The immune system

In a nutshell

The immune system is your body's defence system from foreign or 'non-self' cells. The first stage, known as innate immunity, involves phagocytosis. The second stage, known as adaptive immunity, involves T cells, B cells and the production of antibodies. This leads to the formation of memory cells which provide long-term protection from certain pathogens.

Antigens

Definition

Antigens are molecules that are able to stimulate an immune response when detected. This is because the immune system recognises them as 'foreign' or 'non-self'. Typically, antigens are proteins found on the surface of many cells: this includes pathogens, cancerous cells, infected cells and transplanted cells.

Biology; Microbiology and pathogens; KS5 Year 12; The immune system — 1. Pathogen with antigens (2. and 3.) on it's outer surface.

Phagocytes

Phagocytes are a type of white blood cell that engulf and digest pathogens in a process called phagocytosis.

Macrophages

Macrophages are a type of phagocyte that can engulf and digest pathogens, cancer cells, cellular debris and other foreign substances found in the body. They can also stimulate other immune cells, such as T cells, to carry out an immune response and are found in almost every tissue in the body. Macrophages, therefore, play an important role in innate and adaptive immunity.

Neutrophils

Neutrophils are a type of phagocyte found in the blood. They are normally one of the first responders in an immune response. They are able to protect the body from foreign cells through degranulation, which is the release of antimicrobial compounds, phagocytosis and the production of extracellular traps that bind and trap pathogens.

Phagocytosis

Phagocytosis is an innate defence that involves phagocytes engulfing foreign particles.

Biology; Microbiology and pathogens; KS5 Year 12; The immune system

1.	Chemotaxis	Pathogens (C) produce chemical attractants (B) that encourage immune cells, like phagocytes (A), to move towards them. This is known as chemotaxis.
2.	Activation	Phagocytes recognise foreign antigens on the surface of pathogens.
2.	Binding	Surface receptors on the phagocyte bind to molecules on the pathogen.
3.	Endocytosis	The cytoplasm of the phagocyte moves to surround the pathogen and engulfs it (F), creating a phagocytic vacuole. This is known as endocytosis.
4.	Digestion	Lysosomes (D), which are organelles that contain the enzyme lysozyme, fuse with the phagocytic vacuole to form a phagolysosome (H). This is where lysozymes break down the pathogen (G/I). (E shows the nucleus of the phagocyte)
5.	Antigen presentation	The phagocyte then presents the pathogen's antigens on its surface. It has now become an antigen-presenting cell and it can activate other cells in the immune system to warn them of the pathogen.

Lymphocytes

Lymphocytes are another type of white blood cell. They are the main cell type found in the lymph. Lymphocytes respond to the organism's own cells that contain non-self material, such as a bacterial infection. They are slower in action than phagocytes but can provide long-term immunity. Two types of lymphocytes include T lymphocytes and B lymphocytes. They are both important for the adaptive, or specific immune response.

Curiosity: There is also a third type of lymphocyte known as a natural killer cell. They are part of the innate immune system and have an important role in defence again viruses and tumour cells.

T cells

T cells (T lymphocytes) mature in the thymus. They have receptors on their surface that bind to complementary antigens presented by phagocytes and infected body cells. These antigen-presenting cells are distinguishable from healthy cells because T cells are trained to recognise foreign antigens. There are many different types of T cells.

Cytotoxic T cells

Cytotoxic T cells (T_C) are able to kill abnormal cells and infected body cells by producing a protein called perforin. Perforin makes holes in the cell membrane of these cells so they become freely permeable to all substances and this causes the cell to die.

Cell-mediated immunity

Cell mediated immunity is an adaptive immune response that involves T cells.

Biology; Microbiology and pathogens; KS5 Year 12; The immune system

1.	Pathogens (A) either invade body cells or are taken up by phagocytes during phagocytosis (B).
2.	The pathogen's antigens (C) are then presented on the cell-surface membrane by the phagocyte.
3.	Receptors on a type of T cell (E) known as a T helper cell (T_H)cell are complementary to these antigens (D).
4.	This binding of T cells to antigens activates the T cells to divide rapidly by mitosis. which forms genetically identical cells.
5.	These cloned T cells can then: 6. Develop into memory cells that will allow a rapid response to future infections by the same pathogen. 7. Stimulate phagocytes to engulf pathogens by phagocytosis. 8. Stimulate B cells to divide and secrete antibodies. 9. Activate T_c cells.

B cells

B-cells (B lymphocytes) mature in the bone marrow and are a type of white blood cell associated with humoral immunity. Humoral immunity involves the production of antibodies. Antibodies are proteins that bind to antigens with a complementary shape forming an antigen-antibody complex.

Humoral immunity

Humoral immunity is an adaptive immune response that involves B cells and the production of antibodies.

Biology; Microbiology and pathogens; KS5 Year 12; The immune system

1.	The B cell takes up the surface antigens (A/B) of an invading pathogen.
2.	These antigens are then processed (C) and presented on the surface of the B cell (E). (D shows an antibody on the surface of a B cell.)
3.	Helper T cells (F) that were activated in cell-mediated immunity attach to the antigens that are being presented. This activates the B cell.
4.	The B cell is now activated and divides rapidly by mitosis to produce clones of plasma cells (G). This is known as clonal selection.
5.	These cloned plasma cells produce and secrete specific antibodies that are complementary to the pathogen's antigen.
6.	The antibody can attach to the pathogen and this eventually leads to its destruction.
7.	Some B cells develop into memory cells (H). These memory cells respond to future infections by the same pathogens as they rapidly divide and develop into plasma cells that can secrete antibodies. This is known as a secondary immune response.

Primary and secondary responses

Primary immune response

The primary immune response occurs when the body encounters a pathogen for the first time. It is initially slow as there aren't many B cells that can make the specific antibody needed to bind to the antigens on the pathogen. During this delay, the infected person will show symptoms of infection.

Eventually, enough antibodies will be produced and the immune system will clear the infection. Both T and B cells will produce memory cells which remain in the body for a long time that can recognise the pathogen should there be a future infection. The person is now said to be immune as their immune system can respond quickly to the pathogen.

Secondary immune response

If the immune system encounters the pathogen again, the secondary immune response will be quicker and stronger. The secondary immune response is faster as clonal selection happens more quickly and activated B-cells can divide to produce plasma cells which make antibodies. Memory T cells are activated and quickly get rid of pathogens before symptoms even show.

Biology; Microbiology and pathogens; KS5 Year 12; The immune system — 1. First exposure to the antigen, 2. Second exposure to the antigen. A: Concentration of antibody in blood, B: Time/days, C: Primary response D: Time interval, E: Secondary response.

Autoimmune diseases

Sometimes the immune system doesn't function correctly and recognises normal body cells as 'non-self'. The immune system then produces an immune response against these cells and damages them. This is known as an autoimmune disease.

Examples

Systemic sclerosis is an autoimmune disease caused by the immune system producing an antibody that targets self enzymes involved in connective tissue formation. It causes symptoms such as inflammation and tight joints caused by hardened connective tissue. It can also affect internal organs such as the heart, lungs and digestive system.
Rheumatoid arthritis is another autoimmune disease caused by the immune system attacking joints. This causes pain and inflammation.