Quick questions on Innate and adaptive immune responses (QCE Biology Unit 2)

Physical and chemical barriers stop pathogens entering the body in the first place. Non-specific.

Triggered when pathogens cross the first barrier. Non-specific (responds the same way to any pathogen) but rapid (minutes to hours).

Specific (recognises one antigen) and slow on first exposure (5 to 10 days), but produces memory cells for faster future responses.

Primary response. First exposure to an antigen. Slow (5 to 10 days to peak), produces lower antibody levels, generates memory cells. The host typically experiences symptoms.

- Damaged cells and mast cells release histamine, prostaglandins and cytokines. - Local blood vessels dilate (vasodilation) and become more permeable. - Plasma leaks into the tissue, carrying clotting factors and complement proteins.

- Neutrophils arrive first (within hours); macrophages follow (within a day or two). - The phagocyte engulfs the pathogen, enclosing it in a phagosome. - The phagosome fuses with a lysosome to form a phagolysosome.

- Natural killer (NK) cells. Kill virally infected and cancerous host cells by releasing perforin and granzymes. - Complement system. Plasma proteins that form pores in bacterial membranes (membrane attack complex), coat pathogens for phagocytosis (opsonisation) and amplify inflammation. - Interferons. Released by virally infected cells; signal neighbouring cells to make antiviral proteins.

Phagocytes and dendritic cells display fragments of digested pathogens on MHC class II molecules and travel to a lymph node. There they present the antigen to T cells.

- Each B cell carries a unique B cell receptor (membrane-bound antibody) specific for one antigen. - A B cell that binds its matching antigen, with help from a helper T cell, is activated and undergoes clonal expansion. - Most activated B cells differentiate into plasma cells that secrete large amounts of antibody.

- T cells recognise antigen presented on MHC molecules. - Helper T cells (CD4+). Recognise antigen on MHC II on antigen-presenting cells. Secrete cytokines that activate B cells, cytotoxic T cells and macrophages.

First exposure to an antigen. Slow (5 to 10 days to peak), produces lower antibody levels, generates memory cells. The host typically experiences symptoms.

Second (or later) exposure to the same antigen. Memory cells respond rapidly (within hours to a few days), produce higher antibody levels (mostly IgG) and often clear the pathogen before symptoms appear. This is the basis of long-term immunity and vaccination.

Innate immunity is rapid and powerful; many infections never reach the third line of defence.

Antigen is the foreign substance recognised; antibody is the protein the host makes in response.

Helper T cells coordinate; cytotoxic T cells kill. They are different cell types with different surface markers (CD4 vs CD8) and different MHC recognition.