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.