Phagocytes (white blood cells) destroy microorganisms through the process of phagocytosis. In this process, the cell binds the microorganism and engulfs it.
This process is very specific, on the surface of the phagocyte, there are receptors that allow the cell to recognize the strange microorganisms. There are four types of surface receptors:
-Opsonin receptors: they bind the bacteria or other particles that have been coated with immunoglobulin G antibodies.
-Scavenger receptors bind molecules that have been produced by bacteria.
-Toll-like receptors bind to specific molecules only produced by bacteria
-Antibodies: these receptors are produced by the cells of the immune system (white B cells) and can bind to specific antigens.
When the macrophage comes in contact with a microorganism, it recognizes and binds the microorganism through its surface receptors. Once the microorganism is bound to the cell surface, the macrophage starts to produce cell extensions called pseudopods that surround the microorganism and enclose it in a membrane. once the microorganism is enclosed, the cell pulls inwards and engulfs it. The microorganism inside the cell is now surrounded by a membrane that forms a structure called a phagosome.
Within the macrophage there are organelles called "lysosomes", these organelles are responsible for the digestion of macromolecules, such as old cell parts and microorganisms (or substances produced by microorganisms). The lysosomes have a membrane that keeps an acidic environment within (produced by proton pumps that pump hydrogen ions inside the lysosome), inside it you can find several types of digestive proteins called hydrolytic enzymes. These enzymes are responsible for breaking down macromolecules such as proteins, nucleic acids, and polysaccharides. They are only active in an acidic environment and that is why the interior of the lysosome has a lower pH with respect to the rest of the cell.
Inside the macrophage, the lysosomes fuse with the phagosomes creating a structure called Phagolysosome. Once both organelles are fused, the hydrolytic enzymes start to break down the microorganism neutralizing it (killing it) this way the microorganism won't be able to produce any harm to the cell or the organism.
Once the contents of the phagolysosome are neutralized, it forms a residual body that contains the waste products of the phagocytosis, these products are discharged from the cell through exocytosis.