The mucous membranes lining the digestive, respiratory and urogenital systems have a combined surface area of about 400 m², which is equal to the size of a standard basketball court.  They are the major sites of entry for most pathogens. These vulnerable membrane surfaces are defended by a group of organized lymphoid tissues collectively known as mucosal-associated lymphoid tissue (MALT). It constitutes about 50 per cent of the lymphoid tissue in human body.