Immunity - Session 1

The immune system is the intricate defense network of the body. Think of it as a personal security force of your body. The first line of defense is your body's physical barriers, like your skin and mucous membranes. They prevent pathogens from entering your body in the first place. If pathogens manage to breach the barriers, the immune system fights them. Phagocytes and lymphocytes are the immune cells of the body. They kill any harmful invader.

Phagocytes are a type of white blood cells. Phagocytes undertake a central role in the body's immune system. Their primary function is to engulf, ingest and destroy foreign particles such as bacteria, viruses, fungi and other pathogens. This process is called phagocytosis. It is a vital defense mechanism that helps protect the body from infections and maintain overall health.

There are several types of phagocytes. Each phagocyte has its own unique characteristics and functions. The main types include neutrophils, monocytes and macrophages. Neutrophils are the most abundant type of phagocyte in the bloodstream. They comprise about sixty to seventy percent of all white blood cells.

Neutrophils are often the first responders to sites of infection or tissue damage. They are drawn to the site by chemical signals released by injured cells or pathogens.Neutrophils are highly mobile and can quickly migrate to the site of infection, where they engulf and destroy invading microorganisms. They have a short lifespan and are continually replenished by the bone marrow.

Monocytes are another type of phagocytes. They are a type of white blood cell found in the bloodstream. Monocytes make up about five to ten percent of circulating leukocytes.When stimulated by inflammatory signals, monocytes can exit the bloodstream and enter tissues, where they differentiate into macrophages. They undertake a crucial role in the clearance of pathogens and debris.

Macrophages are large phagocytic cells found in various tissues throughout the body, including the spleen, liver, lungs and lymph nodes. They are derived from monocytes and are highly versatile cells with diverse functions beyond phagocytosis. Macrophages act as scavengers. They clear away the dead cells, debris and pathogens. They also undertake a key role in initiating and regulating immune responses. Additionally, macrophages are involved in tissue repair and remodeling, making them essential for maintaining tissue homeostasis.

Phagocytosis is a process of the immune system in which phagocytes engulf and digest large particles such as bacteria, dead cells and debris. This process is essential for defending the body against infections and for cleaning up dead or damaged tissue. First step of phagocytosis is called chemotaxis. In chemotaxis, the phagocytes are attracted to the site of infection or tissue damage by chemical signals released by pathogens or injured cells. These chemical signals include cytokines and chemokines.

Phagocytes recognize foreign particles or pathogens through receptors on their surface. These receptors are called pattern recognition receptors. They bind to specific molecules on the surface of pathogens. These specific molecules are known as pathogen associated molecular patterns. Once the phagocyte has recognized the target, it binds to it. This binding is facilitated by the receptors on the phagocyte's surface, which attach to molecules on the surface of the pathogen or particle.

The phagocyte extends its cell membrane around the pathogen, enclosing it in a pocket called a phagosome. This is like the phagocyte eating the pathogen by surrounding it completely with its cell membrane. The phagosome, containing the pathogen is fully enclosed within the phagocyte's cytoplasm. The phagosome then fuses with a lysosome. Lysosome is another membrane bound organelle in the phagocyte that contains digestive enzymes and toxic substances. This fusion forms a phagolysosome.

Inside the phagolysosome, the digestive enzymes and toxic substances break-down the engulfed pathogen into small, harmless pieces. Enzymes such as proteases, lipases and nucleases are involved in this degradation process. After the pathogen is digested, the remaining indigestible material is expelled from the phagocyte. The waste is released into the surrounding environment by the process of exocytosis.

An antigen is a substance that the immune system recognizes as foreign or non-self, which triggers an immune response. Antigens are typically proteins or polysaccharides, but they can also be lipids and nucleic acids. They are there on the surface of pathogens such as bacteria, viruses, fungi and parasites. They can also be found on cells and tissues within the body in the case of autoimmune diseases.

Self antigens are molecules or molecular structures that are normally there in the body. They are recognized by the immune system as being part of the body itself.The immune system typically does not mount an immune response against self antigens. This state of non responsiveness is known as the immune tolerance.

Self antigens undertake a role in normal physiological processes and cellular functions. Examples include proteins, glycoproteins and other molecules found on the surface of cells and within tissues. Examples of self antigens are blood group antigens, major histocompatibility complex molecules and other cellular proteins. When the immune system mistakenly recognizes self antigens as foreign, it can result in autoimmune diseases such as rheumatoid arthritis, type one diabetes and multiple sclerosis.

Non self antigens are molecules or molecular structures that originate outside the body. They are recognized by the immune system as foreign. The immune system recognizes and mounts a response against non self antigens to protect the body from infection or harm. These antigens are often associated with pathogens or foreign substances and are key targets for the immune response.

Molecules associated with groups of pathogens, such as bacterial cell wall components and viral proteins are examples of non self antigens. Overactive immune responses to harmless non self antigens such as allergens can cause allergic reactions. Antigens in transplanted organs or tissues from another individual can also be non self antigens. They can be recognized as foreign and result in transplant-rejection.