Lymph nodes are small encapsulated structures that protect the body from foreign antigens. They do this by acting as a filter, trapping the particulate matter in the lymph that percolates through them.
Lymph nodes can be many different sizes and contain macrophages and lymphocytes (two different types of leucocytes) within their sinuses. These are essential in the immune system. During periods of illness, lymph nodes tend to swell and increase in size in response to the infection.