Overdominance the heterozygote has higher fitness than the  fitnesses of the two homozygotes

Underdominance, in which the heterozygote has lower fitness than both homozygotes.

Directional selection leads to fixation of the favored allele and elimination of the other allele, as  long as no other evolutionary forces act on the population.

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T-dependent response

Recognition of protein antigens and requires the participation of CD4 helper T cells

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Plasma cell decision point

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T-independent response

Type 1 T-independent (TI-1) antigens bind to B cells through both Ig and innate immune receptors. LPS from Gram negative organisms binds to B cells via both mIg and TLR, resulting in signaling from both receptors. 

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Type 2 T-independent