• Originate from the stem cells in the bone marrow
  • Stem cells retain the ability to divide by mitosis forming large numbers of cells which differentiate into specialized cells
  • There are two groups of these cells involved in defense:
    • Phagocytes: neutrophils and macrophages
    • Lymphocytes


  • For secondary defense against disease – phagocytosis
  • If a pathogen enters it must be killed before it has time to reproduce and cause symptoms of disease. This is the role of the phagocytes.
  • Are continuously produced by the bone marrow throughout life.
  • They are stored there and leave in the blood to be distributed around the body.
  • They are scavengers and involved in the non-specific response.


2 types of phagocyte:


  • The most common type of white blood cell (60%)
  • Smaller than macrophages
  • Travel throughout the body
  • They have a multi-lobed nucleus
  • Granulocytes – granular cytoplasm
  • Granules contain degradative enzymes
  • They are very short lived few hours in blood, few days in tissue (half life in blood is about 12 hours – perhaps an evolutionary response to the possible infection of parasites living inside this type of cell)
  • They are attracted to areas of cell and tissue damage, probably by chemicals released by the ruptured cells
  • Able to squeeze through walls of blood capillaries and move about tissue spaces (diapedesis)
  • Their numbers increase rapidly during an infection, when they are released from stores in the bone marrow.


The cell with the lobed nucleus [in the centre] is a neutrophill; a type of a phagocyte.



  • Comprise 4% of white blood cells
  • Larger than neutrophils
  • Bean shaped nucleus
  • Agranulocytes – no granules in cytoplasm
  • When they leave the blood they act as neutrophils or they differentiate into macrophages – larger cells that patrol tissues especially lungs, liver, spleen and lymph nodes
  • Can engulf larges particles e.g. malarial parasite (Plasmodium)
  • Some are stationary and line blood spaces in organs such as liver (Kupffer cells)
  • Cytoplasm contains numerous larger lysosomes
  • Long lived – few days in blood, months or even years in tissues
  • Can proliferate in tissues


The cell with a horseshoe-shaped nucleus is a monocyte; from the family of phagocytes.


  • Produced before birth and lave the bone marrow to fill the lymphoid system
  • They are generally not phagocytic but instead secrete antibodies and the hormone-like cytokines.
  • There are two types of lymphocytes:
    • T lymphocytes (often called T cells)
    • B lymphocytes (often called B cells)


The cell with a uniformly circular nucleus is a lymphocyte; it can either by a B-lymphocyte or T lymphocyte. Notice its size; which roughly equals that of a RBC.


  • The comparatively huge cell in the centre is a plasma cell; it is formed upon differentiation [something close to specialization] of B-lymphocytes during an immune response.
  • Both of these must go through a maturation process which starts just before birth.


 Describe the origin, maturation and mode of action of phagocytes;

originate from stem cells in the bone marrow  stem cells divide by mitosis

  • cells differentiate into specialised cells and stored in the bone marrow
  • examples are neutrophils/macrophages/monocytes/polymorphs
  • Monocytes leave the bone marrow before being fully functional and attain maturity in the blood stream.
  • After 40-60 hours of circulation by a mature monocyte, it settles in the tissue and increases in size slightly, now a macrophage [e.g. the alveolar macrophage in the alveoli].
  • On the other hand neutrophils do not leave bone marrow until maturity
  • involved in non-specific responses
  • Phagocytes/neutrophils/macrophages act between initial infection by bacteria and immune response.
  • All three types of phagocytes share the same job, i.e. phagocytosis [killing by engulfing]. The mechanism is described below…



