The Differences Among the Four Types of Immunity

According to the pathway by which it is produced, immunity can be divided into four types: natural active immunity, natural passive immunity, artificial active immunity, and artificial passive immunity.

Among these, natural active immunity is the lasting immunity that the body itself produces after recovering from infection by a pathogen. The characteristic of this immunity is that its effect is long-lasting; contracting a disease once often produces lifelong immunity. For example, after recovering from smallpox virus infection, one will never again contract smallpox.

Natural passive immunity is passed naturally from the mother to her offspring, enabling the fetus and infant to acquire protection passively. This immunity can be obtained through the placenta and breast milk. Breastfed infants have strong immunity and can resist many common pathogens. Maternal sIgA protects the newborn's digestive, respiratory, and genital tract mucosae; sIgA prevents pathogens from adhering to epithelial cells, and is also called the "mucosal barrier antibody." Its deficiency predisposes one to recurrent respiratory and intestinal infections. This is why we promote breastfeeding.

The immunity of natural passive immunity is not lasting; it usually disappears within six months after birth.

Artificial active immunity refers to introducing antigens (vaccines, toxoids, etc.) into the body, so that the immune system is stimulated to produce its own antibodies and memory cells to resist external pathogens. This type of immunity develops slowly, usually requiring 1–4 weeks to produce antibodies. Its main use is in disease prevention, and it is the type of immunization we encounter most in daily life.

Artificial active immunity can produce memory cells and has a relatively long duration of action, usually lasting for several years or even decades. For example, after receiving the hepatitis B vaccine, one can acquire relatively long-lasting immunity against hepatitis B.

Artificial passive immunity refers to introducing preformed antibodies or immune cells into the body (such as antitoxins or gamma globulin). Because antibodies are present in the body immediately after introduction, without needing to stimulate the body to produce them actively, it is called artificial passive immunity.

Artificial passive immunity is usually used for emergency treatment or emergency prevention. For example, when a person is bitten by a rabid dog, constituting a category III exposure, not only must rabies vaccine be injected, but rabies immunoglobulin must also be injected near the bitten tissue to urgently prevent the spread of the rabies virus. As another example, when there is a risk of infection with Clostridium tetani, tetanus antitoxin must be injected urgently—the same principle applies.

Like natural passive immunity, artificial passive immunity can only produce a short-term immune effect and cannot generate memory cells; therefore its duration of action is relatively short.

In summary, in preventing the occurrence of disease, what we use most is still artificial active immunity, that is, vaccination as we commonly call it. By understanding the principles by which the various types of immunity arise, we can better protect our bodies and be spared the affliction of many diseases.