Immune cells play critical roles in the defense of the body against infectious agents and the development and spread of cancer. The immune system involves two primary lines of defense against non-self pathogens. The innate immune system is the first line of defense that quickly responds in a broad way. This involves natural killer cells, macrophages, neutrophils, dendritic cells, mast cells, basophils, and eosinophils. The adaptive immune system takes over if needed and specifically targets and remembers the pathogen to provide long-lasting immunity. The adaptive immune system includes T lymphocytes (T cells), B lymphocytes (B cells), and antibodies.

Tissues and Organs Involved

Many cells work together as part of the innate and adaptive immune system.

One of the primary components of the immune system is the bone marrow, which produces both T and B lymphocytes. While B cells will remain in the bone marrow to mature, T cells continue to another organ, the thymus.

The thymus is a specialized primary lymphoid organ where T cells develop and mature, including the acquisition of antigen receptors such as CD4 and CD8, and differentiate into helper T cells and cytotoxic T cells.

Mature T and B lymphocytes move to other tissues and organs of the immune system, the lymph nodes and spleen, in preparation for activation. The spleen filters the blood, where macrophages engulf and digest cellular debris, pathogens, and other foreign substances, and aids the immune system by identifying pathogenic microorganisms.

Immune Cell Types

Immune cells are derived from blood stem cells through two lineages: myeloid stem cells and lymphoid stem cells.

Myeloid Lineage Cells

  • Granulocytes are leukocytes of the innate immune system that have enzyme containing granules in their cytoplasm.
    • Neutrophils are the most abundant innate immune cells. As first responders, they ingest harmful foreign particles, bacteria, and dead or dying cells in the blood, and also modulate inflammation by various mechanisms.
    • Eosinophils help in killing parasites, tumor cells, and immune system regulation.
    • Basophils travel to the site of infection and release histamines when damaged, which contributes to the inflammatory response, and prostaglandins that help increase blood flow to the infection site.
Immunophenotyping- Myeloid cell maturation

Lymphoid Lineage Cells

  • T cells have several roles and are divided into CD8+ T cells or CD4+ T cells depending on which cluster of differentiation (CD) protein is present on the cell surface.
    • CD8+ T cells (cytotoxic T cells, cytotoxic lymphocytes, or CTLs) recognize and eliminate virus-infected cells and cancer cells.
    • CD4+ T cells are divided into subsets:
      • T-helper (TH)1 cells coordinate immune responses against intracellular microbes.
      • TH2 cells coordinate immune responses against extracellular pathogens such as parasites by alerting B cells, granulocytes, and mast cells.
      • TH17 cells produce interleukin 17 (IL-17) that activates immune and nonimmune cells, and they recruit neutrophils.
      • Tregs (regulatory T cells) monitor and inhibit the activity of other T cells.
    • Killer T cells (cytotoxic T cells) are the primary effector cells of adaptive immunity. They can attach to and kill cancer cells.
  • B cells make antibodies and can also internalize antigens and present them to T cells. Memory B cells are long lived and remember previous exposure to antigens. Plasma B cells are activated B cells, each making one specific antibody in significant quantity.
  • Natural killer (NK) cells are cytotoxic lymphocytes that are critical to the innate immune system. They respond rapidly to infection by viruses and other intracellular pathogens, and tumor formation without the need for antibodies and major histocompatibility complex (MHC).
  • Natural killer T (NKT) cells are a heterogeneous group of T cells that share properties of both T cells and natural killer cells.
Immunophenotyping- Lymphoid cell maturation


Monocytes are produced in the bone marrow and move into tissues throughout the body to differentiate into macrophages and dendritic cells.


Macrophages engulf and digest pathogens by phagocytosis. The macrophages identify the pathogens, which can include cancer cells, microbes, cellular debris, and foreign substances by the presence of nonself surface proteins that distinguish the pathogen from healthy body cells. Macrophages play a critical role in innate immunity and also help initiate specific defense mechanisms (adaptive immunity) by
recruiting other immune cells such as lymphocytes. They can increase inflammation but also have an anti-inflammatory role through the release of cytokines.

Dendritic Cells

Dendritic cells (DCs) are specialized antigen-presenting cells (APCs) that form an interface between innate and adaptive immunity. Once activated, DCs migrate to the lymph nodes and interact with T cells and B cells to initiate and shape the adaptive immune response.

DCs present endogenous and exogenous antigens to T cells in the context of MHC molecules, resulting in antigen tolerance or priming and triggering of an effector T cell response. DCs also help maintain the immune memory of B cells, including the production of cytokines and other factors that promote B cell activation and differentiation.