  • phagocytes act between infection and immune response
  • include macrophages and neutrophils
  • presentation/treatment of antigen by macrophages
  • attracted to site of infection by histamine proteins produced by mast cells – chemotaxis i.e. the process by which cells are attracted to the bacteria.
  • it may be by the materials released by the bacteria or opsonization, and opsonin is a type of antibody that renders bacteria more susceptible to phagocytosis [which may be by coating of the outer membrane of bacteria], or by agglutination [via agglutinins which ‘clump’ together bacteria at wound/area of infection].
  • complements/lymphokins/microbial components attract neutrophils
  • accumulate at wounds/site of infection
  • squeezing through capillary walls (diapedesis)
  • binding of neutrophils direct to bacteria/receptors
  • compliment/antibody/opsonins facilitate binding surface membrane infold/invaginate to surround bacteria/antigen forming a phagosome
  • lysosome fuse with the phagosome
  • forming a phagolysosome
  • bacteria killed by toxic free radicals/hydrogen peroxide
  • intracellular digestion (lysosomal enzymes)
  • products of digestion absorbed into cytoplasm


Distinguish between phagocytes and lymphocytes


  • macrophages and neutrophils involved in non-specific response/engulfing the bacteria/pathogen/antigen  can squeeze through capillary walls



  • B and T cells
  • involved in specific response
  • remain in circulatory system and lymphatic system – produce antibodies


Explain the meaning of the term immune response, making reference to the terms antigen, self and non-self;

  • An immune response is a body’s reaction to an antigen which a marker molecule in the cell surface membrane of foreign bodies that sets off an immune response.
  • The discrimination between self and non-self cells is an integral part of our immune system.
  • This distinguishing is possible by the presence of glycoproteins or other types of recognition molecules.
  • Our body functions normally when no abnormal recognition protein/molecule is encountered by our immune system but when foreign particles exhibiting recognition proteins that our not normally found in our body are encountered then our body’s defense mechanism starts rolling, i.e. an immune response is initiated.
  • This can be a product of two scenarios:

Tissue Transplant leading to tissue rejection because the donor can never have the same recognition proteins as the acceptor. That‘s why, following up a tissue transplant, the acceptors are usually at prescriptions that suppress their immune system from starting an immune response.

Invasion of Bacteria and other foreign particles


Distinguish between B- and T-lymphocytes in their mode of action in fighting infection, and describe their origin and functions;


  • The lymphocytes are the backbone of our immune system without them our immune system would be of no use.
  • Moreover the two main types of lymphocytes, the T and B lymphocytes are interdependent that is why a person infected by the AIDS virus has a severely depleted immune system due to the destruction of T-Lymphocytes.
  • Both of these cells originate in the stem cells of the bone marrow.
  • While the B-cells mature in the bone marrow, the T-cells move as precursors [non-functional form] to the thymus gland where they mature and T-lymphocytes which are over reactive and can cause harm to the body’s own cells are also destroyed here.



  • B-Lymphocytes can differentiate into Memory Cells and Plasma Cells.
  • memory cells act as an immunological memory of the antigen in question after the body is exposed to it for the first time and has countered it and remains in the blood stream for months or even years to initiate a more severe secondary immune response when that antigen is encountered again.
  • plasma cells are there to produce antibodies against a specific antigen and thus have a more developed and extended Rough Endoplasmic Reticulum and Golgi Body.



  • There are of two main types; Helper Cells and Cytotoxic Cells.
  • T- Helper Cells sells act like assistants to the immune system, when they come across an immune cell such as a Dendritic Cell or Macrophage displaying {on their cell membrane like war trophies, after they have destroyed an antigen bearing cell =)] an antigen which they are also specific to, they form a temporary bond at the T-Cell Receptor (TCR) which can be thought of as a binding site on an enzyme and release chemicals called cytokines which simulate other immune system cells like
  • Macrophages and Lymphocytes to take action against the intruder.
  • This simulation can be done in the form of B-Lymphocytes activation and differentiation to produce plasma cells.
  • While Cytotoxic Cells exclusively scan the cell membranes of the bodies’ own cells for changes in the Major Histocompatibility Complex [it can be thought of as a genomic region in a cell responsible for protein synthesis and displaying of the proteins encoded inside the cell on the cell surface membrane], malignant growth [as in cancer], cell invasion by viruses and other intercellular parasites alter the MHC of the cell [i.e. other/more types of proteins will started to get synthesized] and thus Cytotoxic Cells act on it and destroy the cell as a whole.

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