Pathogens are organism that can caused disease. This includes virulent microorganism that cause disease in the body once they get access to.
Opportunistic pathogens are those that live in the body and cause no harm but can cause disease when the body’s defences are weak or when they access the wrong places in the body. Thus E.Coli, which normally lives in the gut can cause complications if it enters blood stream or other cavities where it is usually absent.
Four classes of pathogens
Bacteria: intracellular and extracellular
The circulatory system and the lymphatic system.
The circulatory system and the lymphatic system form the structural basis for the immune system. The blood transports plasma proteins and effector cells of the immune system. The cells of the immune system originate in bone marrow. For further development B cells and T cells travel to spleen, thymus and secondary lymphoid organs
The cells involved in immune response are the white blood cells (WBC) or leukocytes. They are generated by the process called hematopoiesis.
In adults, the bone marrow is the site of generation of immune cells.The stem cells are capable of self renewal and produce three different lineages of blood cells. They are;
- 1. lymphoid progenitor cells
- 2. common myeloid progenitor cells
- 3. common erythroid progenitor cells.
The progenitor cells give rise to all the cell types of immune response.
Phagocytes are the cells that protect the body by ingesting (phagocytosing) harmful foreign particles, bacteria, and dead or dying cells. They are essential for fighting infections and for subsequent immunity. One litre of human blood contains about six billion white blood cells (cells of the immune system of which Macrophages are the most competent phagocytes).
Phagocytes of humans and other animals are called "professional" or "non-professional" depending on how effective they are at phagocytosis. The professional phagocytes include cells called neutrophils, monocytes, macrophages, dendritic cells, and mast cells.
The main difference between professional and non-professional phagocytes is that the professional phagocytes have molecules called receptors on their surfaces that can detect harmful objects, such as bacteria, that are not normally found in the body.
Phagocytes are crucial in fighting infections, as well as in maintaining healthy tissues by removing dead and dying cells that have reached the end of their lifespan.During an infection, chemical signals attract phagocytes to places where the pathogen has invaded the body. These chemicals may come from bacteria or from other phagocytes already present. The phagocytes move by a method called chemotaxis. When phagocytes come into contact with bacteria, the receptors on the phagocyte's surface will bind to them. This binding will lead to the engulfing of the bacteria by the phagocyte.Some phagocytes kill the ingested pathogen with oxidants and nitric oxide.
After phagocytosis, macrophages and dendritic cells can also participate in antigen presentation, a process in which a phagocyte moves parts of the ingested material back to its surface. This material is then displayed to other cells of the immune system. Some phagocytes then travel to the body's lymph nodes and display the material to white blood cells called lymphocytes. This process is important in building immunity. However, many pathogens have evolved methods to evade attacks by phagocytes.
White blood cells, or leukocytes (also spelled "leucocytes"), are cells of the immune system involved in defending the body against both infectious disease and foreign materials. Five different and diverse types of leukocytes exist.
All leukocytes are produced and derived from a multipotent cell in the bone marrow known as a hematopoietic stem cell. They live for about three to four days in the average human body. Leukocytes are found throughout the body, including the blood and lymphatic system.
The number of leukocytes in the blood is often an indicator of disease. There are normally approximately 7000 white blood cells per microliter of blood. They make up approximately 1% of the total blood volume in a healthy adult.
Types of Leucocyte
Neutrophil granulocytes [also known as neutrophils] are the most abundant (40 to 75%) type of white blood cells in mammals and form an essential part of the innate immune system.They are formed from stem cells in the bone marrow.They are short lived and highly motile.
Neutrophils are a type of phagocyte and are normally found in the bloodstream. During the beginning (acute) phase of inflammation, particularly as a result of bacterial infection, environmental exposure, and some cancers, neutrophils are one of the first-responders of inflammatory cells to migrate towards the site of inflammation. They migrate through the blood vessels, then through interstitial tissue, following chemical signals such as Interleukin-8 (IL-8), C5a, fMLP and Leukotriene B4 in a process called chemotaxis. They are the predominant cells in pus, accounting for its whitish/yellowish appearance.
Neutrophils are recruited to the site of injury within minutes following trauma, and are the hallmark of acute inflammation.[
Eosinophil are white blood cells and one of the immune system components responsible for combating multicellular parasites and certain infections in vertebrates. Along with mast cells, they also control mechanisms associated with allergy and asthma.
Eosinophils are also the predominant inflammatory cells in allergic reactions. The most important causes of eosinophilia include allergies such as asthma, hay fever, and hives; and also parasitic infections.
Basophils are chiefly responsible for allergic and antigen response by releasing the chemical histamine causing vasodilation.
Monocytes are a type of white blood cell.They are the largest of all leucocytes. Monocytes play multiple roles in immune function. Such roles include: (1) replenishing resident macrophages under normal states, and (2) in response to inflammation signals, monocytes can move quickly (approx. 8–12 hours) to sites of infection in the tissues and divide/differentiate into macrophages and dendritic cells to elicit an immune response.
Monocytes are produced by the bone marrow from precursors called monoblasts, bipotent cell that differentiated from hematopoietic stem cell. Monocytes circulate in the bloodstream for about one to three days and then typically move into tissues throughout the body. They constitute between three to eight percent of the leukocytes in the blood. Half of them are stored as a reserve in the spleen.
Monocytes which migrate from the bloodstream to other tissues will then differentiate into tissue resident macrophages or dendritic cells.
A lymphocyte is a type of white blood cell in the vertebrate immune system,specifically, a landmark of the adaptive immune system.
