Immune System

The immune system is the most genetically diverse and computationally interesting system in your body. It has to recognize an essentially infinite set of pathogens it has never seen while not attacking any of your own tissues — and it does this with a small number of cell types that learn on the job.

At a glance

What it does

Detects pathogens, neutralizes them, remembers the ones it has seen, and tolerates the ones that are you. It also patrols for cells that have turned cancerous and kills them, and it cleans up damaged tissue after injury. The same system that protects you is also responsible for allergy, autoimmunity, and transplant rejection.

Innate vs adaptive

The innate arm is fast, hardcoded, and non-specific. It recognizes generic pathogen signatures (pattern recognition receptors finding bacterial cell walls, viral RNA, etc.). Key cells: neutrophils, macrophages, dendritic cells, natural killer cells. Also includes the complement cascade and physical barriers like skin and mucus. Response starts within minutes. Does not improve with repeat exposure.

The adaptive arm is slow, specific, and learns. It uses T cells (cell-mediated — cytotoxic T cells kill infected cells; helper T cells coordinate) and B cells (which differentiate into antibody-producing plasma cells). First exposure takes 7-14 days to ramp; repeat exposure triggers a memory response within hours. This is what vaccines exploit.

Key cells

• Neutrophils — most abundant white cell, first responders, short-lived, phagocytose and release enzymes.
• Macrophages — tissue-resident eaters that also present antigens to T cells.
• Dendritic cells — the bridge between innate and adaptive; they sample antigens and present them to T cells in lymph nodes.
• T cells — mature in the thymus, recognize antigens presented on MHC molecules. CD8 cytotoxic, CD4 helper, plus regulatory subtypes that damp responses down.
• B cells — mature in bone marrow, produce antibodies, can class-switch and affinity-mature with repeat exposure.
• NK cells — innate killers that target stressed or missing-self cells, including some tumors.

When it goes wrong

Immunodeficiency goes both ways. Primary immunodeficiencies (inherited) are rare but severe. Secondary — HIV, chemotherapy, chronic steroids, malnutrition — is common and tracks infection risk directly. The opposite failure is autoimmunity, where tolerance breaks and the system attacks self. Type 1 diabetes, rheumatoid arthritis, lupus, Hashimoto's, MS, Crohn's, psoriasis — all are identity errors. Together they affect roughly 5-8% of the population, skew heavily female (for reasons still being worked out), and rising.

Allergy is the immune system reacting strongly to harmless things. Anaphylaxis is the lethal version. Cancer survives partly by evading immune surveillance — modern checkpoint inhibitor immunotherapy takes the brakes off T cells and has rewritten oncology for several tumor types.

Interactions

Cortisol is immunosuppressive, which is why chronic stress blunts responses and why exogenous glucocorticoids are used to treat autoimmunity and transplant rejection. Sleep deprivation reduces vaccine response measurably. Sex hormones influence tolerance — testosterone is net immunosuppressive, estrogen is more complex, and this is part of why autoimmunity skews female. The gut microbiome trains the mucosal immune system, especially in early childhood.

Honest take

Sources

• Janeway's Immunobiology, 10th ed. — the reference on adaptive and innate immunity.
• Murphy et al., Nature Reviews Immunology — on autoimmunity and sex dimorphism.
• Prüss-Ustün et al., WHO reports — on global immunodeficiency burden, including HIV and malnutrition.