Thus, the immune response can cause the autoimmune diseases common to man such as multiple sclerosis, diabetes, rheumatoid arthritis or myasthenia gravis. The normal individual has two levels of defence against foreign agents. The first type is present in neonatal animals and in invertebrates namely natural or innate immunity. This type of immunity is sometimes referred to as non-specific but broadly specific would be a better description. The second type of immunity is adaptive or acquired immunity and is confined to vertebrates.
Physical barriers are the first line of defense against infection.
The skin and mucous membranes provide a continous surface which must be breached and back this up with mechanical protection through cilia and mucous. Physiological factors such as pH, temperature and oxygen tension limit microbial growth. The acid environment of the stomach combined with microbial competion from the commensal flora inhibits gut infection. Protein secretions into external body fluids such as lysozyme also help resist invasion. Phagocytic cells are critical in the defense against bacterial and simple eukaryotic pathogens.