When an antigen is encountered for the first time, it stimulates T cells and B cells to become activated and proliferate, producing clones that attack and destroy the invading antigen. This is the primary immune response, and it also produces memory cells that are able to recognize the same antigen if it should reenter the body. If another invasion of the same antigen occurs at a later date, the memory T cells and memory B cells recognize it and launch a secondary immune response, which is more rapid and intense than the primary immune response. A secondary immune response occurs each time the same antigen is detected by the memory cells. The concentration of antibodies in the secondary immune response is much higher than in the primary response.

Types of Immunity

There is more than one way for a person to develop immunity to a particular pathogen, and these mechanisms may be grouped into two broad categories: active immunity and passive immunity. A person is directly involved in the development of active immunity but not in passive immunity. Active immunity is acquired through the use of a person’s immune response, which leads to the development of memory cells. Passive immunity is acquired without the activation of a person’s immune response, and therefore there is no memory. Further, immunity can be acquired naturally or, in some cases, artificially, through medical intervention.

Naturally acquired active immunity results after a person is exposed to a pathogen, gets sick, and recovers, leaving antibodies and memory B and T cells to fight the pathogen via a secondary immune response if it reenters the body.

Artificially acquired active immunity results after a person receives a vaccine (vak-senO of weakened, dead, or inactivated pathogens or their antigenic parts, which trigger a primary immune response, leaving antibodies and memory B and T cells to fight the pathogen if it reenters the body. Booster shots may be used to trigger a secondary immune response to build up the concentration of antibodies even higher.

Naturally acquired passive immunity occurs in infants who have received maternal IgG via the placenta and IgA in breast milk. Antibodies from breast milk are an important aspect of defense in newborn infants.

Artificially acquired passive immunity results from receiving injections of antibodies produced in another person, an animal, or synthetically. This type of injection, called antiserum, is used in emergency situations when the pathogen (usually a toxin) acts too severely and quickly to wait for natural immunity to act.

Type Mechanism Result
Naturally acquired active immunity Infection by live pathogens Person is ill with the disease; immune response destroys pathogens and leaves memory T and B cells to prevent later infection
Artificially acquired active immunity Receives vaccine of weakened, dead, or inactivated pathogens or their antigenic parts Immune response occurs without the person becoming ill; memory T and B cells remain to prevent later infection
Naturally acquired passive immunity Antibodies passed from mother to fetus in utero or to newborn via breast milk Enables short-term immunity for newborn infant without stimulating an immune response
Artificially acquired passive immunity Receives injection of antibodies against a specific antigen Enables short-term immunity without stimulating an immune response
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