The three major types of lymphocyte are T cells, B cells and natural killer (NK) cells. Lymphocytes can be identified by their large nucleus.
T cells and B cells
T cells (thymus cells) and B cells (bursa-derived cells) are the major cellular components of the adaptive immune response. T cells are involved in cell-mediated immunity, whereas B cells are primarily responsible for humoral immunity (relating to antibodies).
The function of T cells and B cells is to recognize specific “non-self” antigens, during a process known as antigen presentation. Once they have identified an invader, the cells generate specific responses that are tailored to maximally eliminate specific pathogens or pathogen-infected cells. B cells respond to pathogens by producing large quantities of antibodies which then neutralize foreign objects like bacteria and viruses. In response to pathogens some T cells, called T helper cells, produce cytokines that direct the immune response, while other T cells, called cytotoxic T cells, produce toxic granules that contain powerful enzymes which induce the death of pathogen-infected cells.
Following activation, B cells and T cells leave a lasting legacy of the antigens they have encountered, in the form of memory cells. Throughout the lifetime of an animal these memory cells will “remember” each specific pathogen encountered, and are able to mount a strong and rapid response if the pathogen is detected again. The T Helper Cell is a sub-type of the T cell that is able to activate all these lymphocytes.
Dendritic cells (DCs) are antigen-presenting cells, of the mammalian immune system. Their main function is to process antigen material and present it on the cell surface to the T cells of the immune system. They act as messengers between the innate and the adaptive immune systems.
Dendritic cells are present in those tissues that are in contact with the external environment, such as the skin (where there is a specialized dendritic cell type called the Langerhans cell) and the inner lining of the nose, lungs, stomach and intestines. They can also be found in an immature state in the blood.
Once activated, they migrate to the lymph nodes where they interact with T cells and B cells to initiate and shape the adaptive immune response. At certain development stages they grow branched projections, the dendrites that give the cell its name.
A mast cell (also known as a mastocyte or a labrocyte) is a resident granulocyte of several types of tissues that contains many granules rich in histamine and heparin.
Although best known for their role in allergy and anaphylaxis, mast cells play an important protective role as well, being intimately involved in wound healing and defense against pathogens.
The mast cell is very similar in both appearance and function to the basophil, a type of white blood cell. However, they are not the same, as they arise from different cell lines.
innate immune system
The innate immune system, is first line of defense, is a subsystem of the overall immune system that comprises the cells and mechanisms that defend the host from infection by other organisms in a non-specific manner. This means that the cells of the innate system recognize and respond to pathogens in a generic way, but unlike the adaptive immune system (which is only found in vertebrates), it does not confer long-lasting or protective immunity to the host.
Innate immune systems provide immediate defense against infection, and are found in all classes of plant and animal life. They include both humoral immunity components and cell-mediated immunity components.
The innate immune system is an evolutionarily older defense strategy, and is the dominant immune system found in plants, fungi, insects, and in primitive multicellular organisms.
The major functions of the vertebrate innate immune system include:
- Recruiting immune cells to sites of infection, through the production of chemical factors, including specialized chemical mediators, called cytokines.
-Activation of the complement cascade to identify bacteria, activate cells and to promote clearance of dead cells or antibody complexes.
-The identification and removal of foreign substances present in organs, tissues, the blood and lymph, by specialised white blood cells.
-Activation of the adaptive immune system through a process known as antigen presentation.
-Acting as a physical and chemical barrier to infectious agents.
Adaptive immune system
The adaptive immune system, also known as the acquired immune system or, more rarely, as the specific immune system, is a subsystem of the overall immune system that is composed of highly specialized, systemic cells and processes that eliminate or prevent pathogen growth. One of the two main immunity strategies found in vertebrates (the other being the innate immune system), acquired immunity creates immunological memory after an initial response to a specific pathogen, leading to an enhanced response to subsequent encounters with that same pathogen.
This process of acquired immunity is the basis of vaccination. Like the innate system, the adaptive system includes both humoral immunity components and cell-mediated immunity components.
In acquired immunity, pathogen specific receptors are "acquired" during the lifetime of the organism (whereas in innate immunity pathogen-specific receptors are already encoded in the germline)... The acquired response is said to be "adaptive" because it prepares the body's immune system for future challenges (though it can actually also be maladaptive when it results in autoimmunity).[n 1]The system is highly adaptable because of somatic hypermutation (a process of accelerated somatic mutations), and V(D)J recombination (an irreversiblegenetic recombination of antigen receptor gene segments). This mechanism allows a small number of genes to generate a vast number of different antigen receptors, which are then uniquely expressed on each individual lymphocyte. Because the gene rearrangement leads to an irreversible change in the DNA of each cell, all of the progeny (offspring) of that cell will then inherit genes encoding the same receptor specificity, including the Memory B cells and Memory T cells that are the keys to long-lived specific immunity.
Apoptosis is the process of programmed cell death (PCD) that may occur in multicellular organisms. Biochemical events lead to characteristic cell changes (morphology) and death. These changes include blebbing, cell shrinkage, nuclear fragmentation, chromatin condensation, and chromosomal DNA fragmentation.
programmed cell death—is a normal healthy function of cells
Major Histocompatibility Complex
Phagocytosis is the process of engulfing a solid particle by a phagocyte or a protist to form an internal phagosome.
Phagocytosis is involved in the acquisition of nutrients for some cells, and, in the immune system, it is a major mechanism used to remove pathogens and cell debris. Bacteria, dead tissue cells, and small mineral particles are all examples of objects that may be phagocytized.