Your immune system is a multifaceted, complex, self-regulating system, designed to protect you from any invasion by pathogens or parasites. One way to visualise it is as a your body’s Department of Defence, with various subdivisions, such as border control, police force, army, air force etc. Let’s take a whirlwind tour.
The first line of defence is strong borders, in this case the health of your mucosa lining the nose, sinuses, throat, whole gastrointestinal tract and lungs. There is a layer of mucous secreted by the cells of the lining, which traps foreign objects, and propels them outward by the synchronised beating of tiny cilia on the cells. Within this mucous is found Secretory IgA, a type of specialised antibody (think of this as the border police)which plays an important role in detecting foreign invaders and presenting them to specialised dendritic cells (regular police). These dendritic cells, together with big macrophages, literally ‘big eaters’, engulf these invaders and whisk them off to the lymph nodes (local police station). They also call other immune cells, such as neutrophils, a type of white blood cell, to the scene by releasing chemicals such as cytokines (radio for reinforcements). This constitutes your innate immune system, which needs no prior exposure to a particular virus or bacterium to spring into action.
Once in the lymph nodes, the pathogens are carefully examined (interrogated), and over a few days, specialised T lymphocytes are cloned in large numbers (trained) and sent to the site of the invasion. These cells are highly specialised to target only the particular invader, rather liked trained snipers which will recognise and take out a known identified threat. This is important, because they must only destroy the virus, without damaging surrounding healthy cells (bystander damage).Appropriately named natural killer cells, which are a bit like rangers in the army, also get in on the action. Concurrently, specialised B cells are deployed, which recognise and bind to the pathogen (arrest and handcuff them). Further cells called mast cells release chemicals such as histamine in to the tissue ( a bit like teargas, but more damaging to surrounding tissue).
It is important to manage this process, as many immune cells, among other functions, release very damaging enzymes to directly kill viruses and bacteria (mini bombs), which causes death of surrounding cells. This regulation is managed by aptly named T regulatory cells (the guys at HQ), which dampen down the inflammatory process at the appropriate time. Thus, proper immune function requires timed and co-ordinated action by the various components, leading to good activation, appropriate level of response, and eventual well-timed dampening down.