# 58 The immune system - Phagocytes

Credit: Pass My Exams.

The human immune system is made up of the organs and tissues involved in destroying pathogens inside the body.
There are 2 main groups of cells involved:
• phagocytes: ingest and digest pathogens or infected cells;
• lymphocytes: 
- recognise specific pathogens through interaction with receptors in their cell surface membranes
- respond in one of several ways, for example by secreting antibodies.


Phagocytes

Phagocytes are produced in the bone marrow by the mitotic division of precursor cells. This produces cells that develop into monocytes or neutrophils.

Monocytes are inactive cells which circulate in the blood. They eventually leave the blood, often as the result of encountering chemical signals indicating that bacteria or viruses are present. As monocytes mature, they develop more RER, Golgi apparatus and lysosomes. When they leave the blood they become macrophages.

They engulf bacteria by endocytosis and digest them inside phagosomes. Monocytes and macrophages can live for several months.

Macrophage engulfing bacteria. Credit: Biology Helper

Similar precursor cells in bone marrow produce neutrophils. These also travel in blood. They leave the blood in large numbers at sites of infection and engulf and digest bacteria in a similar way to macrophages. A neutrophil lives for only a few days.

 Coloured scanning electron micrograph (SEM) of a neutrophil white blood cell (green)
engulfing methicillin-resistant Staphylococcus aureus bacteria (MRSA, pink).
Credit: Science Photo Library.

Phagocytes are able to act against any invading organisms. Their response is non-specific.

Several different types of cell, including macrophages, place antigens of pathogens they have encountered in their cell surface membranes, where there is a good chance that a B-lymphocyte or T-lymphocyte may encounter them. These cells are called antigen-presenting cells (APC).

Syllabus 2015   • The immune system (a) [PA] recognise phagocytes and lymphocytes under the light microscope; (b) state the origin and describe the mode of action of phagocytes (macrophages and neutrophils);

Syllabus 2016 - 2018 11.1  The immune system The immune system has non-specific and specific responses to pathogens. Auto-immune diseases are the result  of failures in the system to distinguish between self and non-self. a)   state that  phagocytes (macrophages and neutrophils) have their origin in bone  marrow and describe their mode of action c)   describe and explain the significance of the increase in white blood cell count  in humans with infectious diseases and leukaemias