Page 106 - Concise Pathology for Exam Preparation ( PDFDrive )

5 Diseases of Immunity 91

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• �The complement system has several important functions n innate immunity and

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consists t least 20 serum glycoproteins, which are activated in a cascade sequence,

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meaning that activation of a single molecule will lead to thousands of molecules being

generated and therefore amplification of the response.
2.
Acquired or adaptive
• �The adaptive immune system is the second line of defence against pathogens that

are able to evade or overcome innate immune defences. This is an antigen-specific
immune response.
• �There are two types of adaptive immune responses: humoral immunity, mediated

by antibodies produced by B lymphocytes and cell-mediated immunity (CMI),
mediated by T lymphocytes.
• �The T and B cells of the adaptive immune response are responsible for long-term
memory. Upon secondary exposure to a specific antigen, the cells of the adaptive
immune response exert their effects in a stronger and quicker way than the natural
or innate response.

The immune system has several functions, most important of which is self-recognition

and non-self-recognition. When the process self-recognition breaks down and the

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immune system begins to attack self-antigens, the condition is labelled as autoimmunity.

Examples of autoimmune diseases include systemic lupus erythematosus (SLE),

rheumatoid arthritis (RA) and diabetes mellitus (DM).

Q. Write briefly on the cells of immune system.
Ans. Cells of the immune system include the following:
Lymphocytes
Lymphocytes express specific receptors for antigens. Their maturation takes place before
exposure to this antigen. This is referred to as ‘clonal selection’. Lymphocytes of the same
specificity are said to belong to the same clone and express the same antigen receptors.
T Lymphocytes
• �Thymus-derived cells, which are mediators of CMI.
• �Mature T cells constitute 60–70% of circulating lymphocytes and are also present in the
paracortical region of lymph node and periarteriolar sheath of the spleen.
• �Each T cell is genetically programmed to recognize a specific cell-bound antigen by
means of an antigen-specific T cell receptor (TCR).

• �In 95% T cells, ‘TCR’ consists of a disulphide linkage made up of a and b polypeptide

chains. n 5% T cells, the disulphide linkage is made up of g and d polypeptide chains. The
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ab TCR recognizes peptide antigens presented by major histocompatibility antigens. The gd
TCR recognizes peptides, lipids and small molecules without the need for antigen presenta-

tion y major histocompatibility antigens. gd cells are present in the epithelial surfaces (skin,

b

mucosa, GIT) and mainly protect from microbes entering through epithelial surfaces.

• A subset of T cells expresses markers that are also found on NK cells (NK-T cells). The

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NK-T cells recognize glycolipids displayed by major histocompatibility complex (MHC)

like molecule CD1 and their function is inadequately defined.

• �‘TCR diversity’ is generated by somatic rearrangement of genes coding for a, b and g,

d chains.
• �The enzyme in developing lymphocytes that mediates rearrangement of TCR is the
product of RAG1 and RAG2 (recombination activating genes). Inherited defects in RAG
proteins results in failure to generate mature lymphocytes.

• �Each TCR noncovalently linked five polypeptide chains, which form the CD3
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complex and z-chain dimer (TCR complex). CD3 complex and z-chain dimer are iden-

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tical cells.
• �In addition to CD3 proteins, T cells express a variety of other molecules, ie, CD4, CD8,
CD2, CD11a, CD28, CD40 and integrins.

• �CD4 is expressed in 60% of mature CD31 T cells and CD8 is expressed in 30% of
mature CD31 T cells.
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