Food, Nutrition and Immunity

5.1 Nutritional Requirements

Macronutrients

• Carbohydrates: Primary energy source. 4 kcal/g. Starch (plant storage), glycogen (animal storage), cellulose (plant structural — dietary fibre).
• Proteins: 4 kcal/g. Building blocks = 20 amino acids (9 essential amino acids from diet). Functions: enzymes, hormones, antibodies, structural (collagen, keratin).
• Fats: 9 kcal/g. Saturated fats (animal sources) — solid at RT; unsaturated fats (vegetable oils) — liquid. Trans fats (hydrogenated oils) — associated with cardiovascular disease.

Micronutrients (Vitamins and Minerals)

Nutrient	Type	Function	Deficiency Disease
Vitamin A	Fat-soluble	Vision, immunity	Night blindness, xerophthalmia
Vitamin B₁ (Thiamine)	Water-soluble	Carbohydrate metabolism	Beriberi
Vitamin B₁₂	Water-soluble	RBC formation, nerve	Pernicious anaemia
Vitamin C (Ascorbic acid)	Water-soluble	Collagen synthesis, antioxidant	Scurvy
Vitamin D	Fat-soluble	Ca²⁺ absorption from gut	Rickets (children), Osteomalacia (adults)
Vitamin K	Fat-soluble	Blood clotting (coagulation factors)	Excessive bleeding
Iron	Mineral	Haemoglobin, oxygen transport	Anaemia
Iodine	Mineral	Thyroxine synthesis	Goitre, cretinism
Calcium	Mineral	Bones, teeth, muscle contraction, clotting	Osteoporosis, tetany
Zinc	Mineral	Enzyme cofactor, wound healing, immunity	Growth retardation, poor immunity

5.2 Immunity in Detail

Innate (Non-specific) Immunity

• First line: Physical barriers — skin, mucus, cilia, stomach acid.
• Second line: Internal cellular defences — phagocytes (neutrophils, macrophages engulf pathogens), natural killer cells (destroy virus-infected cells), complement system (protein cascade causing bacterial lysis), inflammation (increased blood flow, phagocyte recruitment), fever (reduces pathogen replication).

Adaptive (Specific) Immunity

• Humoral Immunity (B lymphocytes): B cells activated → differentiate into plasma cells (secrete specific antibodies) and memory B cells (for future rapid response).
• Antibodies (Immunoglobulins): IgG (most common; crosses placenta), IgA (mucosal surfaces, breast milk), IgM (first to be produced in infection), IgE (allergy, parasites).
• Cell-mediated immunity (T lymphocytes): T helper cells (CD4+ — activate B cells and cytotoxic T cells); T cytotoxic cells (CD8+ — kill virus-infected and cancer cells); T regulatory cells (suppress autoimmunity).

HIV and AIDS

HIV (Human Immunodeficiency Virus) is a retrovirus that specifically attacks CD4+ T helper cells, causing progressive immune failure leading to AIDS (Acquired Immune Deficiency Syndrome). AIDS is diagnosed when CD4 count falls below 200 cells/mm³ (normal: 500–1500). It is treated with antiretroviral therapy (ART) — not cured but controlled. India has ~2.4 million PLHIV (2023).

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