Immunology
Immune Cells
Lymphocytes, neutrophils, macrophages — the cellular army defending against pathogens
Immune cells are white blood cells (leukocytes) that defend the body against pathogens, infected cells, and tumors. Two arms — innate (fast, nonspecific) and adaptive (slow, specific, memory). Innate: neutrophils (phagocytes; 50-70% of WBCs), macrophages (tissue resident), dendritic cells (antigen presentation), NK cells (kill virus-infected and tumor cells). Adaptive: B cells (make antibodies), T cells (CD4 helpers, CD8 killers). Total ~10¹² leukocytes in adult human; produced in bone marrow (~10¹¹/day). Defects cause immunodeficiency (HIV depletes CD4 T cells); overactivity causes autoimmunity (rheumatoid arthritis, lupus).
- Total leukocytes4,000-11,000 per μL blood
- Neutrophils50-70% of WBCs (most abundant)
- Lymphocytes20-40% (B cells, T cells, NK)
- Production rate~10¹¹ leukocytes per day in marrow
- T cell maturationThymus (origin of "T")
- B cell maturationBone marrow (origin of "B")
Interactive visualization
Press play, or step through manually. The visualization is yours to drive — try it before reading on.
Watch the 60-second explainer
A condensed visual walkthrough — narrated, captioned, under a minute.
Why immune cells matter
- Infection. Bacterial, viral, fungal, parasitic defense.
- Vaccination. Generates memory B and T cells.
- Cancer. Immune surveillance; checkpoint inhibitors unleash T cells.
- Transplantation. T cells reject grafts; immunosuppressants required.
- Autoimmunity. Cells attack self — RA, lupus, type 1 diabetes.
- Allergy. Mast cells and IgE drive hypersensitivity.
- HIV/AIDS. CD4 T cell depletion defines disease progression.
Common misconceptions
- White cells are one type. Many — each with distinct role.
- Antibodies kill bacteria directly. Mostly opsonize for phagocytes or activate complement.
- T cells make antibodies. B cells do; T cells regulate or kill.
- Innate immunity is primitive. Sophisticated pattern recognition; saves life in first hours.
- More immune activity is better. Excess causes autoimmunity, sepsis, cytokine storm.
- Memory means lifelong. Some wane (pertussis ~5-10 yrs); boosters needed.
Frequently asked questions
What's the difference between innate and adaptive immunity?
Innate is fast (minutes to hours), nonspecific, no memory — neutrophils, macrophages, NK cells, complement, barriers. Adaptive is slow (days), highly specific (recognizes one antigen), has memory (faster on second exposure) — B and T lymphocytes. Innate detects general patterns (PAMPs like bacterial LPS via TLR receptors); adaptive uses unique receptors generated by V(D)J recombination giving ~10¹⁵ possible specificities.
What do neutrophils do?
First responders to bacterial infection. Live ~5 days; circulate then enter tissues at infection sites following IL-8 gradient. Phagocytose bacteria; release reactive oxygen species, defensins, and NETs (neutrophil extracellular traps — chromatin webs that ensnare bacteria). Pus = dead neutrophils. Low neutrophils (neutropenia, e.g., chemo patients with ANC <500) → severe infection risk; febrile neutropenia is medical emergency.
What's the role of macrophages?
Tissue-resident phagocytes — Kupffer cells (liver), microglia (brain), alveolar (lung), osteoclasts (bone). Engulf pathogens, debris, apoptotic cells. Present antigen on MHC II to CD4 T cells. Secrete cytokines (TNF-α, IL-1, IL-6) driving inflammation and fever. M1 phenotype = pro-inflammatory; M2 = tissue repair. Dysregulation in atherosclerosis (foam cells), tuberculosis (granulomas), and cancer (tumor-associated macrophages).
How do T cells differ from B cells?
B cells make antibodies (secreted Y-shaped proteins binding antigens) — humoral immunity. T cells use cell-surface receptors (TCR) — cellular immunity. CD4 helper T cells coordinate response (release IL-2, IL-4, IFN-γ); CD8 cytotoxic T cells kill infected/tumor cells via perforin and granzymes. Both undergo clonal selection — only cells whose receptor matches antigen proliferate.
What do natural killer (NK) cells do?
Innate lymphocytes that kill cells lacking MHC class I — typically virus-infected or tumor cells (which downregulate MHC to evade T cells). "Missing self" hypothesis. Use perforin/granzyme like CD8 T cells but without antigen specificity. Also kill antibody-coated cells via ADCC (antibody-dependent cellular cytotoxicity) through CD16 receptor. Important in herpes virus and CMV control.
Where are immune cells made?
All from hematopoietic stem cells (HSC) in bone marrow. HSC differentiates into myeloid progenitor (→ neutrophils, monocytes/macrophages, dendritic cells, eosinophils, basophils, mast cells) or lymphoid progenitor (→ B, T, NK cells). T cells leave marrow as immature thymocytes, mature in thymus (positive and negative selection eliminates ~98%). B cells mature in marrow itself.
What happens in HIV/AIDS?
HIV uses CD4 and CCR5/CXCR4 co-receptors to infect CD4 T cells. Progressively depletes them — normal CD4 count is 500-1500/μL; AIDS defined at <200. Loss cripples adaptive immunity — opportunistic infections (PCP pneumonia, CMV retinitis, candida esophagitis) and cancers (Kaposi sarcoma). ART (antiretroviral therapy) suppresses viral replication; CD4 count recovers; life expectancy now near-normal if treated early.