Expected Levels of Immune Cells in Blood

Level (x 109/L)
Cell Type Adult Pediatric (4-7 years old) Neonate (0-1 days old)
WBC 3.6–10.6 5.0–17.0 9.0–37.0
Neutrophils 1.7–7.5 1.5–11.0 3.7–30.0
Lymphocytes 1.0–3.2 1.5–11.1 1.6–14.1
Monocytes 0.1–1.3 0.1–1.9 0.1–4.4
Eosinophils 0.0–0.3 0.0-0.7 0.0–1.5
Basophils 0.0–0.2 0.0–0.3 0.0–0.7

Source: Modified from Wikipedia.

Complement System — A Non-cellular Part of the Immune System

The complement system, or cascade, enhances (complements) the ability of antibodies and phagocytic cells to clear microbes and damaged cells from an organism, promote inflammation, and attack the cell membranes of pathogens. Complement components in circulation are synthesized by the liver while complement components in tissues are mainly synthesized by macrophages.

The complement system comprises several related proteins that form a self-amplifying proteolytic cascade. Products later in the cascade can create pores in cell membranes, leading to cell lysis, while many of the proteolytic fragments generated during the cascade have other useful biological activities. The complement cascade can be activated by an antibody first binding to a particular antigen, direct
binding to microbial surfaces, or by circulating proteins of the innate system that bind to pathogenassociated molecular patterns (PAMPs).

The Role of Immune Cells in Infectious Diseases

The immune response depends on the type of microorganism together with the nature of the pathogenicity (extracellular, intracellular, toxin-based, etc.) and virulence level or, in the case of parasites, the life cycle. For example:


  • Neutralizing antibodies are synthesized by the B cells if the bacterial pathogenicity is due to a toxin.
  • Opsonizing antibodies destroy extracellular bacteria.
  • The complement system is activated, especially by gram-negative bacterial lipid layers.
  • Phagocytes kill most bacteria.
  • CD8+ T cells can kill cells infected by intracellular bacteria.


  • The innate immunity includes defensins and phagocytes.
  • CD4+ T helper cells are responsible for the adaptive immune response against fungi.
  • Dendritic cells secrete IL-12 after ingesting fungi, and IL-12 activates the synthesis of gamma interferon which activates the cell-mediated immunity


  • Interferon, NK cells, and phagocytes prevent the early spread of viruses.
  • Specific antibodies and complement proteins neutralize viruses and can limit spread and reinfection.
  • The adaptive immunity is most important in protecting against viral infection, including CD8+ T cells that kill viruses and CD4+ T cells that dominate the effector cell population.


  • Parasitic infection stimulates various mechanisms of immunity due to their complex life cycle.
  • Both CD4+ and CD8+ T cells protect against parasites.
  • Macrophages, eosinophils, neutrophils, and platelets can kill protozoa and worms.
  • Eosinophils and the stimulation of IgE by Th2 CD4+ T cells are involved in killing intestinal worms.
  • Inflammatory responses also combat parasitic infections.

The Role of the Immune System in Cancer

Immune cells play key roles in both tumor proliferation and elimination. Chronic inflammation is a necessary consequence of cancer progression that includes genomic and epigenomic instability, immune evasion, angiogenesis, and metastasis. Chronic inflammation can also be a primary cause of cancer.

An aberrant innate and adaptive immune response contributes to tumorigenesis by selecting for aggressive cells, inducing immunosuppression, and stimulating cancer cell proliferation and metastasis. During early tumor development, cytotoxic immune cells such as natural killer (NK) cells and CD8+ T cells recognize and eliminate the more immunogenic cancer cells.

Selected cancer cells that survive progress to clinically detectable tumors and adopt different strategies of peripheral immune tolerance and recruitment of immunosuppressive immune cells, such as regulatory T cells, pro-tumoral macrophages, and pro-metastatic neutrophils that can subdue other tumoricidal cells.

Macrophages are important drivers of chronic cancer-associated inflammation and are involved in every step of cancer progression, from early neoplastic transformation throughout metastatic progression to therapy resistance.

Processes such as angiogenesis, extracellular matrix (ECM) remodeling, and immune evasion are mediated by tumor-associated macrophages (TAMs), tumor-associated neutrophils (TANs), and immature dendritic cells (DCs), resulting in rapid tumor progression and metastasis.

In contrast, cytotoxic macrophages and neutrophils, natural killer (NK) cells, and mature DCs eliminate tumor cells in primary sites and after dissemination.

T cells are the second most frequent immune cell type found in human tumors besides TAMs and modulate immunity and direct inflammatory or anti-inflammatory processes. CD8+ T cells are the most prominent anti-tumor cells. After being primed by APCs, the CD8+ T cells differentiate into cytotoxic T lymphocytes (CTLs) that attack the tumor.

The constant selective pressure of the effector response results in the selection of tumor variants that escape immune recognition and enter a phase of outgrowth that is not blocked by effector immune cells. At the same time, the tumor induces the recruitment of regulatory CD4+ T cells (Tregs) that counteract anti-tumor immune cells by diverse mechanisms. Tumors with high Treg infiltration have the
worst prognosis.


