DEFINITION: Phagocytosis is one of the most important process of our body’s first line defence though which the microbes that entered into our body are engulfed and destroyed by some particular immune cells. They are engulfed into a large vesicle of the cells through endocytosis. The cells that perform that process called phagocytes. Phagocytosis is the most important process of our innate immune system. It helps to destroy the microbe or break them down into small pieces and present them to the other immune cells and trigger further immune responses.
CELLS THAT PERFORM PHAGOCYTOSIS: Macrophages, neutrophils, and dendritic cells in the tissues and monocyte cells in the blood are the main cells which carry out phagocytosis. They are known as phagocytes.
MECHANISM OF PHAGOCYTOSIS: Phagocytosis is initiated by the recognition of microbes or other pathogenic organisms and infected cells and are finished by the destruction of these particle by phagocytes.
This process is done through various steps. These are –
- At first the recognition of the invading microbes is done by the phagocytes. Phagocytic cells contain a specific receptor on their surface called pattern recognition receptor which help them to recognize a particular molecular pattern present on the surface of the microbes.
- After recognition the microbes are engulfed by the phagocytes and are entered into a vesicle, which called phagosome.
- Then the phagosomes diffuse with the lysosomes present in the cytoplasm of the phagocytes and form phagolysosome. Phagolysosome contain various degradative enzymes or microbicidal substances such as Lysozymes, Nucleases, Proteases, Lipases etc.
- The degradative enzymes present in the phagolysosome start to degrade the trapped microbes and break them down into small particle. Macrophages, Neutrophils, Dendritic cells contain Major Histocompatibility Complex molecules (MHC-I and MHC-II) through which they express the small pathogenic peptides on their surface by forming a complex. And present them to the other immune cells mainly to the T-lymphocytes.
- The phagocytic cells completely degrade the ingested microbes into phagolysosome and then expel them out from the cell through the process called exocytosis.
HOW PHAGOCYTIC CELLS RECOGNIGE THE MICROBE OR PATHOGEN ?
: Phagocytic cells or phagocytes express a variety of receptor on their surface which are known as pattern recognition receptors (PRRs). PRRs can recognize different and specific molecular pattern of different microbes which are associated on the surface of the pathogen, and are known as pathogen associated molecular patterns or PAMPs. After this recognition endocytosis occur which initiates the process of phagocytosis and destroy the pathogenic particles or other infected target cells.
WHAT IS PAMPs AND PRRs ?
PRRs or Pattern Recognition Receptors are one type of surface protein present on the surface of the phagocytic cells. These receptors recognize a specific molecular pattern present on the surface of the pathogen entered in to the body or some molecules released by the damaged cells. PRRs play a vital role in providing the first line defence by the immune cells against the microbes.
PAMPs or Pathogen Associated Molecular Pattern are generally termed as microbe associated molecular patterns (MAMPs) are a conserved structure located on the surface of the pathogens.
NOTABLE: The activation of phagocytosis can also occur indirectly through the recognition of the soluble proteins (that have bound the microbial surface) by the phagocytes. The soluble proteins, mainly antibody and complement protein C3b act as the enhancer of the phagocytosis. The process of enhancing phagocytosis is known as opsonization and the phagocytosis enhancing proteins are called opsonin.