White Blood Cells (HL IB Biology)

What do phagocytes do?

• Phagocytes are white blood cells that are produced continuously in the bone marrow
• They are responsible for removing dead cells and invasive microorganisms; a non-specific immune response
• Phagocytes move in an amoeboid movement to the site of infection and attach to pathogens
  • The cell surface membrane of the phagocyte extends out and around the pathogen, engulfing it by endocytosis
• They then digest the pathogen using enzymes which are stored within lysosomes (in their cytoplasm)

Phagocytosis diagram

Phagocytic cells ingest pathogens and digest them using enzymes

What are lymphocytes?

• There are two types of lymphocyte that play a particular role in the specific immune response
  • T cells
  • B cells
• Note that lymphocytes are a type of white blood cell found both in the lymph nodes and circulating in the blood

T cells

• T cells, sometimes known as T lymphocytes, are produced in the bone marrow and finish maturing in the thymus, which is where the T in their name comes from
• Mature T cells have specific cell surface receptors called T cell receptors
• These receptors have a similar structure to antibodies and are each specific to a particular type of antigen

Production of T cells diagram

Mature T cells have many different types of receptor on the cell surface membrane; these receptors will bind to different antigens on antigen presenting cells

• T cells are activated when they encounter and bind to their specific antigen on the surface of an antigen-presenting cell 
  • This antigen-presenting cell might be a macrophage, an infected body cell, or the pathogen itself
• These activated T cells divide by mitosis to increase in number
  • Dividing by mitosis produces genetically identical cells, or clones, so all of the daughter cells will have the same type of T cell receptor on their surface

B cells

• B cells, also known as B lymphocytes, are a second type of white blood cell in the specific immune response
  • B cells remain in the bone marrow as they mature, hence the B in their name
• B cells have many specific receptors on their cell surface membrane
  • The receptors are in fact antibodies, and are known as antibody receptors
  • Each B cell has a different type of antibody receptor, meaning that each B cell can bind to a different type of antigen

Production of B cells diagram

Mature B cells each have different types of antibody receptors on their cell surface membrane

• If the corresponding antigen enters the body, B cells with the correct cell surface antibodies will be able to recognise it and bind to it 
  • When the B cell binds to an antigen it forms an antigen-antibody complex
• The binding of the B cell to its specific antigen, along with the cell signalling molecules produced by T helper cells, activates the B cell
• Once activated, the B cells divide repeatedly by mitosis, producing many clones of the original activated B cell
• There are two main types of B cell
  • Effector cells, which differentiate into plasma cells
    • Plasma cells produce specific antibodies to combat non-self antigens
  • Memory cells
    • Remain in the blood to allow a faster immune response to the same pathogen in the future