Immunophenotyping is a powerful tool that uses flow cytometry to gain insight into the composition and dynamics of cell populations in an immune response. The technique involves identifying cell types in heterogeneous cell populations by using different fluorescently-tagged antibodies that target various CD markers. Different combinations of antibodies can be used to identify different groups and sub-groups of cells. For example, CD3 can be used as a general marker for T cells followed by other CD markers to identify regulatory T cells (CD4 and CD25), T helper cells (CD4), and cytotoxic T cells (CD8). Immunophenotyping can be used for discovery and can also be applied in clinical settings, for example to help diagnose and classify a leukemia or lymphoma, or refine the analysis of cancer cell lines by determining the presence or absence of cancer cell markers that correlate with different degrees of severity.

Antibodies as Tools in Immunophenotyping

Leinco Technologies provides a range of antibodies that can be used in immunophenotyping in research (see tables).


Monoclonal Abs
Target Alternative Name Polyclonal Unconjugated FITC PE APC PerCP DyLight® Reactivity Clone
Stem and Progenitor Cells
CD117 c-Kit; SCF-R S579 Human
CD71 Transferrin Receptor C163 C165 C328 Available Human T56/14
C275 C277 C278 C355 C1933 Mouse R17217
Megakaryocyte and Platelets
CD61 Integrin β3 C191 C193 C194 C327 C1822 Available Human F11
C616 C617 C618 Human PM6/13
Granulocyte: Basophil
CD123 IL3RA I-651 Human 32703
I-719 Mouse 151231
Granulocyte: Eosinophil
CD44 C183 C185 C186 C320 C1804 Available Human E1/2
C562 C563 C564 Human F10-44-2
C251 C253 C254 C349 C1894 Available Human/Mouse IM7
Granulocyte: Neutrophil
CD15 C157 C159 C160 C316 C2015 Available Human TG1
CD16 C6850 C6854 C6851 C6852 C6853 Available Human 3G8
C465 Human GRM1
C247 C249 C250 C348 Available Mouse 2.4G2
CD11b C133 C135 C136 C307 C1727 Available Human ICRF44
C227 C229 C230 C341 C1906 Available Mouse M1/70
CD14 C153 C155 C156 C315 C1867 Available Human UCHM-1
Myeloid Dendritic cells: MDC2
CD141 Thrombomodulin T537 T547 Human 501733


Monoclonal Abs
Target Alternative Name Polyclonal Unconjugated FITC PE APC PerCP DyLight® Reactivity Clone
Stem and Progenitor Cells
CD117 c-Kit; SCF-R S579 Human
T lymphocytes (T Cells)
CD2 C109 C111 C112 C308 C1839 Available Human G11
CD3 C105 C107 C108 C309 C2033 Available Human UCHT-1
C207 C2839 C1737 Available Mouse 500A2
C1757 C2234 C2037 C2036 Available Mouse 145-2C11
T lymphocytes (Cytotoxic)
CD8 C117 C119 C120 C312 C2068 Available Human UCHT-4
C297 C299 C300 C301 C1664 Available Mouse 2.43
C215 C217 C218 C339 C1746 Available Mouse 53-6.7
T lymphocytes (Helper)
CD4 C9210 Human OKT-4
C211 C213 C214 C338 C1840 Available Mouse GK1.5
T lymphocyte (Regulatory)
CD4 See Above See Above See Above See Above See Above See Above See Above See Above
CD25 IL2Rα LT300 LT304 LT303 Available Human HU107
C1189 C1191 C1192 C1193 C1924 Available Mouse PC61
T lymphocyte (NK/T)
CD3 See Above See Above See Above See Above See Above See Above See Above See Above
T lymphocyte (Activated)
CD25 IL2Rα See Above See Above See Above See Above See Above See Above See Above See Above
CD30 C1384 Human 81337
B lymphocyte (B cell)
CD19 C141 C143 C144 C317 C1978 Available Human SJ25-C1
C2088 C2105 C2107 C2106 C2108 Available Mouse 1D3
CD20 C1602 C1654 C1895 C1896 C1682 Available Human 2H7
B lymphocyte (Activated)
CD19 See Above See Above See Above See Above See Above See Above See Above See Above
CD25 IL2Rα See Above See Above See Above See Above See Above See Above See Above See Above
CD30 See Above See Above See Above See Above See Above See Above See Above See Above
Natural Killer (NK) Cell
CD56 NCAM See Above See Above See Above See Above See Above See Above See Above See Above

Find Out More

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  3. Alexandra C. Costa, Joana M.O. Santos, Rui M. Gil da Costa, Rui Medeiros, Impact of immune cells on the hallmarks of cancer: A literature review, Critical Reviews in Oncology/Hematology, Volume 168, 2021, 103541, ISSN 1040-8428, (
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