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What is Passive Immunity?

A short-lived immunity acquired by a person when he receives antibodies from a host, is termed passive immunity.
Sonia Nair
Immunity is the resistance power of the body to fight disease-causing organisms. The body attains this ability due to the action of the immune system, which is a complex network of cells, proteins, tissues, organs, etc. These constituents jointly fight the infectious organisms that attack the body. Immunity is classified into different types. It can be innate or acquired, active or passive, and natural or artificial. In some cases, the immunity can be a combination of the above said categories, like natural passive immunity or artificial active immunity.
Types of Immunity
Basically, immunity is classified as innate and adaptive (acquired). Innate immunity is the immediate response of the immune system, as soon as it detects the attack of infectious organisms. Innate immunity is present in the human body from the time of birth itself, and the action of innate system is not specific to the disease-causing organism. In case of adaptive or acquired immunity, the action is specifically against the specific pathogen. This acquired or adaptive immune response may take a few days to form and fight the pathogen. However, acquired immune system maintains an immunological memory, that can identify a pathogen, that has been encountered on a previous occasion. In such cases, if the same pathogen attacks the person again, the acquired immune system acts immediately and fights it.
Acquired or adaptive immunity is subdivided into artificial and natural. If the antibodies (proteins that fight diseases) are transferred to the fetus by the mother, then the immunity is natural. If it acquired through artificial means, like vaccination, then it is artificial immunity. If the body develops immunity to a particular disease after contact with a pathogen, it is also natural immunity. Both artificial and natural immunity can be further divided into passive and active. If the body is induced to develop the immunity or has developed it on its own, then it is active immunity, which can last longer. If the immunity is obtained through transfer of antibodies from a host, it can be short-lived, and is called passive immunity.
Passive Immunity
The immunity generated in a person through the transfer of specific antibodies from a host is called passive immunity. In some cases, antibodies from animals, like horses are used in humans. This is done, when a person is unable to develop an immune response or at times, when controlling the infection is very imminent, and there is very less time for the body to develop an immune response. In such circumstances, antibodies are transferred from another host, so as to offer an immediate protection to the patient. This type of immunity can be artificial or natural. If the antibodies are transferred from the mother to the fetus through the placenta; or to the baby, through colostrum, then it is natural passive immunity. If the antibodies are transferred from another person or animal, then it is artificial passive immunity. However, passive immunity (both natural and artificial) is short living in most cases.
Passive Immunity Vs. Active Immunity
In general, it can be said that passive immunity is obtained by the transfer of antibodies from another person, and not by the antibodies produced by the body of the patient. The latter condition is active immunity, wherein the person develops antibodies in his body, fights the disease, and gets immune. Even active immunity can be natural and artificial. In natural active immunity, the person is attacked by a pathogen, develops the disease, triggering immune response by producing antibodies that fight the disease, and the person becomes immune. In case of artificial active immunity, the antigen is introduced to the body through vaccination and the antigen triggers the immune system to create antibodies, without developing any symptom of the disease.