THE SECOND LINE OF DEFENCE
The second line of defence is enforced when the first line of defence has failed. It involves non-specific responses to pathogens that have invaded the internal environment of the organism.
The second line of defence involves specialist cells and white blood cells such as phagotcytes, macrophages, neutrophils, natural killer cells, dendritic cells, complement proteins and mast cells.
Phagocytes are leucocytes (white blood cells) that are made in the bone marrow and circulate through the circulatory and lymphatic systems however can also leave and travel to the site of pathogen entry. Phagocytes engulf pathogens via a process known as phagocytosis. The phagocyte, containing the engulfed pathogen fuses with the membrane of the lysosome its enzymes digest and destroy the pathogen. In the human body, the macrophage is the most important phagocyte.
Macrophages originate in the bone marrow as monocytes and differentiate into macrophages when they leave the bone marrow. Macrophages detect chemical trails of microorganisms and pathogens and engulf them. Upon engulfing the pathogens, the macrophage will present the pathogen’s antigens (proteins) to stimulate recognition on this part of the immune system. They are commonly found in high concentration in the liver and kidney.
Neutrophils can engulf pathogens via endocytosis and are involved in the formation of pus. Neutrophils release chemicals to kill bacteria. Because the neutrophils cannot produce new lysosomes, they will die and form pus. Natural killer cells attack and kill infected cells of the body and directly kill cancer cells by releasing perforins (chemical), causing perforations in the cell membranes and causing the pathogenic cells to lyse.
Dendritic cells are important in processing antigen material and presenting them on their surface to arouse action from other cells of the immune system. They exist in small numbers in tissue that is in contact with the external environment. Complement proteins are proteins which assist phagocytes in the recognition of and presence of pathogens, continually circulating the bloodstream. Complement proteins bind to antibodies which have bound to a antigen matter, attracting phagocytes. Complement proteins can lyse the cell membrane of the pathogen to further stimulate phagocytic processes. Special chemicals such as interferon may also be secreted by cells when pathogens have entered them. interferon is a class of cytokine (messenger protein or glycoprotein molecules secreted for cell communication) secreted to reduce the chance of infection by adjacent uninfected cells by triggering these cells to make particular enzymes which prevent the virus from replicating itself inside the cell. #NonSpecificImmunity #WhiteBloodCells