The air-breathing vertebrates emerged about 300 million years ago. From the very first inspiration the air, with different pollutants (toxic gas, bacteria, viruses, fungus spores) can enter the lung. The cellular composition of the lung should be in alert and defensive against harmful agents, particularly viral infection, which is not easily captured by the bronchus-associated lymphoid tissue (BALT). BALT develops in late neonatal life, therefore the lung itself should be ready for innate immune response, which works without delay, unlike the adaptive immune system.

The surfactant consists of 80 % phospholipid, 10% neutral lipid and 10 % protein. The surfactant protein (SP) are A, B, C, D. The amount of SP-A is very low unlike SP-B, which is amphipatic and essential for the life, interact with lipid at the air-liquid surface. SP-C is lacking in the birds but the so-called chicken lung lectin and SP-D contribute to the opsonization of bacteria and enhance the phagocytosis. The type II pneumocytes do not express MHC class II antigen, therefore their antigen presentation is doubtful, and the cells do not function as the M cells in the intestine.

The phagocytic cells, like macrophages, undifferentiated dendritic cells and thrombocytes are essential elements of the innate immunity. In addition to the cellular elements the surfactant proteins SP-A and D - as soluble factors, collectins and chicken lung lectin - can opsonize bacteria and facilitate their phagocytosis. The activated macrophages expressing MHC class II antigen are capable for initiating adaptive immune response with T or B cell stimulation. It was surprising that 74.3 monoclonal antibody (Jeurissen, 1992) did not recognize dendritic cells in the lung, but udentified type II pneumocytes in the parabronchus and atrium, but not in the air capillaries of chickens.

Our systemic immunohistochemical study indicate, that the lung is being endowed by cells which are capable for both innate and adaptive immune responses.