The Immune Response

• Infected cells release chemicals (e.g. histamine) attracting neutrophils
• Histamine also causes capillaries to become more leaky, meaning more fluid leaves the area of infection (making it red & swollen)
• This means more tissue fluid passes into the lymphatic system, leading pathogens towards macrophages waiting in the lymph nodes

• When pathogen enters body it is recognised as foreign by antigens on its outer membrane
• Antibodies attach to foreign antigens and membrane-bound receptors on phagocyte bind to antibodies
• Phagocyte then engulfs pathogen, folding the membrane inwards, trapping the pathogen in a phagosome
• Lysosomes fuse with the phagosome (forming a phagolysosome) and release lysins into it which digest the bacterium
• Harmless end products of digestion are then absorbed by the cytoplasm

Clonal selection

• Selection of correct T lymphocytes for immune response
• Antigen presentation first takes place by phagocytes and cells attacked by pathogen (don't fully digest pathogen & incorporate its antigens into cell surface to attract lymphocytes)
• T lymphocyte receptors bind to these antigens, activating the T lymphocyte

Clonal expansion

• T lymphocyte divides by mitosis to produce clones (and increase numbers to become effective) which then differentiate into different types of lymphocytes with different functions; 
• T helper cells - release cytokines which stimulate differentiation of B lymphocytes
• T memory cells - stay in the body for years and act as immunological memory, v. active in secondary response 
• T killer cells - search body for pathogens, attach to foreign antigens, and produce toxic substances

Clonal selection

• Correct B lymphocytes are first selected for (like T lymphocytes) the immune response by clonal selection

Clonal expansion

• T helper cells release cytokines which stimulate the differentiation of the B lymphocytes by cell signalling (T cells are signalling to B cells that there is a pathogen in the body)
• Activated B cell divides by mitosis into;
• B plasma cells - manufacture and release antibodies very quickly into the blood
• B memory cells - remain in the body for years and act as immunological memory, respond v. quickly to secondary response

• Manufacture and release antibodies into the blood, specific to the antigen
• Antibodies bind to the antigens on the surface of the pathogen , forming antigen-antibody complexes

Agglutination

• Large antibody binds many pathogens together
• The group of pathogens is too large to enter the host cell

Neutralisation

• Antibodies bind to pathogen's antigens (binding sites) thus preventing the pathogen from binding to and entering a host cell