Cells of the Immune System: Innate and Adaptive Defenders

A single scraped knee can introduce thousands of bacteria into the body at once. Within minutes to hours, one type of cell arrives to attack them. If those cells cannot finish the job, a second type takes over within hours. If the threat hides inside your own cells where neither can reach it, a third group is called in that can inspect a cell from the outside and decide whether to kill it. The immune system is not one defense. It is a sequence of defenders, each one answering a problem the previous one could not solve.

This article introduces the cells of the immune system in that order: the cells that respond first and without training, then the cells that must be trained before they act. Understanding which cell answers which threat is the foundation for everything else in immunology, so this page is the starting point that the more detailed topics build on.

The immune system is a collection of organs, tissues, and cells spread throughout the body and connected by blood vessels and lymphatic channels. Its cells communicate using protein signals such as cytokines, complement, and antibodies, and together they defend the body against pathogens.

Two kinds of defense: innate and adaptive

Before meeting the individual cells, it helps to know the two teams they belong to. Every immune cell is either part of the innate response or the adaptive response, and this single distinction organizes the whole system.

The innate immune system acts fast, within minutes to hours. Its cells recognize general features shared by many pathogens, such as bacterial cell wall components. They need no prior exposure and no training, and they keep no memory of what they fought. Neutrophils, macrophages, dendritic cells, the granulocytes, and natural killer cells belong here.

The adaptive immune system acts later, over days, but it is precise. Its cells recognize one specific target each, they must be trained before they can act, and they retain memory so the second encounter with the same pathogen is faster and stronger. B lymphocytes and T lymphocytes belong here.

The two systems are not separate. Dendritic cells and macrophages sit at the bridge: they fight as part of the innate response, then present what they find to the adaptive cells, starting the specific response. That handoff is a theme this whole article returns to.

	Innate	Adaptive
Speed	Minutes to hours	Days
Recognition	General pathogen features	One specific target each
Training needed	No	Yes
Memory	No	Yes
Main cells	Neutrophils, eosinophils, basophils (the granulocytes), macrophages, dendritic cells, mast cells, NK cells	B and T lymphocytes

Some of these cells, such as B-cells and T-cells, require “training” or education before carrying out their effector functions. These cells are chief mediators of adaptive or acquired immunity. While other cells can carry out their functions without the need for training such immune cells are neutrophils, macrophages, natural killer (NK) cells. These cells are part of the innate immune system.

All these specialized cells of the immune system arise from bone marrow’s hematopoietic stem cells.

Hematopoietic stem cells

Adult hematopoietic (blood-forming) stem cells are multipotent cells, have the potential to differentiate and mature into the different cells of the immune system. They are responsible for the constant renewal of blood, the production of billions of new blood cells each day.

Hematopoietic stem cells are the most important cell in bone marrow transplants as they can generate the entire immune system. This transplants are used to treat patients with cancers and other disorders of the blood and immune systems.

About 1 in every 100,000 cells in the marrow is a stem cell; other cells present include stromal cells, stromal stem cells, blood progenitor cells, and mature and maturing WBCs and RBCs.

Hematopoietic stem cell differentiate along one of two pathways, giving rise to either a common lymphoid progenitor cell or a common myeloid progenitor cell. Each of them differentiates further into various cell types. The presence of types and amounts of growth factors in the microenvironment of a particular stem cell or progenitor cell control its differentiation.

Myeloid progenitor cells

Progenitor cells have lost the capacity for self-renewal and are committed to a particular cell lineage. Myeloid stem cells generate progenitors of red blood cells, white blood cells (neutrophils, eosinophils, basophils, monocytes, mast cells, dendritic cells), and platelets.

When appropriate factors and cytokines are present, progenitor cells proliferate and differentiate into corresponding cell types.

Neutrophils

Neutrophils, also called polymorphonuclear leukocytes (polys or PMNs), are the most abundant circulating white blood cells, making up roughly 50 to 70 percent of the total. They develop from hematopoietic stem cells in the bone marrow and are among the first cells recruited to a site of infection, often arriving within a few hours of microbes entering the tissue. Their lifespan in circulation is short, around 6 to 12 hours.

Neutrophils appear on lab reports as part of a complete blood count (CBC with differential). These increase in number in the bloodstream during infection and are in large part responsible for the elevated white blood cell count seen with some infections.

Neutrophils may migrate to sites of infection within a few hours after the entry of microbes and are responsible for the formation of “pus.” Neutrophils mediate the earliest phase of inflammatory reactions, ingest bacteria or fungi, and kill them. They have little role in the defense against viruses.

Monocytes

Monocytes are closely related to neutrophils and are found circulating in the bloodstream. They make up 5-10 percent of the white blood cells. They also line the walls of blood vessels in organs like the liver and spleen. Here they capture microorganisms in the blood as the microorganisms pass by. When monocytes leave the bloodstream and enter the tissues, they change shape and size and become macrophages.

