Where immune cells come from
Nearly all immune cells begin as stem cells in the bone marrow. Some mature there, while others, the T cells, finish maturing in the thymus, a small organ behind the breastbone. They then circulate in blood and lymph or settle in tissues. Lymph nodes, the spleen and patches of tissue in the gut and airways act as meeting places where immune cells sample what is passing through and compare notes.
The cells discussed here are all kinds of leukocytes, commonly called white blood cells. They make up only a small fraction of blood by count, compared with red cells, and many immune cells live mostly in tissues rather than the bloodstream.
The innate cells: quick and general
Innate cells respond without needing to have met an invader before.
- Neutrophils are the most abundant white blood cell and are usually first to arrive at a site of infection. They engulf microbes and release antimicrobial substances. They are generally short-lived, and pus is largely made up of dead neutrophils and debris.
- Monocytes and macrophages: Monocytes circulate in the blood, and when they enter tissues they develop into macrophages (“big eaters”). Macrophages engulf microbes and dead cells, clear debris and release signals that coordinate inflammation. Some tissue macrophages are long-lived and take on specialized roles.
- Dendritic cells are scouts that capture pieces of invaders, called antigens, and carry them to lymph nodes, where they present them to T cells. They are the bridge between the innate and adaptive branches.
- Eosinophils and basophils (with related mast cells) are linked with defense against parasites and also with allergic reactions, in which they release chemicals such as histamine.
- Natural killer (NK) cells patrol for cells that are infected or have turned abnormal, and can kill them without prior sensitization. They help limit viral infections and may help remove some abnormal cells, an area studied in cancer research.
The adaptive cells: slower, precise, with memory
The adaptive system relies on lymphocytes, each carrying a receptor that fits one particular shape.
- B cells recognize antigens on their surface receptors. With help from T cells, they multiply and become plasma cells, which are antibody factories. Antibodies are Y-shaped proteins that bind a target. They can block a microbe from entering cells, neutralize toxins, clump microbes together and flag them for engulfment, or activate complement proteins.
- Helper T cells coordinate. When they recognize an antigen presented by a dendritic cell or other presenting cell, they release signaling proteins that direct B cells, killer T cells and macrophages.
- Cytotoxic (killer) T cells recognize infected cells displaying fragments of viral proteins on their surfaces, and destroy them. This matters for viruses, which hide inside cells where antibodies cannot reach.
- Regulatory T cells dampen immune responses, helping to prevent excess damage and attacks on the body’s own tissues.
The recognition of “self” and “non-self” is shaped during development, when cells that react too strongly to the body’s own molecules are largely eliminated or controlled.
How immune memory works
When a naive lymphocyte first meets its antigen, it takes several days to build up a response. During it, some cells become short-lived effectors that fight the infection. Others become long-lived memory cells that remain in the body after the threat has gone.
If the same antigen appears again, memory cells respond faster and in larger numbers, and the antibodies produced often bind more tightly, since B cells refine their receptors through mutation and selection during the first response. This is why an infection may be milder, or may not cause symptoms at all, the second time. Vaccines make use of this principle, and vaccine mechanisms are described in more detail separately.
Memory does not always last for life. For some pathogens, protection fades over months to years, and others change their surface features, so earlier memory only partly fits. The duration and strength of memory varies by pathogen and by person, and this is an active research area.
Cell types and the problems they can cause
The same cells that defend the body can contribute to harm when misdirected. Overactive responses involving mast cells and eosinophils underlie allergy. When lymphocytes mistakenly target the body’s own tissues, autoimmune conditions can arise. Persistent activation of macrophages is implicated in chronic inflammation. Conversely, when parts of the system are missing or weakened, as in some inherited conditions or infections that affect immune cells, people become vulnerable to infections that others handle easily.
Differences between viruses and bacteria also matter, since different immune cell types take the lead against each.
This article is a general overview of immune cell biology, not medical advice. Questions about immune health are best directed to a qualified clinician.