The Human Immune System

Two groups of white blood cells work together: phagocytes engulf and digest pathogens, while lymphocytes provide a response targeted at particular antigens. This combines a general defence with a specific one.

Phagocytes engulf and digest pathogens

A phagocyte surrounds a pathogen with its cell membrane and takes it into the cell. Digestive enzymes then break down the pathogen. This process is called phagocytosis.

Phagocytosis is a non-specific response: phagocytes use the same basic engulfing and digesting process against different pathogens. They do not need to produce a different antibody for each type of pathogen.

Antibodies target particular antigens

Pathogens have molecules called antigens on their surfaces. These can be recognised as foreign by the immune system. Different pathogens can carry different antigens.

B-lymphocytes produce antibodies, which are Y-shaped proteins. An antibody has binding sites with a shape complementary to a particular antigen: the shapes fit together. It can therefore attach to that antigen, but not to an antigen with an unsuitable shape. This is why antibody production is a specific immune response.

For example, an antibody that fits an antigen on one pathogen will not necessarily bind to a different pathogen. Protection against one infection therefore does not automatically protect someone against every other infection.

Antibodies can link pathogens together into clumps, a process called agglutination. Clumped pathogens cannot move as easily. Antibody binding also helps mark pathogens for destruction by phagocytes, which engulf and digest them. The specific and non-specific responses therefore cooperate rather than acting as separate defences.

Neutralising harmful toxins

Some bacteria release toxins, which are poisonous substances that damage the body and contribute to disease symptoms. Lymphocytes also produce antitoxins that neutralise these toxins.

This tackles a different part of the problem: destroying a bacterium removes the pathogen, whereas neutralising its toxin prevents the toxin from causing harm.

The first response takes time

When a pathogen first enters the body, it may multiply before the specific response becomes strong enough to control it. Phagocytes can begin engulfing pathogens, but a lymphocyte that recognises the pathogen’s antigen must also be activated.

That lymphocyte multiplies to produce many matching cells, allowing large quantities of the specific antibody to be made. Antibodies bind to the pathogens, and phagocytes help remove them. Because this primary immune response takes time to develop, the person may experience symptoms before recovering.

Memory cells make later responses faster

After an infection, some lymphocytes remain as memory cells. These provide lasting recognition of the particular antigen.

If the same pathogen enters again, memory cells enable the matching antibody to be produced much faster and in larger quantities. This secondary immune response can control the pathogen before it multiplies enough to cause symptoms. This explains how a previous infection can provide immunity to the same pathogen: the protection depends on a faster, specific response, not on an impenetrable barrier.