Macrophages

Macrophages are mononuclear, immune cells that arise from the myeloid progenitors of the hematopoietic system. They are involved in the detection, phagocytosis, and destruction of pathogens. Macrophages in different tissues have given special names, for example, microglial cells, kupffer cells, alveolar macrophages, and osteoclasts.

The step-by-step mechanism by which these cells ingest and kill microbes is covered in a separate article on phagocytosis.

When the first responders cannot cope: antigen-presenting cells

Neutrophils and macrophages handle many infections on their own. But some threats are too specific, or hide inside cells, and need the precise adaptive response. The problem is that adaptive cells recognize only small fragments of a pathogen, not the whole microbe. Something has to capture the pathogen, break it into fragments, and display those fragments to the adaptive cells.

That is the job of antigen-presenting cells (APCs). Dendritic cells, macrophages, and B cells can all present antigen, and they form the bridge from the fast innate response to the specific adaptive one. How they display those fragments, using MHC molecules, is a mechanism in its own right and is covered in a dedicated article on MHC and antigen presentation.

Dendritic cells

Dendritic cells are the most important antigen-presenting cells (APCs) for activating naïve T cells. These cells arise from both myeloid and lymphoid progenitors. They have long membranous projections and phagocytic capabilities. They display microbial antigens to T lymphocytes.

Dendritic cells play major roles in innate responses to infections and in linking innate and adaptive immune responses.

Basophils

Basophils are blood granulocytes (these granules bind with basic dyes) with many structural and functional similarities to mast cells. They are derived from bone marrow progenitors and constitute less than 1% of blood leukocytes.

Basophils are not present in tissues (normally) but may be recruited in some inflammatory reactions. Basophils express IgG and IgE receptors, bind IgE, and can be triggered by antigen binding to the IgE.

Mast cells

Mast cells are tissue-resident granulocytes that share a common origin and many functions with basophils but differ in where they live and how long they last. Basophils circulate in the blood for a few days; mast cells leave the marrow as immature precursors, settle in tissues such as skin, airways, gut, and the lining of blood vessels, and can survive there for weeks to months. Both carry high-affinity IgE receptors and release histamine, so both drive immediate (type I) hypersensitivity. Mast cells are the main effector cell of tissue allergic reactions such as hives, asthma, and anaphylaxis, while basophils reinforce the reaction from the circulation.

Basophil versus mast cell in one line: same weapons, different address. Basophils circulate and are the rarest blood leukocyte (under 1 percent); mast cells sit in tissue and are not normally found in blood.

Eosinophils

Figure: Eosinophils

These are blood granulocytes that express cytoplasmic granules containing enzymes that are harmful to the cell walls of parasites. Eosinophils are responsible for participating in immediate allergic reactions, modulating inflammatory responses.

The granules of Eosinophils contain basic proteins that bind acidic dyes such as eosin. Eosinophils are normally present in peripheral tissues, especially in mucosal linings of the respiratory, gastrointestinal, and genitourinary tracts.

Lymphoid Progenitor Cell

Common lymphoid progenitor cells give rise to B, T, and NK (natural killer) cells and some dendritic cells. Lymphocytes are the central cells of the immune system, responsible for adaptive immunity and immunologic memory. Other important attributes of lymphocytes are diversity, self/non-self recognition, and specificity.

Lymphocytes constitute 20-40% of the body’s white blood cells and 99% of the cells in the lymph. There are approximately 10^11 lymphocytes in our body. These lymphocytes continually circulate in our blood and lymph and migrate to infection sites whenever they get signals.

B-Cells

B-cells, also known as B-lymphocytes, are specialized cells of the immune system whose major function is to produce antibodies (also called immunoglobulins). B lymphocyte-derived its letter designation from its site of maturation, in the bursa of Fabricius in birds. B-cells develop from hematopoietic stem cells in the bone marrow where they also get immunological training and maturation.

When B-cells encounter foreign antigens, they respond by transforming into antibodies secreting cells, called plasma cells. B-cells can also mature into memory cells, which are responsible for rapid and heightened secondary immune response.

How B cells mature, get activated, and produce antibody is covered in a separate article on humoral immunity.

T-Cells

T-cells, also known as T-lymphocytes, develop from hematopoietic stem cells in the bone marrow but complete their development in the thymus (the “T” stands for the thymus). Like B lymphocytes, these cells have membrane receptors for antigens called T-cell receptors (TCR). T-cells attack cells infected with viruses, and they also act as regulators of the immune system.

T cells fall into several subsets based on their function:

Cytotoxic T cells (CD8+) kill cells that are infected by viruses or otherwise altered, such as tumor cells. They recognize fragments of protein displayed on the surface of the target cell.

