Host defense mechanisms refer to the various strategies employed by the immune system to protect the host from pathogens and other foreign invaders. These mechanisms can be broadly divided into two categories: innate and adaptive.
Innate defense mechanisms are the first line of defense against invading pathogens and include physical and chemical barriers, such as the skin and mucous membranes, as well as cellular and molecular components, such as neutrophils, natural killer cells, complement proteins, and pattern recognition receptors. These mechanisms provide immediate protection against a wide range of pathogens, but are not specific to individual pathogens.
Adaptive defense mechanisms, on the other hand, are specific to individual pathogens and develop over time as a result of exposure to the pathogen or through vaccination. The adaptive immune response is mediated by lymphocytes, primarily B and T cells, and involves a complex interplay between antigen recognition, T cell activation, B cell activation and antibody production, and memory cell formation.
Host defense mechanisms also include non-immunological processes such as the physical removal of pathogens from the body, such as coughing or sneezing, as well as the activities of other organs such as the liver and spleen in filtering and removing foreign material from the blood. Additionally, the host may employ strategies such as fever, which can inhibit the growth of some pathogens, and the recruitment of additional immune cells to the site of infection.
Overall, the host defense mechanisms are a complex and highly integrated system that work together to protect the host from a wide range of pathogens and foreign invaders.