Biology

Immunity

6,353 words, 10 sections

The body defends itself in layers, and this chapter is the account of every one of them: the barriers and phagocytes that work without instruction, the lymphocytes that work on instruction, the antibodies they secrete, the proteins that amplify both, and the ways the whole arrangement misfires in allergy, autoimmunity and immunodeficiency. The papers hold 166 distinct questions on this chapter across 63 official papers between 2009 and 2026, drawn from six boards, and the practice bank carries 695 live questions on it. Biology is 81 of the 180 marks in the paper, and a mean item difficulty of 1.37 puts this chapter among the more straightforward marks available. The syllabus attaches a single learning outcome to it, 12.1, on specific defence mechanisms, and that outcome is far narrower than what the papers ask: the topic filed as Immune Defense Mechanisms alone accounts for 135 of the 166 questions. Where the outcome and the paper disagree, this note follows the paper, and says so.

There are two systems, and the cleanest way to hold them is to ask what each one already knows. Innate immunity, also called inborn immunity, knows nothing in particular. It follows from your genetic make-up, it exists before any infection is acquired, and it answers every invader in the same way. It is built from physical barriers such as skin and mucous membranes, cellular barriers such as macrophages, neutrophils, mast cells, eosinophils and basophils, and biochemical barriers such as cytokines, interferon and interleukins. Adaptive immunity, also called acquired immunity, is specific and it remembers. It comes into action after the innate line has been breached, it is carried out by T and B lymphocytes, it is slow to begin and long lasting because of memory, sometimes for life, and it varies from person to person.

The adaptive army is the lymphocytes. They make up $20$ to $40$ per cent of circulating white blood cells and are found in the blood as well as in the tissues. Both kinds are antigen-specific and carry their own receptors, the T cell receptor and the B cell receptor. Their average life is about a week to a few months, though a few survive for years, and the long-lived ones are precisely the cells that carry immunological memory from one infection to the next.

Within the innate line the order of arrival matters. Neutrophils are the most abundant white blood cells and the first to reach a site of bacterial infection, moving there quickly and phagocytosing what they meet. Macrophages come after them and finish the job, clearing the debris and the pus that accumulate as the battle ends. Kupffer cells are the phagocytic macrophages of the liver.

Worked example

The question asks which of four items is not a first line of defence. The options are tears, mucous membranes, ciliated cells and cytotoxic T cells. Tears, mucous membranes and ciliated cells all act on the surface of the body or in its secretions, stopping a pathogen before it gets in; all three are physical or mechanical barriers, and mucous membranes in particular are classified as physical barriers rather than chemical ones. Cytotoxic T cells carry a T cell receptor and a $CD8$ co-receptor, they cannot act until antigen is presented to them on $\mathrm{MHC\text{-}I}$ , and their entire purpose is to kill infected cells once infection is established. They belong to the third line, so they are the answer. The step students get wrong is treating any immune cell as a barrier. A barrier keeps the pathogen out; a cell deals with it after it is in.

8 more sections in this chapter

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Introduction

1. Innate and adaptive immunity, and the cells on each sideThere are two systems, and the cleanest way to hold them is to ask what each one already knows. Innate immunity, also called inborn immunity, knows nothing in p