The Immune System
The immune system is a host defence system comprising many biological structures and processes within an organism that protects against disease. In order to function well, the immune system must detect a wide variety of agents, known as pathogens which can range from viruses to parasitic worms, and must have the ability to distinguish them from the host's healthy tissue. In most species the immune system consists of an innate and an adaptive immune response.
The innate immune response is stimulated when a pathogen successfully enters the host. It is the first line of defence and is required for the elimination of the pathogen from the host. Some of the key players in the innate immune response are macrophages, neutrophils and dendritic cells. A more in-depth overview of these cells can be found here.
The adaptive immune response is stimulated if the innate immune response is not effective at killing the pathogen. This response is antigen-specific and requires the recognition of specific "non-self" antigens during a process called antigen presentation. Some of the key players in the adaptive immune response are B cells, T cells and dendritic cells. More information about these cells can be found here.
Below is a schematic of some of the key immune cells in the immune response. It conveys the interaction between some of the essential cells required for mounting an immune response.