The immune system is composed of two semi-independent parts: a network of lymphoid vessels, and several lymphoid tissues and organs, distributed throughout the body. The lymphoid vessels function to return the fluid and proteins which have been filtered out of the blood capillaries, back to the circulatory system. In addition, the vessels take up the fat absorbed at the small intestine, and transport it to the circulatory system. The lymphoid organs contain phagocytes and lymphocytes, which play essential roles in triggering immune responses against foreign antigens, microorganisms, tumour and transplanted cells, and viruses.
Mathematical modelling is increasingly used to address various immunological phenomena, in particular the within-host population dynamics of immune responses to infectious agents. From the mathematical models, scientists hope to gain a better understanding of the dynamics involved in the pathogen-host system. In addition, other models look at T-cell dynamics, and the quantitative events that underlie the immune response to pathogens.