2.5.1 Cell Recognition

Key Features of Cell Recognition:

1. Cell Surface Proteins and Glycoproteins:

• Cell membranes contain unique proteins and glycoproteins that act as antigens.
• These antigens enable the immune system to identify:
• Self-cells: Normal body cells.
• Non-self cells: Pathogens, infected cells, or foreign material (e.g., transplanted tissue).
• Abnormal cells: Cancerous or damaged cells.
• Toxins: Produced by pathogens.

2. Role of Antigens:

• An antigen is a protein or glycoprotein on the surface of a cell or virus that triggers an immune response.
• Each cell has specific antigens, making it unique.

3. Recognition by Lymphocytes:

• T lymphocytes (T cells):
• Recognise antigens presented on the surface of infected or abnormal cells.
• Trigger an immune response, including the destruction of infected cells.
• B lymphocytes (B cells):
• Bind to specific antigens using antibodies.
• Produce antibodies to neutralise pathogens.

Process of Cell Recognition:

1. Detection of Foreign Antigens:

• Lymphocytes patrol the body and examine the antigens on the surface of cells.
• Foreign antigens are recognised as non-self, initiating an immune response.

2. Clonal Selection:

• Specific lymphocytes that recognise the foreign antigen are activated and divide rapidly.
• This process ensures a targeted response to the pathogen.

3. Destruction of Non-Self Cells:

• Cytotoxic T cells destroy infected cells.
• Antibodies produced by B cells bind to pathogens, marking them for destruction by phagocytes.

Applications of Cell Recognition:

1. Immune Defence:

• Identifying and eliminating pathogens (e.g., bacteria, viruses, fungi).
• Neutralising toxins.

2. Transplant Rejection:

• Transplanted organs have foreign antigens, which the immune system may recognise as non-self, leading to rejection.

3. Autoimmune Diseases:

• Occur when the immune system mistakenly recognises self-antigens as foreign, attacking the body's own cells (e.g., type 1 diabetes, rheumatoid arthritis).