Complement and dendritic cell

Second-line defences

The innate immune system gets to work fast

Once barriers like the skin and mucosal membranes are breached, the immune system’s second line of defence against invading pathogens is its non-specific (innate) response.

This response is rapid, short-lived and generic, and it lacks any memory of previous infections. Through pattern recognition receptors, cells involved in the innate response can recognise pathogens like bacteria and viruses but can’t distinguish between, say, the viruses that cause measles and flu. Important components of innate immunity include:

Natural killer cells

True to their name, natural killer (NK) cells’ natural inclination is to kill everything they meet. What stops them is the recognition of self markers – major histocompatibility complex (MHC) proteins – on the surface of the cells they make contact with. Virus-infected cells and cancer cells have fewer of these markers and so are more likely than healthy self cells to be killed. NK cells release chemicals called cytokines, which alert and attract other immune cells.


Cells that engulf – or phagocytose – microbes or other cells that are infected, damaged or dying. Most cells are capable of phagocytosis, but the immune system employs specialist phagocytes like macrophages and neutrophils to deal with foreign matter. They begin by wrapping themselves around the offender, enclosing it in a vesicle called a phagosome before breaking down the contents with hydrolytic enzymes. The remains are presented (so-called antigen presentation) to other specialised immune cells that initiate a more targeted immune response.


When the receptors of cells involved in the non-specific immune system are engaged by pathogens, the cells release molecules that trigger inflammation. Increased blood flow brings in more cells to deal with the problem, and also leads to swelling and pain, which alert you to the fact that something is wrong. Greater blood flow also causes an increase in temperature, which can inhibit the replication of some bacteria and viruses.


A set of around 30 proteins in the blood plasma that can be activated by the presence of microbes or antibody–antigen complexes. Complement (see drawing, above left) can destroy pathogens and activate phagocytic cells.

Lead image:

Complement and dendritic cell.

Bret Syfert, ‘Big Picture’ CC BY


About this resource

This resource was first published in ‘Immune System’ in January 2015.

Microbiology, Immunology
Immune System
Education levels:
16–19, Continuing professional development