As mentioned before phagocytosis is used to destroy pathogens, this is done by a white blood cell called a phagocyte that ingests and destroys the pathogen. Phagocytosis is the process that engulfs bacteria in cells in the form of vesicles formed from the cell surface membrane. Phagocytes provide an important an important defence against pathogens that manage to enter the body, some phagocytes travel in the blood but can move out of blood vessels into other tissues. During phagocytosis adherence or attachment is the first phase, when the firm contact between the cell membrane of the phagocyte and the microbe is established. In the next phase, ingestion occurs, when by means of pseudopodia the phagocyte engulfs the microbe and kills it with the help of lysosome.
- Chemical products of the pathogen act as attractants, causing phagocytes to move towards the pathogen.
- Phagocytes attach themselves to the surface of the pathogen.
- They engulf the pathogen to form a vesicle known as a phagosome.
- Lysosomes move towards the vesicle and fuse with it.
- Enzymes within the lysosomes break down the pathogen. The process is the same as that for digestion of food in the intestines.
- The soluble products from the breakdown of the pathogen are absorbed into the cytoplasm of the phagocyte.
Whilst the initial response of the body is non-specific. The next phase is the specific response that confers immunity. This specific response depends upon a type of blood cell called a lymphocyte. There are 2 types of lymphocyte each with their own immune response. B lymphocytes are associated with humoral immunity involving antibodies that are present in body fluids; T lymphocytes are associated with cell mediated immunity. Both lymphocytes are formed from stem cells found in the bone marrow.
Cell-mediated immunity is an immune response that does not involve antibodies but rather involves the activation of macrophages, natural killer cells, antigen-specific cytotoxic T-lymphocytes, and the release of various cytokines in response to an antigen. The stages in the response of T cells are as follows:
- The phagocyte places antigens from the pathogen on its cell-surface membrane.
- Receptors on certain T helper cells fit exactly onto these antigens.
- This activates other T cells to divide rapidly by mitosis to form a clone.
- The cloned T cells: a) develop into memory cells that enable a rapid response to future infections by the same pathogen. b) stimulate phagocytes to engulf pathogens by phagocytosis. c) stimulate B cells to divide. d) kill infected cells.
The role of the receptors on T cells is important. The receptors on each T cell respond to a single antigen. It follows that there are a vast number of different types of T cell, each one responding to a different antigen.
Humoral immunity is a type of immunity which is conferred through the release of antibodies which are used to target cells for destruction by the body when these cells are viewed as potentially dangerous. This type of immunity is a complement to cellular immunity, in which cells release toxins to kill unwanted invaders, or attack the invaders directly to kill them. Together, humoral and cellular immunity are designed to defend the body against a wide variety of threats which could compromise it. B cells divide by mitosis to form clones of identical B cells. For each clone the cells will develop into one of two types of cell:
- Plasma cells, these cells secrete antibodies directly. They survive for only a few days but each can make around 2000 antibodies every second during its brief lifespan. These antibodies destroy the pathogen and any toxins it produces. The plasma cells are therefore responsible for the immediate defence of the body against infection. This is known as the primary immune response.
- Memory cells live considerably longer than plasma cells. These cells circulate in the blood and tissue fluid, when they encounter the same antigen at a later date they divide rapidly and divide rapidly and develop into plasma cells and more memory cells. The plasma cells produce the antibodies needed to destroy the pathogen, while the new memory cells circulate in readiness for future infection. In this way the memory cells provide long term immunity against the original infection. This is known as the secondary immune response. It is both more rapid and of greater intensity than the primary response. It ensures that a new infection is repulsed before it can cause any harm –and individuals are completely unaware they have been infected.
The way in which memory cells function explains why most of us only develop diseases like chickenpox only once in our lifetime. The pathogens causing each of these diseases are of a single type. Therefore they are quickly identified by the memory cells when they invade the body on subsequent occasions. However some pathogens have over 100 different strains as the antigens that the viruses are made of are constantly changing. This is known as antigenic variability. Any subsequent infections are therefore highly likely to be caused by different varieties of the pathogen. These antigens will not correspond to the antibodies formed during previous infections.
B cells respond to pathogens by producing antibodies, antibodies are proteins synthesized by B cells. When the body is invaded by non-self material, a B cell produces antibodies. These antibodies react with antigens on the surface of the non-self material by binding to them precisely, in the same way a key fits a lock. Antibodies are therefore very specific, each antigen having its own antibody. The massive variety of antibodies is possible because they are made of proteins. They are made of four polypeptide chains. The chains of one pair are called heavy chains, while the chains of the other pair are shorter and known as light chains. The binding site is different on every antibody and is therefore called the variable region. Each site consists of a sequence of amino acids that form a specific 3D shape that binds directly to a type of antigen. The rest of the antibody is the same in all antibodies so its called the constant region. This binds to receptors on cells
Matt Barraclough 12 THG