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Tag words: immunity, pathogen, immunology, immune system, immunological system, immune response, adaptive immunity, acquired immunity, active immunity, passive immunity, antigen, antigen presentation, antibody, antibodies, lymphokine, complement, opsonization, antibody-mediated immunity, AMI, cell mediated immunity, CMI, IgG, IgA, IgM, IgE, B cells, T cells, NK cells, IL-1, IL-2, IL-4.

Kenneth Todar currently teaches Microbiology 100 at the University of Wisconsin-Madison.  His main teaching interest include general microbiology, bacterial diversity, microbial ecology and pathogenic bacteriology.

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Immune Defense against Bacterial Pathogens: Adaptive or Acquired Immunity (page 1)

(This chapter has 6 pages)

© Kenneth Todar, PhD

Human Blood Cells; erythrocytes, neutrophils, eosinophils, basophils, monocytes and lymphocytes. The white blood cells are central players in the immune responses.

Adaptive Immunity

Innate immunity, by itself, may not be sufficient to protect a host against an invading pathogen or to prevent disease from occurring. However, if innate immunity fails, the organism may yet be detected and attacked by the mechanisms of adaptive immunity.

The innate and adaptive immune responses both function to protect against invading organisms, but they differ in a number of ways. (1) The innate immune system is constitutively present and reacts immediately to infection. The adaptive immune response to an invading organism takes some time to develop. (2) The innate immune system is not specific in its response and reacts equally well to a variety of organisms, whereas the adaptive immune system is antigen-specific and reacts only with the organism that induced the response. (3) The adaptive immune system exhibits immunological memory. It "remembers" that it has encountered an invading organism (antigen) and reacts more rapidly on subsequent exposure to the same organism. The innate immune system does not possess a memory.


An antigen (Ag) is a foreign substance (i.e., not part of the animal's tissues) of relatively high molecular weight (>12,000 daltons) that induces a specific immunological response in the form of AMI or CMI or both. Because of their complex macromolecular structure, a single microorganism consists of multiple antigens (e.g. surface structures such as cell wall components, fimbriae, flagella, etc., or extracellular proteins, such as toxins and extracellular enzymes). The coat proteins and some of the envelope proteins of animal viruses are also antigenic. The animal host is able to respond specifically to each and every antigen to come into contact with the components of the immunological system.

Adaptive immunity is a function of the immunological system (Figure 1). The immunological system is able to recognize specific antigens and react in such a way that the host generates  antibody-mediated immunity (AMI), cell-mediated immunity (CMI), or both. AMI and CMI are the two great arms of the immunological response discussed below.

Active vs passive immunity

Although adaptive immunity develops in an animal which is undergoing a specific immunological response to an antigen, the immune cells and factors generated can be shared among two or more animals. Hence, adaptive immunity can be acquired by an animal in two ways: active immunity and passive immunity.

1. In the case of active immunity, the animal undergoes an immunological response to an antigen and produces the cells and factors responsible for the immunity, i.e., the animal produces its own antibodies and/or immuno-reactive lymphocytes. Active immunity can persist a long time in the animal, up to many years in humans.

2. Passive immunity is the acquisition by an animal of immune factors which were produced in another animal, i.e., the host receives antibodies and/or immuno-reactive lymphocytes originally produced during an active response in another animal. Passive immunity is typically short-lived and usually persists for only a few weeks or months.

Furthermore, either active or passive immunity may be acquired by natural means (e.g. self production of antibodies during infection or transfer of antibodies from mother to offspring) or by artificial means (i.e., vaccination and other immunization procedures). Some familiar examples of active and passive immunity are given in the table below.
Table 1. Examples of Active and Passive Immunity
Type of Immunity How Acquired by Host Examples
Active Immunity As a result of exposure to an infectious agent or one of its products (antigens) Natural: Antibodies are produced by the host in response to the infectious agent itself (e.g. recovery from the disease).

 Artificial: immunization (vaccination) with some product derived from the infectious agent (e.g. toxoid, killed cells, structural components of cells, inactivated or attenuated viruses, etc.).

Passive Immunity As a result of the acquisition of antibodies which have been produced in another animal (by active means) or derived from cells grown in tissue culture (e.g. monoclonal antibodies) Natural: Transplacental transfer of antibodies from mother to fetus; transfer of antibodies from mother to infant in milk by nursing.

Artificial: Injection of immune serum from an individual previously immunized or recovered from disease, e.g. hepatitis; injection of serum from an animal hyperimmunized with tetanus toxoid.

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Kenneth Todar has taught microbiology to undergraduate students at The University of Texas, University of Alaska and University of Wisconsin since 1969.

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