Cells of the Immune System

These are undifferentiated cells that have the potential to become a variety of blood cells. They initially become:

Myeloid stem cells become:

Lymphoid Stem Cells travel to the thymus gland and differentiate into:

B lymphocytes (B cells) (mature in the bone marrow) that differentiate into:
- Plasma Cells
- Memory B Cells
T lymphocytes (T cells) (mature in the thymus) that differentiate into:
- CD4 cells (T helper cells)
- CD8 cells (Cytotoxic T Cells)
Natural Killer Cells

Immature red blood cells
Remnant RNA material called reticulum in cytoplasm
Released from bone marrow
Normally 1% of red blood cells are reticulocytes
Higher percentage indicates rapid turnover of blood, such as in:
- Acute blood loss
- Haemolysis
Loose their reticulum after a few days – becoming:

RBCs contain large quantities of haemoglobin, and carry oxygen from the lungs to the tissues
They survive about 3 months

Important in responding to parasites
They normally sit waiting in tissues
Contain granules with:
- Proinflammatory cytokines
- Chemicals toxic to pathogens, such as “major basic protein“
They have two functions:
- Exocytosis of their granules
- Phagocytosis (but less than neutrophils and macrophages)

Also called B cells
Mature in the bone marrow
Cell surface lined with antibodies
Each B cell has a specific type of antibody for a specific antigen
There are millions of different possible antibodies and therefore millions of different specific B cells
B cells can only recognise antigens that match their antibodies
Once activated B cells become either:
- Plasma cell – secrete antibodies (also called immunoglobulins)
- Memory B cell – await a later infection with the same pathogen

Also called T cells
Pro-thymocytes leave the bone marrow and travel to the thymus gland
Mature in the thymus gland into T cells
Cell surface lined with T-cell receptors
Each T cell has a specific type of T-cell Receptor (TcR) for a specific antigen
- CD4 cells have CD4 TcRs that recognise MHC class 2 receptors
- CD8 cells have CD8 TcRs that recognise MHC class 1 receptors
CD4 cells become T-helper cells when activated
T helper cells help other cells become activated, including CD8 cells
CD8 cells become cytotoxic T cells when activated
Cytotoxic T cells destroy infected cells via
- Granule exocytosis causing destruction of the cell
- Activating the Fas pathway causing cell apoptosis

Originate from several cell lines
Specialist dendritic cells found in the skin are called Langerhans cells
Take up antigens, process them and display them on their cell surface
Have both MHC class I and class II molecules
Act as messengers, taking antigens from infected tissues to lymph nodes to activate T cells