How does your body remember a specific pathogen?

Immunological memory of the memory B cells and memory T cells during adaptive immunity. When antigen presenting cells phagocytose pathogens, antigen peptide fragments are presented on the surface of these cells attached to MHCs. These will signal T cells to recognise these pathogen antigens and then adapt a specific immune response to the specific antigen aka the specific pathogen by phagocytose these pathogen and at the same time, memory T cells are formed which carries immunological memory for the specific antigen by the pathogen. The same goes for B cells just that B cells produce specific antibodies which bind to the specific pathogens. Memory B cells and Memory T cells will be activated upon Recognising a specific pathogen and perform function of each cell. This will enable the body to have a quicker and effective immune response to the repeated pathogen infection the second time.

Blood transfusions were rarely done and often fatal; they were outlawed until the nineteenth century.