SCID or severe combined immunodeficiency disease is one of the most widely studied immunodeficiency diseases. Two types of severe combined immunodeficiency disease (SCID) exist with different modes of inheritance and gens involved.

Differences between the two are discussed below.

X- LINKED SCID

It is the commonest form of SCID accounting for 50-60%

Since inheritance is X-linked, it is more common in boys.

Defect is caused due to mutation in the common gamma chain subunit of cytokine receptors.

Defect in T cells are more pronounced compared to B cells. Defects in NK cell is also present

Thymus is small and devoid of lymphoid follicles and contains lobules of undifferentiated epithelial cells resembling fetal thymus.

Gene therapy has been successful.

AUTOSOMAL RECESSIVE SCID

Most common form of AR SCID
Defect in T> B cells

Thymus is small and devoid of lymphoid follicles and remnants of Hassall’s corpuscles can be found.

Hematopoietic stem cell transplantation is the mainstay of treatment. Enzyme therapy and gene therapies have been tried.

SOME OTHER FORMS OF AUTOSOMAL RECESSIVE SCID

Mutations in recombinase-activating genes (RAG)

Mutations in JAK 3 have similar effects as mutations in the γc chain.

Several mutations have been described in signaling molecules associated with the T-cell antigen receptor and components of calcium channels.

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