The types of cells that are involved in the response depends on the type of stimulus. This is because each cell type is suited to combat the initiating stimulus. For example, neutrophils are equipped to destroy bacteria while eosinophils prefer parasites and allergic stimuli.
Neutrophils
Neutrophils belong to the granulocyte group of white blood cells (leukocytes). They have an eosinophilic (pink) staining, granular cytoplasm with lobulated nuclei (they are polymorphonuclear).
Neutrophils are the body's first response to bacterial infections and are recruited by chemoattractants. They are aggressive cells which may phagocytose bacteria. They degranulate when in contact with bacteria which releases bacteriocidal enzymes and free radicals which digest microbes and tissue debris, degrade bacterial walls and activate other inflammatory cells. Neutrophils die when they degranulate and so they don't live for very long.
Eosinophils
Eosinophils also belong to the granulocyte group of leukocytes. They are motile cells that are a bit larger than erythrocytes and have a very eosinophilic staining, granular cytoplasm with multiple nuclei (polymorphonuclear).
These cells are involved in parasitic and allergic responses. Degranulation results in the release of several products, including:
- Enzymes: such as peroxidases, collaginases and histaminases
- Toxic Proteins: such as major basic protein and eosinophil cationic protein which are toxic to parasites.
- Cytokines and chemokines: these attract more neutrophils to the area
- Leukotrienes: these increase vascular permeability and cause smooth muscle contraction.
Another member of the granulocyte group of leukocytes are the mast cells. These cells are located in the connective tissue around blood cells and respond to allergens. The cells contain immunoglobin molecules on their surface which act as receptors to antigens. When an antigen binds to the receptor molecules, the mast cells rapidly degranulate and this releases vasoactive substances such as:
- Heparin: which prevents thrombosis and blood stasis
- Histamine: which increases vascular permeability and acts as a chemoattractant for neutrophils and eosinophils.
- Leukotrienes: which cause an increase in vascular permeability and smooth muscle contraction.
Macrophages are mononuclear cells. They are large cells that have large nuclei and a pale-looking cytoplasm. They travel in the blood as monocytes and differentiate when they reach the tissues. Here they are the major tissue phagocytic cell and phagocytose cells, bacteria, fungi, protozoa, tissue debris etc. Macrophages may live for months.
They produce pro-inflammatory cytokines such as interleukin-1 and tumour necrosis factor.
Lymphocytes
Lymphocytes are also mononuclear cells and circulate the body through the blood and lymph. They circulate in an inactive state and are activated when they come in to contact with antigens. These cells are about twice the size of erythrocytes and have very large nuclei with little cytoplasm. They are very important cells that are involved in antibody formation and cell mediated immunity.
Lymphocytes have a long memory and may produce T cells and B cells.
Phagocytosis
A few steps are involved in the important process of phagocytosis. Firstly, the leukocyte recognises the microbe when it binds to its phagocyte receptors located on the surface of the cell. The phagocyte membrane then zips up around the microbe to completely engulf it and form a phagosome. Lysosomes from the leukocytes cytoplasm then fuse with the phagosome. The lysosomal enzymes then kill the microbe. This video shows a pretty cool representation of the process:
That's all for now, if you have any questions let me know in the comments section below :)
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