Immunity and vaccines

Immunity is the body's ability to fight disease-causing organisms. There are two kinds.

Innate immunity is non-specific: it works against anything foreign, and is present from birth. It has four kinds of barriers:

• Physical: the skin, and the mucus lining the respiratory, digestive and urogenital tracts, which traps microbes.
• Physiological: acid in the stomach, saliva in the mouth, tears from the eyes.
• Cellular: white blood cells such as neutrophils, monocytes, natural killer cells, and macrophages in tissues, which swallow and destroy microbes.
• Cytokine: cells infected by a virus release proteins called interferons, which protect nearby cells from infection.

Acquired immunity is specific to each pathogen, and it has memory. The first time we meet a pathogen, the body reacts slowly and weakly: the primary response. If we meet it again, the response is fast and strong: the secondary (or anamnestic) response. That is why most people get diseases such as measles only once.

Two kinds of white blood cells carry it out:

• B-lymphocytes make antibodies, proteins released into the blood. Each antibody molecule has four chains, two heavy and two light, so it is written H₂L₂. There are several types: IgA, IgM, IgE, IgG. Because antibodies are in the blood, this is the humoral immune response.
• T-lymphocytes do not make antibodies, but help B-cells and destroy infected cells directly: cell-mediated immunity. This is also what rejects a transplanted organ that does not match; tissue and blood group matching are done before a transplant, and the patient takes drugs to suppress the response.

Immunity can be gained in two ways:

• Active immunity: the body makes its own antibodies, after an infection or a vaccine. It takes time but lasts long.
• Passive immunity: ready-made antibodies are given. A baby gets IgA antibodies in colostrum, the first milk, and IgG from its mother across the placenta. In an emergency, ready-made antibodies are injected, for example anti-tetanus serum after an injury, or anti-venom after a snake bite. It acts at once but does not last.

Vaccines use the immune system's memory. A vaccine contains a weakened or killed pathogen, or parts of it. The body responds as to a real infection and keeps memory cells, so when the real pathogen comes, the secondary response quickly destroys it. Today, recombinant DNA technology lets us make some vaccines in bacteria or yeast, such as the hepatitis B vaccine, produced in yeast.