How Does the Immune System Recognise Cells?
Learn how antigens help the immune system recognise cells, and how T cells and B cells produce cell-mediated and humoral responses in AQA A Level Biology.
The immune system recognises cells by the molecules on their surfaces. Some of these molecules act as antigens: they can be recognised by immune cells and trigger an immune response. Your own cells normally carry familiar surface molecules, while pathogens, transplanted cells and abnormal cells may carry antigens that the immune system recognises as different.
How recognition starts
A phagocyte can engulf a pathogen and display its antigens on its surface. It is then an antigen-presenting cell. A helper T cell with a complementary receptor binds to the displayed antigen. This helps activate other immune cells. Recognition is specific: a receptor must fit the antigen it responds to.
Cell-mediated response
The cell-mediated response involves T lymphocytes. Activated helper T cells stimulate cytotoxic T cells, which destroy infected or abnormal body cells displaying the relevant antigen. This is particularly useful when a pathogen is inside a cell, where circulating antibodies cannot reach it directly.
Humoral response
The humoral response involves B lymphocytes and antibodies in body fluids. A B cell with a complementary receptor binds to an antigen. With stimulation from a helper T cell, it divides to form a clone. Some cells become plasma cells, which secrete antibodies specific to that antigen. Antibodies bind to pathogens, causing them to clump together (agglutination) or making them easier for phagocytes to engulf. Other B cells become memory cells, helping produce a faster response if the same antigen is encountered again.
Exam mistake to avoid: cell-mediated does not mean “any response involving cells”, and antibodies do not directly kill pathogens. Distinguish T cells acting on affected cells from B cells producing antibodies that bind to antigens.
