1. Immediate nonspecific response
The skin serves as an outer barrier that blocks out some of the microorganisms. If the microorganisms enter the body through the respiratory system, reactions like coughing, sneezing and tearing will clear out some of them. Mucus secreted by the respiratory system also helps trap microorganisms, and the enzymes in mucus digests microorganisms too. Inflammation is also one of the immediate nonspecific response. If microorganisms enters body tissue, cells around that area will secret eicosanoids and cytokines, which produce fever to slow down bacteria reproduction and attract white blood cells to attack bacteria.
The complement system works like this: antibodies attach to carbohydrates on the surfaces of microbs. The recognition signal will trigger the production of all kinds of protease. This produces a catalytic cascade, which results in the production of peptides that attract immune cells.
Phagocytosis: Cells called phagocytes engulf pathogens and then form a vesicle with the pathogen in it. The vesicle then combine with lysosome, and pathogen is digested by enzymes.
2. Activates T and B cells in response to an infection
T Cells and B Cells are major lymphocytes. Killer T Cells recognize non-self cells by detecting the type of antigen with its receptor. If the antigen does not match up to self, Killer T Cells produces enzymes that puts the non-self cell into apoptosis. The Helper T Cell does not kill non-self cells itself. It uses receptors to detect the type of non-self cells and help immune system to react in the correct way. B Cells attach the antibodies on its surface to the antigens of non-self cells and attract Helper T Cells. They then produces antibodies that specifically signals this type of non-self cell. The antibodies circulates in lymph system and blood and bind to these non-self cells as markers for the immune system to recognize and destroy.
3. Responds to a later exposure to the same infectious agent
Vaccines basically help to expose B Cells and Helper T Cells to virus and produce antibodies that would bind to the virus and mark them as non-self the next time they see it and help the immune system to react.
4. Receptors on the surface of cells can only recognize self antigens. When receiving a mismatching antigen, it recognize the thing as non-self.
When cancer cells forms a tumor, they usually carry different antigens, which would help Killer T Cells to recognize them as non-self and attack them. However, sometimes the immune system cannot recognize the tumor cells because they produces chemicals that inhibits the immune system responses.
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