Macrophages engulf viruses and present virus antigens to a helper T lymphocytes during the process of antigen presentation. Helper T lymphocytes bind viral antigens in association with MHC type II proteins that are found on macrophages. These lymphocytes activate B lymphocytes through the production of cytokines. B cells similarly recognize viral antigens via a cell surface receptor and differentiated lymphocytes (plasma cells) synthesize antibody. Antibodies attach to viruses preventing their entry into cells and opsonize them for phagocytosis. Primary antigen recognition and immune responses usually result in the formation of memory cells that persist, making antigenic recognition much faster during reinfection.
Reactions to a blood group intolerance involve antibody-mediated destruction of foreign red blood cells (RBC). Every human has antibodies in blood against the blood groups that he or she does not possess. These antibodies react with antigens on the incompatible RBC. Binding of antibodies to RBC antigens activates complement reactions that result in lysis of antibody-coated RBC. Reactions to a blood group intolerance involve less components and steps in comparison with antiviral immune response.