Two layers of defence
The immune system has two linked parts. Innate immunity is present from birth, acts within minutes to hours, and responds in a similar way to many invaders. Adaptive immunity takes days to build up, is aimed at one specific antigen, and leaves memory.
Innate defences
- Barriers. Skin, mucus, stomach acid, tears and the normal microbiota block entry.
- Phagocytes. Neutrophils and macrophages engulf microbes. They recognise common microbial patterns (for example LPS) through pattern recognition receptors.
- Inflammation. Chemical signals widen vessels and bring in more cells.
- Complement. A set of blood proteins that tag microbes and can punch holes in them.
- Interferons. Proteins released by virus-infected cells that warn neighbours to raise antiviral defences.
- Natural killer cells. They destroy abnormal cells that lack normal marker molecules.
Adaptive defences
B cells carry receptors that bind antigens. Once activated, they become plasma cells that secrete antibodies. IgG is the most abundant antibody in blood, and IgA protects mucosal surfaces such as the gut. T cells come in two main kinds. Helper T cells (CD4) recognise antigen displayed on MHC II by antigen-presenting cells and direct the response. Cytotoxic T cells (CD8) recognise antigen on MHC I, found on nearly all nucleated cells, and kill infected cells.
Memory and vaccination
After a first exposure (the primary response), memory cells remain. A later exposure triggers a faster, stronger secondary response. Vaccines present an antigen in a safe form, so that memory forms without the person suffering the full disease. Active immunity comes from the body's own response, by infection or vaccination. Passive immunity is borrowed antibody, such as antibody that crosses the placenta, and fades because no memory is made.
Worked example
Follow a viral infection through the body, step by step.
- A virus infects a cell. The cell releases interferon, and neighbouring cells become harder to infect. This is innate and fast.
- A dendritic cell takes up viral protein and shows pieces on MHC II to a helper T cell. The helper T cell activates a B cell and a cytotoxic T cell that match the antigen.
- Plasma cells secrete antibodies that bind free virions. Cytotoxic T cells find infected cells showing viral pieces on MHC I and kill them.
- After clearance, memory B and T cells persist. A second encounter weeks later brings antibodies within a day or two rather than a week or more.