Introduction to the Immune System (A-level Biology)

Introduction to the Immune System

The immune system exists in many different forms across many different species. In this section, we will focus on understanding the immune system found in most mammalian species. The immune system works through a system of recognition mechanisms which enable it to distinguish native cells (self) from pathogens (foreign bodies), and subsequently eliminate these pathogens.

Overview of Immune System

Before we go into depth with various parts of the immune system, we will do a quick overview of the whole immune response, to understand how everything fits in to the puzzle.

The immune system can be divided into two major mechanisms:

Innate Immune System

This is the primary response:

1. Pathogen enters the blood.
2. The body detects the foreign antigens (chemical markers) on the surface of pathogens.
3. Neutrophils in the blood break down the pathogens by phagocytosis.
4. Macrophages in the lymph nodes also carry out phagocytosis.

Adaptive Immune System

This is the secondary response:

1. Phagocytes activate three types of T cells.

• • T-helper (Th) cells activate B cells and T-killer cells.
  • Cytotoxic T (Tc) cells kill pathogens.
  • T-memory (Tm) cells provide long-term immunity.

1. B cells (activated by Th) produce antibodies to break down pathogens.
2. B cells produce plasma cells which produce further antibodies.

We will first learn how the immune system recognises the difference between foreign pathogens and the body’s own cells.

Antigens

Antigens are molecules present on the surface of cells, which trigger an immune response. Antigens include proteins on cell surface membranes.

• The immune system detects antigens in order to identify a cell. Antigens are typically unique and specific to particular cells and are also specific to particular pathogens. The majority of antigens found in pathogens do not exist in normal cells.

Self and Non-Self

• The immune system doesn’t want to attack self-cells. The immune system does not want to attack the body’s own native cells (self). Attacking normal cells can lead to disastrous consequences, and diseases of the immune system which cause the immune system to target normal cells are known as autoimmune diseases. These autoimmune disorders can often be fatal for an organism.

• MHC proteins are found on self cells. Major Histocompatibility Complex (MHC) is a protein marker used to distinguish native cells (self) from foreign bodies (non-self). MHC proteins enable the immune system to be able to specifically attack foreign pathogens as well as abnormal cells, while sparing normal cells.

There are two types of MHC molecule:

• • MHC I molecules are found on normal body cells. They bind to endogenous antigens (normal antigens produced in normal cells) and display them on the cell surface membrane so that they can be identified as self-cells to the immune system. MHC class I molecules are also found on immunological cells and allow them to recognise each other and communicate with each other.

• • MHC II molecules are found on immune system cells. MHC II is found specifically in immunological cells such as macrophages, neutrophils, and dendritic cells. MHC class II molecules bind to foreign antigens to activate an immune response against a particular invading pathogen.

Autoimmune disorder

• Autoimmune disorders can occur. Autoimmune disorders are disorders in which the body’s own immune system destroys its own tissues.
• Autoimmune diseases can be acute or chronic. Certain autoimmune diseases are long term diseases that are relatively treatable, for example Type I diabetes. Other examples include rheumatoid arthritis which is an autoimmune disorder of the joints, and systemic lupus erythematosus, where virtually any organ or joint in the body can be affected.

Non-self and Rejection

• Sometimes the immune system is counterproductive. The immune system responds to non-self and foreign pathogens, but sometimes this can be counterproductive.
• Organ transplants can be rejected. Patients often need organ transplants. But the immune system of the host can immunologically reject the donor organ. Therefore when transplants are done, the donor organ must closely match the immunological antigen profile of the recipient individual.
• MHC genes vary between individuals. It is important to understand that MHC genes, as well as other antigen genes, are different between individuals, except identical twins. As such, foreign cells, even if they are human cells, can be recognised as non-self by the immune system. This causes the immune system to attack the foreign cells.

FAQs