Leukocytes
- collective name for the various types of white blood cells that make up the immune system
- formed partially in the bone marrow (granulocytes and monocytes) and partially in the lymph tissue (lymphocytes and plasma cells)
- the value of WBC is that most of them are specifically transported to areas of infection and inflammation, thereby providing a rapid and potent defense against infectious agents
- 7000 WBC’s per microliter of blood
Types of WBC in the blood and their concentrations
- Polymorphonuclear neutrophils: 62.0%
- Polymorphonuclear eosinophils: 2.3%
- Polymorphonuclear basophils: 0.4%
- Monocytes: 5.3%
- Lymphocytes: 30.0%
- Blood platelets: 300 000 per microliter of blood
Myeloid cells
- all cells begin as myoblasts
- granulocytes and monocytes
- formed only in the bone marrow
- megakaryocytes are also formed in the bone marrow, where they fragment and form the blood platelets (thrombocytes)
- Differentiate into:
1. neutrophils
2. basophils
3. eosinophils
4. monocytes - monocytes differentiate into macrophages
- myeloid-derived cell: dendritic cells
- Mast cells are derived from the differentiation of a unique set of bone marrow myeloid cells and share many characteristics with basophils
- particularly important is their ability to secrete inflammatory mediators and to function as phagocytes, which is any cell capable of phagocytosis
Life span of myeloid cells
Granulocytes
- 4 to 8 hours circulating in the blood after being released
- 4-5 days in the tissues where they are needed
Monocytes
- 10 to 20 hours in the blood
- once in the tissues they swell into macrophages, and live for months unless destroyed while performing their phagocytic functions
Lymphoid cells
- begin as lymphoblasts
- lymphocytes and plasma cells
- produced mainly in the various lymphogenous tissue- lymph glands, spleen, thymus, tonsils and various pockets of lymphoid tissue elsewhere in the body
- bone marrow and Peyer’s patches underneath the epithelium in the gut wall
- Differentiate into the following:
1. B lymphocytes (B cells)
2. T lymphocytes (T cells)
3. Natural killer (NK) cells
4. Plasma cells - plasma cells are not really a distinct cell type but differentiate from B lymphocytes during the immune response
- serve as recognition cells in adaptive immune responses and are essential for all aspects of these receptors
Life span of lymphocytes
- lymphocytes enter the circulatory system continually, along with the drainage of lymph from the lymph nodes and other lymphoid tissue
- after a few hours, they pass out of the blood back into the tissues via diapedesis
- they then re-enter the lymph and return to the blood again and again
- thus, there is continual circulation of lymphocytes through the body
- the lymphocytes have life spans of weeks or months, depending on the body’s need for these cells
Cytokines
- protein messengers from certain immune (and other) cells that regulate immune responses, by providing for cross talk between different immune system cells
- regulate host cell division (mitosis) and function in both innate and adaptive immune responses
- aren’t produces by distinct specialized glands, but instead by a variety of individual cells
- mainly act as autocrine and paracrine substances
- can also circulate in the blood to exert hormonal (endocrine) effects on distant organs and tissues involved in host defenses
- there is great redundancy in cytokine action- that is, different cytokines can have very similar effects
- Interleukins act between leukocytes and non-leukocytes
- Tumor necrosis factors elicit inflammatory responses to kill tumor cells
- Interferons interfere with processes like viral replication
Featured selected cytokines
- Interleukin 1
- Tumor necrosis factor - alpha
- Interleukin 6
- Interleukin 2
- Interferons Type I
- Interferons Type II
- Chemokines
Source of Cytokine: Antigen-presenting cells such as macrophages
Interleukin 1
Target cells: Helper T cells
Function: Stimulate IL-2 receptor expression
Tumor necrosis factor-alpha
Target cell: certain brain cells
Function: induce fever
Interleukin 6
Target cell: numerous systemic cells
Function: stimulate systemic responses to inflammation, infection and injury
Source of Cytokine: Most immune cells
Interleukin 2
Target cells: Helper T cells; cytotoxic T cells; NK cells; B cells
Functions: Stimulate proliferation (increase the amount of cells); promote conversion to plasma cells
Source of Cytokine: Most cell types
Interferon Type I
Target cells: most cell types
Functions: stimulate cells to produce antiviral proteins (innate response)
Source of Cytokine: NK cells and activated helper T cells
Interferon Type II
Target cell: NK cells and macrophages
Functions: Stimulate proliferation and secretion of cytotoxic compounds
Source of cytokine: Damaged cells, including endothelial cells (cells that make up the lining of the blood vessels)
Chemokines
Target cells: neutrophils and other leukocytes
Functions: Facilitate accumulation of leukocytes at sites of injury and inflammation
Immune system
A diverse collection of disease-fighting cells found in the blood and lymph and in tissues and organs throughout the body
Immunology
the study of the physiological defenses by which the body (the host) recognises itself from nonself (foreign matter). In the process, foreign matter, both living and nonlinving, is destroyed or rendered harmless
Functions of immune system
- Protect against infection by pathogens- viruses and microbes including bacteria, fungi and eukaryotic parasites
- isolate or remove foreign substances
- destroy cancer cells that arise in the body, known as immune surveillance
Innate immune response
defend against foreign substances or cells without having to recognise their specific identities. Innate responses include the response to injury or infection known as inflammation, and a family of antiviral proteins called interferons.
Adaptive immune response
depend upon specific recognition by lymphocytes of the substance or cell to be attacked
Bacteria
- Cellular structure: unicellular organisms that have an outer wall in addition to a plasma membrane but no intracellular membrane bound organelles
- Larger in size than viruses
- Can reproduce on its own both inside and outside the body
- Genetic material: have a single, circular DNA molecule
- Response to antibodies: can be killed or inhibited by antibiotics
- Cellular damage: cause tissue damage by releasing toxins
Viruses
- Cellular structure: consist of nucleic acids surrounded by a protein coat. No cell wall or membrane
- Smaller in size than bacteria
- Can only replicate by invading the host cell and directing it to synthesise the proteins required for viral replication. Has no enzymes for metabolism or ribosome for protein synthesis
- Genetic material: have either DNA or RNA, but not both
- Response to antibiotics: not affected by antibiotics, antiviral drugs are used instead
- Cellular damage: transform host cells into cancer cells
Neutrophils
When are they released
- in response to inflammation or infection
Functions
- Phagocytosis
- Release chemicals involved in inflammation (vasodilators, chemotaxins, etc.)
Basophils
When are they released
- Allergic reactions, parasitic infections, inflammation
Functions
- Release histamine and other chemicals involved in inflammation into the blood
Eosinophils
When are they released
- Allergic reactions, parasitic infections, asthma and atopic diseases
Functions
- Destroy multicellular parasites
- Participate in immediate hypersensitivity reactions
Monocytes
When are they released
- Infections or inflammation. Released into the blood and migrate to tissues where they differentiate into macrophages or dendritic cells
Functions
- Enter tissue and transform into macrophages
Macrophages
When are they released
- Present in tissues
Functions
- Phagocytosis
- Extracellular killing via secretion of toxic chemicals
- Process and present antigens to helper T cells
- Secrete cytokines involved in inflammation, activation and differentiation of helper T cells, and systemic responses to infection or injury (the acute phase response)
