Blood Groups, Immunity and Vaccines: Notes for Exams

Blood is a fluid connective tissue that carries oxygen, food, hormones, heat and waste, and also carries the defence of the body. About fifty-five per cent of it is plasma, a straw coloured fluid that is about ninety per cent water with proteins such as albumin, globulin and fibrinogen dissolved in it, and the remaining forty-five per cent is the formed elements, that is red cells, white cells and platelets. This note covers the three parts of the subject that examiners ask together: the blood groups, the way the body defends itself, and the vaccines that train that defence.

The cells of blood

Clotting is a chain of reactions. Platelets release thromboplastin, which with calcium ions converts prothrombin into thrombin, and thrombin converts the soluble plasma protein fibrinogen into insoluble threads of fibrin that trap the cells and seal the wound. Vitamin K is needed for the liver to make prothrombin, so a deficiency of it delays clotting. Heparin, made in the body, is a natural anticoagulant, and blood banks keep stored blood fluid with citrate. Haemophilia, in which clotting fails, is a sex-linked recessive disease carried on the X chromosome.

The ABO and Rh blood groups

Karl Landsteiner discovered the ABO system in 1900 and received the Nobel Prize for it in 1930; his birthday, 14 June, is kept as World Blood Donor Day. The group depends on the antigen present on the red cell and the antibody present in the plasma.

The Rh factor, found by Landsteiner and Wiener in 1940 in the rhesus monkey, is a separate antigen, the D antigen. A person who has it is Rh positive and one who does not is Rh negative. If an Rh negative mother carries an Rh positive child, antibodies formed in her blood can attack the red cells of a later child, a condition called erythroblastosis foetalis, which is prevented by giving the mother anti-D immunoglobulin. The rare Bombay blood group, in which even the H antigen is absent, was found at Mumbai in 1952.

Innate and acquired immunity

Immunity is of two kinds. Innate or non-specific immunity is present from birth and acts against everything alike: the unbroken skin, the mucus of the airways, the hydrochloric acid of the stomach, the lysozyme of tears and saliva, the phagocytic white cells, the inflammatory response and the interferons released by virus-infected cells. Acquired or specific immunity is built against a particular germ and remembers it. It has two arms: humoral immunity, in which B lymphocytes become plasma cells and pour out antibodies, and cell-mediated immunity, in which T lymphocytes kill infected cells and regulate the response. The primary lymphoid organs, where the lymphocytes are made and mature, are the bone marrow and the thymus; the secondary organs, where they meet the germ, are the spleen, the lymph nodes, the tonsils, the appendix and the Peyer's patches of the intestine.

Acquired immunity may be active or passive. In active immunity the body makes its own antibodies, either after an infection or after a vaccine; it takes time to build but lasts long because memory cells remain. In passive immunity ready-made antibodies are given from outside, as in an antiserum against snake venom or tetanus, or as in the antibodies a child receives through the placenta and through the first milk, the colostrum; protection is immediate but short lived. Antibodies are proteins called immunoglobulins, each made of four polypeptide chains, two heavy and two light. IgG is the most abundant in blood and the only class that crosses the placenta, IgA is the one in milk and in secretions, IgM is the largest and appears first in an infection, IgE is involved in allergy and IgD on the surface of B cells.

Vaccines

Edward Jenner showed in 1796 that matter from cowpox protected against smallpox, which is why he is called the father of immunology and why the word vaccine comes from vacca, the Latin for cow. Louis Pasteur went on to prepare weakened germs as vaccines and made the rabies vaccine in 1885. A vaccine works by showing the immune system a harmless form of the germ or a part of it, so that memory cells are ready before the real infection arrives.

India's Universal Immunisation Programme, which grew in 1985 out of the expanded programme on immunisation of 1978, gives these free, and Mission Indradhanush of 2014 was launched to reach the children left out. Most vaccines must be kept between two and eight degrees Celsius, which is what the cold chain means. Smallpox was declared eradicated from the world in 1980, and the South-East Asia Region, India with it, was certified free of polio in 2014. When enough of a population is immune the germ cannot find a chain of hosts, and even the unvaccinated are protected; this is herd immunity.