<P> Respiratory epithelium can secrete a variety of molecules that aid in the defense of the lungs . These include secretory immunoglobulins (IgA), collectins, defensins and other peptides and proteases, reactive oxygen species, and reactive nitrogen species . These secretions can act directly as antimicrobials to help keep the airway free of infection . A variety of chemokines and cytokines are also secreted that recruit the traditional immune cells and others to site of infections . </P> <P> Surfactant immune function is primarily attributed to two proteins: SP - A and SP - D. These proteins can bind to sugars on the surface of pathogens and thereby opsonize them for uptake by phagocytes . It also regulates inflammatory responses and interacts with the adaptive immune response . Surfactant degradation or inactivation may contribute to enhanced susceptibility to lung inflammation and infection . </P> <P> Most of the respiratory system is lined with mucous membranes that contain mucosa - associated lymphoid tissue, which produces white blood cells such as lymphocytes . </P> <P> The lungs make a surfactant, a surface - active lipoprotein complex (phospholipoprotein) formed by type II alveolar cells . It floats on the surface of the thin watery layer which lines the insides of the alveoli, reducing the water's surface tension . </P>

Explain the process of respiration in man with a labelled diagram