The host immune system retains immunological memory of B and T cells stimulated by previous infections. Upon later inoculation, a host rapidly builds defense from its memory cells. Each host acquires a unique memory profile based on its infection history.
With respect to the antigenic response of pathogen, the host varies genetically. MHC alleles are highly polymorphic. The germ line genes that contribute to the T cell receptor have some polymorphisms that influence recognition, but the germ line B cell receptor genes do not carry any known polymorphisms.
The immune profiles of host system describe the degree of distribution of specific antigenic variants. Host systems that are previously affected by the pathogen tend to have broader profiles as these are experienced with infections. Maternal antibodies provide short-term resistance to infants, and certain antibody along with TCR provides temporary protection to recently infected hosts. The host system may vary spatially in their prior exposure to different epitopes, creating a spatial mosaic in the selective pressures that favor different antigenic variants.