These two types of lymphocyte are used in 3 ways to fight infection. B cells recognise microbes because they express membrane bound antibody which acts as an antigen receptor. At time of first infection there is no antibody in blood and level does not begin to increase until between 7-10 days afterwards. When we have an infection we produce antibodies which recognise many different types of structure on bacterial surface. Thus serum from an immune individual contains many different types of antibodies each of which recognise different structures on surface of membrane.
Antibody is soluble and diffuses through tissues to target extracellular micro-organisms.
Phagocytosis by neutrophils and macrophages with intracellular killing. These cells recognise macrophages containing intracellular bacteria by means of T cells antigen receptor, which is not an antibody. Viruses are obligate intracellular pathogens, can infect any type of cells, and most cells do not possess antimicrobial mechanisms. During intracellular replication virus proteins appear on the surface of the infected cell.