Helper T cells (CD4+) do not kill directly. They coordinate the response by helping B cells make antibody and by activating macrophages. They are the organizers of adaptive immunity.

Regulatory T cells dampen the response and help prevent the immune system from attacking the body's own tissues.

Other subsets, including gamma-delta T cells and memory T cells, are also recognized. How a T cell is activated, starting when its receptor engages a presented antigen, is covered in a separate article on T lymphocyte activation.

Natural Killer (NK) Cells

Natural killer (NK) cells are so named because they easily kill cells infected with viruses without the requirement of thymic education (that T-cells require). NK cells kill virally infected cells and altered self cells (cancer cells).

NK cells are large granular lymphocytes that are CD3 negative and typically CD56 and/or CD16 positive. They are present in relatively low numbers in the bloodstream and in tissues. They are large, granular lymphocytes that do not express the set of surface markers typical of B or T cells.

How to remember which cell does what

First responders, no training needed (innate). Neutrophils arrive first and in the largest numbers, ingest bacteria and fungi, and form pus. Macrophages, the mature form of monocytes that have entered tissue, are the long-lived cleaners that ingest microbes and dead cells and also present antigen. Think of neutrophils as the rapid strike force and macrophages as the resident garrison.

The granulocytes, named by what their granules do. Eosinophils target parasites and drive allergic reactions; their granules stain with acidic eosin. Basophils and mast cells release histamine and drive allergic and inflammatory responses. A memory hook: Eosinophils for Extra-large parasites, Basophils and mast cells for Big allergic reactions.

The bridge cells (innate into adaptive). Dendritic cells are the most important antigen-presenting cells for waking up naive T cells. They are the messengers that carry news of an infection from the tissue to the adaptive system.

The trained specialists (adaptive). B cells make antibodies. T cells come in two headline types: CD8 killers that destroy infected cells, and CD4 helpers that organize everyone else. A common mnemonic: CD8 ate (8) the infected cell; CD4 helpers coordinate the (4)ce.

The exception that proves the rule. NK cells are lymphocytes, like B and T cells, but they need no training and keep no memory, so they act as part of the innate system. They kill infected and cancerous cells that have stopped displaying the normal self signal.

Key exam facts in one table

Cell	Lineage	Innate or adaptive	Defining job
Neutrophil	Myeloid	Innate	First responder; ingests bacteria and fungi; forms pus
Monocyte / Macrophage	Myeloid	Innate (also APC)	Ingests microbes and dead cells; presents antigen
Dendritic cell	Myeloid	Bridge	Most important APC for activating naive T cells
Eosinophil	Myeloid	Innate	Targets parasites; allergic reactions
Basophil / Mast cell	Myeloid	Innate	Histamine release; allergic and inflammatory responses
B lymphocyte	Lymphoid	Adaptive	Produces antibodies; becomes plasma or memory cell
T lymphocyte (CD8)	Lymphoid	Adaptive	Kills infected and altered cells
T lymphocyte (CD4)	Lymphoid	Adaptive	Helps B cells and macrophages; coordinates response
Regulatory T cell	Lymphoid	Adaptive	Dampens the response; maintains self-tolerance
NK cell	Lymphoid	Innate	Kills infected and tumor cells without training

Where students get confused

"Macrophages and monocytes are two different cells." They are the same cell at two stages. A monocyte circulates in the blood; when it enters tissue it matures into a macrophage. Same lineage, different location and name.

"NK cells are a kind of T cell because they kill infected cells like CD8 T cells do." No. NK cells are lymphocytes but not T cells. They use different receptors, they are not restricted by MHC, and they need no thymic training. CD8 T cells must be trained and recognize a specific presented antigen; NK cells kill any cell that has lost its normal self signal.

"Neutrophils fight viruses." Not really. Neutrophils are built for bacteria and fungi. Viruses are handled mainly by NK cells early and cytotoxic T cells.

"Dendritic cells are just another phagocyte." They can phagocytose, but their defining job is different. Their purpose is to carry captured antigen to the lymph node and present it to naive T cells, starting the adaptive response. They are messengers more than they are killers.

"All lymphocytes are adaptive." Most are, but NK cells are the exception: a lymphocyte that behaves like an innate cell.

References and further readings

1. Abbas AK, Lichtman AH, Pillai S. Cellular and Molecular Immunology. 10th ed. Elsevier; 2022.
2. Punt J, Stranford SA, Jones PP, Owen JA. Kuby Immunology. 8th ed. W.H. Freeman; 2019.
3. Delves PJ, Martin SJ, Burton DR, Roitt IM. Roitt's Essential Immunology. 13th ed. Wiley-Blackwell; 2017.
4. Patel AA, Ginhoux F, Yona S. Monocytes, macrophages, dendritic cells and neutrophils: an update on lifespan kinetics in health and disease. Immunology. 2021;163(3):250–261. https://doi.org/10.1111/imm.13320