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The various arms of the immune system can be categorized in several different ways. One of these is in terms of innate and adaptive immune responses.
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The Innate Immune Response
The innate immune response is a non-specific response, meaning it is activated simply by the presence of a pathogen. Cells of the innate system respond to pathogens in a very general, generic way. Their response does not change, regardless of the type of pathogen involved. Another key feature of the innate immune response is that it has no memory, and it cannot trigger the development of memory.
The innate response is, in evolutionary terms, a much older defense system than the adaptive response – plants, fungi, insects, and more primitive multicellular organisms have innate immune defense systems.
In vertebrates (including humans), the innate response has several functions:
- Recruitment of innate and adaptive immune cells to sites of infection and inflammation
- Activation of complement, a cascade of reactions which promotes the killing of invading bacteria
- Removal of foreign matter in organs, tissues, blood, and lymph
- Activation of the adaptive immune response
The earliest of the innate responses to infection is inflammation (which is a general response to any harmful stimuli, whether or not pathogens are involved). When cells are injured, they release chemical factors which attract innate immune cells to the site, where they begin carrying out their functions – ingesting cellular debris, killing infected cells, and presenting antigen. Cells involved in the innate response include natural killer cells, as well as phagocytes, granulocytes, and antigen-presenting cells.
Presenting antigen is arguably the most important function of the innate immune response, as it is antigen presentation which triggers the activation of the adaptive response.
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The Adaptive Immune Response
The adaptive immune response is characterized by two main things: specificity, and memory. These two features of the adaptive response are what make the immune system so powerful in terms of providing long-term protection.
The adaptive immune response is triggered when T cells recognize antigen which has been presented on antigen-presenting cells. This part of the process hinges on an immunological concept called specificity, which is a vital key of the adaptive response.
Specificity typically refers to the fact that cells of the adaptive immune system—namely T and B cells—are highly specific in that each cell recognizes only one type of antigen. These cells are activated only when they recognize their specific antigen, and only when it has been presented to them by an antigen-presenting cell.
Adaptive immune responses are stronger and more effective than innate responses, due to the involvement of T and B cells. Helper T cells produce cytokines which direct and control the response, while cytotoxic T cells destroy host cells which have been infected with intracellular pathogens. B cells produce antibody, which is particularly effective at targeting bacterial cells for destruction.
Adaptive responses also tend to be more highly specific in terms of generating a response tailored to destroy the specific pathogen involved in the infection. In cases of viral infection, for example, the cytotoxic response is stronger than the antibody response (in fact, the antibody response may be non-existent), as the former is more effective at combating intracellular infection.
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Adaptive Immunity Leads to Immunological Memory
Finally, there is the concept of immunological memory, which can develop only when the adaptive immune response has been triggered. This, too, hinges on the specificity of T cells and B cells.
When these cells are activated by a pathogen and mount an immune response, some will become memory cells. These memory cells form part of a ‘database’ of cells which can recognize pathogens that the immune system has encountered. The cells linger long after the initial infection has been dealt with – often, for an entire lifetime.
Each memory cell specifically recognizes a single antigen—part of a pathogen—and when it recognizes that antigen again, the cell quickly becomes activated. This immunological memory is the reason why the second and subsequent infection with the same pathogen results in an immune response which is stronger, and more quickly activated, than the initial response.
There are exceptions, of course, the most common of these being the viruses which cause colds and flu. The reason for this is that these viruses have evolved a mechanism by which they can evade the adaptive immune response: they mutate very quickly. From one year to the next, the antigens expressed by these viruses are rarely the same – so even though you might have immunological memory against last year’s flu, that doesn’t guarantee you immunity to the types of flu that will be around in the coming winter.
Innate and Adaptive Immunity
This five-part series looks at different types of immune responses, including innate and adaptive, and cell-mediated and antibody-mediated responses.