9% (physiological saline). Homeostasis is maintained jointly by the nervous and endocrine systems, kidneys, lungs and liver.
§2. Organs and cells of the immune system
Central immune organs:
red bone marrow — all blood cells, including lymphocytes, are made here; B lymphocytes mature here;
the thymus — T lymphocytes mature and are “trained” here; it is large in childhood and shrinks later.
Peripheral organs: lymph nodes, the spleen, the tonsils, lymphoid tissue in the gut.
Immune cells are white blood cells (leucocytes):
phagocytes (neutrophils, monocytes and the macrophages formed from them) engulf and digest microbes;
lymphocytes — B lymphocytes and T lymphocytes — provide specific immunity.
An antigen is a substance the body recognises as foreign and that triggers an immune response (microbial proteins, toxins, molecules on foreign cells). An antibody (immunoglobulin) is a special protein made by B lymphocytes in response to an antigen; it binds only to its own antigen, like a key to a lock.
§3. Mechanisms of immunity
1. Non-specific (innate) defence — against any foreign agent:
skin and mucous membranes — a mechanical barrier;
the enzyme lysozyme in saliva, tears and mucus — breaks down bacterial walls;
hydrochloric acid in gastric juice — kills microbes;
inflammation and fever;
phagocytosis — phagocytes engulf and digest microbes. Phagocytosis was discovered by I. I. Mechnikov, who created the phagocytic theory (Nobel Prize 1908, shared with P. Ehrlich).
2. Specific (acquired) immunity — against a particular antigen:
humoral immunity — B lymphocytes turn into plasma cells and release antibodies into the blood; antibodies bind and neutralise antigens, clump microbes together and make phagocytosis easier (humoral theory — P. Ehrlich);
cell-mediated immunity — T lymphocytes directly destroy virus-infected, tumour and foreign (transplanted) cells; helper T cells activate B lymphocytes.
3. Immune memory — after the first meeting with an antigen, memory cells remain in the body. When the same antigen enters again, the response is much faster and stronger, and the disease does not develop or is mild. Vaccination is based on this.
§4. Types of immunity
Immunity: innate (species) and acquired (natural or artificial; active or passive).
Innate (species) immunity — inherited and typical of the whole species: for example, humans do not catch canine distemper, and many animals do not catch many human infections.
Natural active — develops after recovering from a disease (after measles or chickenpox, often for life).
Natural passive — ready-made antibodies pass from mother to fetus across the placenta and to the baby in breast milk; it lasts a few months.
Artificial active — after a vaccine. A vaccine contains weakened or killed microbes, their parts or inactivated toxins (toxoids). The body makes antibodies itself and forms memory cells; immunity develops slowly (1–2 weeks) but lasts a long time. Vaccines are given to prevent disease.
Artificial passive — after a therapeutic serum (antiserum). A serum contains ready-made antibodies from the blood of an immunised animal or person. It acts at once but briefly (a few weeks). Serum is given to treat a disease or poisoning: tetanus, diphtheria, snakebite.
History of vaccination: in 1796 E. Jenner inoculated a boy with cowpox and protected him from smallpox; L. Pasteur made vaccines against anthrax and rabies. Mass vaccination has eradicated smallpox worldwide.
Immune disorders: HIV (human immunodeficiency virus) attacks helper T cells and destroys immunity, leading to AIDS; the patient may die even from ordinary infections. HIV spreads through blood, sexual contact and from mother to child. Allergy is an excessive immune response to harmless substances (pollen, foods).
§5. Blood groups
In 1900 K. Landsteiner discovered the AB0 blood group system. Red cells carry antigens (agglutinogens A and B) on their surface, and plasma contains antibodies (agglutinins α and β). When matching antigen and antibody meet (A and α, B and β), the red cells clump together — agglutination.
Group I (0): no antigens, antibodies α and β — universal donor.
Group II (A): antigen A, antibody β.
Group III (B): antigen B, antibody α.
Group IV (AB): antigens A and B, no antibodies — universal recipient.
Transfusion rule: the donor’s red cells must not clump in the recipient’s plasma (the small amount of antibody in donor plasma is diluted in the recipient’s blood). In practice only blood of the same group and Rhesus type is transfused, after a compatibility test.
§6. The Rhesus factor
The Rhesus factor (Rh) is another red-cell antigen. People who have it are Rh⁺ (about 85%); those who do not are Rh⁻ (about 15%).
If an Rh⁻ person receives Rh⁺ blood, anti-Rhesus antibodies form; a second transfusion causes a severe reaction.
Rhesus incompatibility: if the mother is Rh⁻ and the fetus is Rh⁺ (from the father), fetal red cells enter the mother’s blood at birth and trigger antibody formation. The first baby is usually unharmed, but in a second Rh⁺ pregnancy the mother’s antibodies cross the placenta and destroy the fetus’s red cells. To prevent this, the mother is given a special anti-D immunoglobulin after delivery.
Worked example
Which blood groups can in theory be given to a person with group III?
The recipient’s (group III) plasma contains antibody α.
Antibody α clumps red cells carrying antigen A (groups II and IV).
Red cells with antigen B (III) or with no antigen (I) do not clump.
Answer: groups I and III.
Worked example
A child has been bitten by a snake. Will a vaccine or a serum be given, and why?
The venom acts quickly — protection is needed immediately.
After a vaccine the body needs 1–2 weeks to make antibodies — too late.
A serum contains ready-made antibodies that bind the venom at once.
Answer: a serum — it gives artificial passive immunity.
Worked example
A town has 120 000 inhabitants, about 85% of whom are Rh⁺. Roughly how many are Rh⁻?
Share of Rh⁻: 100% − 85% = 15%.
120 000 × 0.15 = 18 000.
Answer: about 18 000 people.
Check yourself
What type of immunity does a vaccine produce?
Where do T lymphocytes mature?
Which blood group is the universal recipient?
What type of immunity do antibodies in breast milk give?
Who proposed the phagocytic theory?
When is Rhesus incompatibility dangerous?
Which cells produce antibodies?
If about 15% of a group of 300 people are Rh⁻, how many is that?
Common mistakes
Mixing up vaccine and serum — a vaccine prevents (active), a serum treats (passive).
Calling group IV the universal donor — the universal donor is group I.
Mixing up where antigens and antibodies are — antigens on red cells, antibodies in plasma.
Calling immunity from breast milk artificial — it is natural passive.
Writing that Rhesus incompatibility is severe in the first pregnancy.
Calling antibiotics a type of immunity — they are drugs, not immunity.