White cells defend the body against infection and clear up damaged tissue. This page introduces the cell types, innate and adaptive immunity, inflammation as the first step of healing, and what current evidence says about exercise and immune health.
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Part 6 · Blood and Immunity · Chapter 27 of 47
Learning objectives
By the end of this page you should be able to:
Name the white cell types and their main roles.
Compare innate and adaptive immunity.
Describe the stages of inflammation and their role in tissue repair.
Explain how macrophages switch roles during muscle regeneration.
Describe the immune response to acute and regular exercise.
Apply this to injury management and to exercise in people at risk of infection.
Why it matters
Every soft tissue injury physiotherapists treat begins with inflammation. Knowing that inflammation is the start of repair, not just a problem to suppress, shapes how we use rest, ice, anti-inflammatory drugs and loading.
Patients and athletes also ask whether exercise weakens immunity. The evidence now answers that question more clearly than older textbooks suggest.
The white cells
Figure 1 · The white cell family
Each type has a job. A raised neutrophil count suggests bacterial infection or tissue injury; raised lymphocytes suggest a viral infection. Typical adult ranges.
White cells are made in the bone marrow; lymphocytes mature in the thymus, lymph nodes and spleen. Most white cells spend only a short time in blood and do their work in tissues. A blood count gives a snapshot of cells in transit.
Innate and adaptive immunity
Figure 2 · Two arms of defence
They work together. Innate cells present fragments of microbes to T cells, and antibodies help innate cells find their targets.
Element
Example
Clinical link
Physical barrier
Intact skin, airway cilia and mucus
Pressure ulcers, smoking and immobility weaken barriers
Phagocytes
Neutrophils, macrophages
Low neutrophils after chemotherapy raise infection risk
Antibodies
IgG, IgA, IgM from B cells
Vaccination builds antibody memory
T cells
Helper and cytotoxic T cells
HIV reduces helper T cells
Inflammation
Inflammation is the innate response to injury or infection. Damaged cells and microbes release signals that dilate local vessels and make capillaries leaky. Redness, heat, swelling and pain follow, and white cells move into the tissue.
Neutrophils arrive first, within hours, and clear debris and microbes. Monocytes follow and become macrophages. Over days the response resolves and repair takes over.
Figure 3 · Inflammation builds the repair
The switch matters. Inflammatory macrophages clear debris and then switch to an anti-inflammatory type that drives muscle regeneration; removing them prevents repair [1]. Schematic timing.
Arnold and colleagues showed that in injured muscle, recruited monocytes start as pro-inflammatory cells that clear debris [1]. After eating the debris, they switch to an anti-inflammatory type that drives the formation of new muscle fibres [1]. When monocytes were removed at the time of injury, the muscle did not regenerate [1].
What this means for injury management
Early inflammation is part of healing, not only a problem. Protect the tissue early, then introduce progressive loading as the response settles. Use anti-inflammatory drugs and prolonged icing thoughtfully, since blocking inflammation completely may interfere with the repair sequence.
Exercise and the immune system
A single bout of vigorous exercise sends lymphocytes and natural killer cells into the blood. One to two hours later their numbers fall below resting levels. This was once read as a temporary ‘open window’ of immune suppression.
Figure 4 · Lymphocytes during and after exercise
Redistribution, not suppression. The fall after exercise reflects cells moving to tissues where infections start [2]. Schematic.
Campbell and Turner re-examined the evidence. They found limited reliable support for increased infection risk after vigorous exercise, and argued that the fall reflects cells moving into tissues where infections begin [2]. Studies of vaccination showed heightened antibody responses after exercise bouts [2].
Nieman and Wentz trace how the field has moved from cell counts to systems biology, and summarise links between regular activity, lower inflammation and reduced chronic disease [3].
Figure 5 · Activity and the immune system
The old ‘open window’ worry is overstated. Reviews conclude that exercise most likely improves immune function across the lifespan [2, 3].
Situation
Practical guidance
Mild cold, symptoms above the neck
Light activity is usually acceptable; reduce intensity
Fever, body aches, symptoms below the neck
Rest until recovered; fever with exercise risks heart muscle inflammation
Low neutrophils during cancer treatment
Avoid crowded settings and infection sources; follow oncology team advice
Older adult wanting to stay well
Regular moderate activity supports immune health [2, 3]
Innate: fast, non-specific; adaptive: slow first time, specific, with memory.
Inflammation: neutrophils first, then macrophages.
Macrophages switch from clearing debris to building muscle [1].
Post-exercise lymphocyte fall = redistribution, not suppression [2].
Regular activity supports immune health across life [2, 3].
Further reading
Source
Why it earns its place
Arnold L et al. — J Exp Med, 2007
Macrophages and muscle repair [1]
Campbell JP, Turner JE — Front Immunol, 2018
Rethinking exercise and immunity [2]
Nieman DC, Wentz LM — J Sport Health Sci, 2019
Overview of exercise immunology [3]
References
Arnold L, Henry A, Poron F, et al. Inflammatory monocytes recruited after skeletal muscle injury switch into antiinflammatory macrophages to support myogenesis.Journal of Experimental Medicine. 2007;204(5):1057–1069.
doi:10.1084/jem.20070075PMID 17485518Experimental study
Campbell JP, Turner JE. Debunking the myth of exercise-induced immune suppression: redefining the impact of exercise on immunological health across the lifespan.Frontiers in Immunology. 2018;9:648.
doi:10.3389/fimmu.2018.00648PMID 29713319Review
Nieman DC, Wentz LM. The compelling link between physical activity and the body’s defense system.Journal of Sport and Health Science. 2019;8(3):201–217.
doi:10.1016/j.jshs.2018.09.009PMID 31193280Review
