Cells of immune system

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1. Cells of immune system Muhammad Kamil Khan Microbiologist

2. Introduction The cells that serve specialized roles in innate and adaptive immune responses are phagocytes, dendritic cells, antigen specific lymphocytes, and various other leukocytes that function to eliminate antigens. The cells of innate and adaptive immune system are normally present as circulating cells in the blood and lymph, means collections in lymphoid organs and as scattered cells in all tissues. The HSC is considered adult stem cell because it can differentiate into all the types of blood cells. The immune system faces numerous challenges to generate effective protective responses against infectious pathogens.

3. First, the system response rapidly to small numbers of many different microbes that may be introduced at any site in the body. 1 HSC is present per 5*10 4 cells in the bone marrow and their numbers are strictly con trolled by a balance of cell division, death, and differentiation. Second, in the adaptive immune response, very few naive lymphocytes specifically recognize and respond to any one antigen. Third, the effectors mechanisms of the adaptive immune system (antibodies and effector T cells) activate and destroy microbes.

4. The ability of the immune system to meet these challenges and to perform its protective functions mainly dependent on several properties of its cells and tissues. Mainly we have three types of blood cells i.e RBC (erythrocytes), WBC (leucocytes) and platelets (thrombocytes). The major cells and tissues of the immune system and their important roles are the following: Macrophages Neutrophils Monocytes Dendritic cells Lymphocytes Eosinophils Mast cells Basophiles Natural killer cells Platelets

5. Hematopoiesis

6. 1. Eosinophils Motile phagocytic and granulocytic cells having bilobed nucleus that can migrate from the blood into the tissue spaces. Volume in blood 3%. GM,CSF,IL3&5 involved for the maturation of eosinophils play important role in the defense against multicellular parasitic organisms, including worms. They can be found clustering around invading worms, whose membranes are damaged by the activity of proteins released from eosinophilic granules. Eosinophils may also secrete cytokines that regulate B and T lymphocytes thus influencing the adaptive immune response. Also involved in Variety of inflammatory reaction and sometime leading to asthma and allergic symptoms. Some eosinophils are normally present in peripheral tissues, especially in respiratory system, gastrointestinal and genitourinary tracts.

7. 2. Macrophages Phagocytic APC’s Larger than monocytes Activate against pathogen because they have TLR’s. Present in mouth for one year. Ingest the death host cell thus clean up process.

8. 3. Neutrophils Having multilobe nucleus i.e up to 5 lobe called polymorphonuclear leucocytes. Volume in blood 60-70%. Number increased by G-CSF during infection and inflamation . Spherical 12-15 micrometer in diameter. Life span 8 days in tissue 6-12hr in circulation. 1 st cell type that defense against any infectious agent i.e bacteria and fungi. Two granules i.e 1. azurophillic granules (cathelicidins and defensins) that are microbicidal in nature. 2. specific granules lysozyme, collegenase and elastase . These granules do not stain strongly with either basic or acidic dyes ( hematoxylin and eosin, respectively), which distinguishes neutrophil granules from basophils and eosinophils.

9. 4. Monocytes Monocytes make up about 5% to 10% of white blood cells having 15micrometer diameter and are a heterogeneous group of cells that migrate into tissues and differentiate into a diverse array of tissue-resident phagocytic cells, including macrophages and dendritic cells During hematopoiesis in the bone marrow, granulocyte- monocyte progenitor cells differentiate into promonocytes , which leave the bone marrow and enter the blood, where they further differentiate into mature monocytes.

10. 5. Dendritic Cells Part of APC’s and myeloid lineage of hematopoietic cells that are developed from the differentiation of monocytes. Flt3 that bind to flt3 tyrosine kinase receptors on the precursor cells to produce the dendritic cells. activate naive T cells therefore it linked both innate and adaptive immunity. Widely distributed in lymphoid tissue, mucosal epithelium and organ parenchyma. Two types Conventional DCs present in skin, mucus, organ parenchyma. It can be activated against microbes and display each antigen for T cell. Plasmacytoid DCs show response against viral infection.

11. Follicular Dendritic Cells FDCs are not derived from precursors in the bone marrow and are unrelated to the dendritic cells that present antigens to T lymphocytes. FDCs trap antigens complexed to antibodies or complement products and display these antigens on their surfaces for recognition by B lymphocytes. Important for the selection of activated B lymphocytes whose antigen receptors bind the displayed antigens with high affinity.

12. 7. Mast Cells Mast cells are bone marrow–derived cells that are present in the skin and mucosal epithelium and contain abundant cytoplasmic granules filled with cytokines histamine, and other mediators. Stem cell factor is a cytokine that is essential for mast cell development. Normally, mature mast cells are not found in the circulation but are constitutively present in healthy tissues, usually adjacent to small blood vessels and nerves. Human mast cells vary in shape and have round nuclei, and the cytoplasm contains membrane-bound granules which contain acidic proteoglycans that bind basic dyes. Mast cells express IgE and IgG antibodies. These antibodies on the mast cell surface also bind antigen, signaling events are induced that lead to release of the cytoplasmic granule contents into the extracellular space. The released contents of the granules, including cytokines and histamine, promote changes in the blood vessels that cause inflammation. Mast cells also express other activating receptors that recognize complement proteins, neuropeptides , and microbial products. Mast cells provide defense against helminths .

13. 8. Basophiles Basophils are blood granulocytes with many structural and functional similarities to mast cells. Basophils are derived from bone marrow progenitors, mature in the bone marrow, and circulate in the blood. Volume less than 1% in blood. Normally they are not present in tissues but may be recruited to some inflammatory sites. Basophils contain granules that bind basic dyes and they are capable of synthesizing many of the same mediators as mast cells. Like mast cells, basophils express IgG and IgE receptors, bind IgE , and can be triggered by antigen binding to the IgE because basophil numbers are low in tissues, their importance in host defense and allergic reactions is uncertain.

14. 8. Lymphocytes Unique cells involved in adaptive immune system. Small, non- phygocytic and mono-nuclear leucocytes. Originated from bone marrow pluripotent stem cells and mature inside bone marrow thymus In healthy adult total amount of leukocytes are 5*1 11. Two types i.e B cells and T cells

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